Class 10 Science Chapter LightReflection And Refraction
Question 1. Define the principal focus of a concave mirror.
Answer:
It is a point on the principal axis at which a beam of incident light parallel to the principal axis converges after reflection from the concave mirror.
Question 2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer:
Focal length = \(\frac{1}{2}\) x Radius of curvature
= \(\frac{1}{2}\) × 20 cm
= 10 cm
Question 3. Name a mirror that can give an erect and enlarged image of an object.
Answer: Concave mirror
Question 5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
As the radius of curvature and focal length of a convex mirror are positive, so
R = +32 cm
and f= \(\frac{R}{2}\)
f = +16 cm
Question 6. A concave mirror produces a times magnified (enlarged) real image of an object placed 10 cm in front of it. Where is the image located?
Answer:
As the image is real, so magnification m must be negative.
∴ \(m=\frac{h^{\prime}}{h}=\frac{v}{u}=3\)
Or v = 3u
But u = 10 cm
∴ v = 3 x (10)
v = 30 cm
Thus, the image is located at a distance of 30 cm from the mirror on the object side of the mirror.
Question 7. A ray of light traveling in air enters obliquely into water. Does the light ray bends towards the normal or away from the normal? Why?
Answer:
Light travels faster in the rarer mediums as air and slower in the denser mediums as water. As a ray of traveling in air enters obliquely into the water, it slows down and bends towards the normal.
Question 8. Light enters from air to glass and has an ing refractive info of dex 1.50. What is the speed of light in the glass? The speed of light in a vacuum is 3 x 10^{8} ms^{1}.
Answer:
The refractive index of glass, n_{g} = 1.50
Speed of light in vacuum, c = 3 × 10^{8} ms^{1}
Speed of light in glass = \(\frac{3 \times 10^8 \mathrm{~m} / \mathrm{s}}{1.5}\) = 2 × 10^{8} ms^{1}
Question 9. Find out, the medium having the highest optical density. Also, find the medium with the lowest optical density.
Answer:
The diamond has the highest refractive index (2.42), so it has the largest optical density. Air has the lowest refractive index (1.0003), so it has the lowest optical density.
Question 10. You are given kerosene, turpentine, and water. In which of these does the light travel fastest? Use the information given.
Refractive indices of some material media (concerning vacuum):
Answer:
For kerosene, n = 1.44
For turpentine oil, n = 1.47
For water, n = 1.33
As water has the lowest refractive index, so light travels faster in this optically rarer medium than in kerosene and turpentine oil.
Question 11. The refractive index of a diamond is 2.42. What is the meaning of this statement?
Answer:
The refractive index of a diamond is 2.42. It indicates that the ratio of the speed of light in air
to that in diamond is 2.42.
Question 12. Define 1 dioptre of power of a lens.
Answer: One dioptre is the power of a lens whose focal length is 1 meter.
Question 13. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
Answer:
Here, v = +50 cm [v is positive for real image]
As the real image is of the same size as the object, so
⇒ \(m=\frac{h^{\prime}}{h}=\frac{v}{u}=1\)
Or u = v=50 cm
Now,\(\frac{1}{f}=\frac{1}{v}\frac{1}{u}=\frac{1}{+50}\frac{1}{50}=+\frac{2}{50}=+\frac{1}{25}\)
Or f = 25 cm = 0.25 m
P = \(\frac{1}{f}=\frac{1}{+0.25 \mathrm{~m}}\)= + 4 D
Question 14. Find the power of a concave lens of focal length 2 m.
Answer:
As the focal length of a concave lens is negative, so f = − 2 m
Power,
P = \(\frac{1}{f(\text { in } \mathrm{m})}\)
⇒ \(\frac{1}{2 \mathrm{~m}}=0.5 \mathrm{D}\)
Question 15. Which one of the following materials cannot be used to make a lens?
 Water
 Glass
 Plastic
 Clay
Answer: 4. Clay being opaque, does not transmit light.
Question 16. The image formed by a concave mirror is observed to be virtual, erect, and larger than the object. Where should be the position of the object?
 Between the principal focus and the center of curvature.
 At the center of curvature.
 Beyond the center of curvature.
 Between the pole of the mirror and its principal focus.
Answer: 4. Between the pole of the mirror and its principal focus.
Question 17. Where should an object be placed in front of a convex lens to get a real image of the size of the object?
 At the principal focus of the lens.
 At twice the focal length.
 At infinity.
 Between the optical center of the lens and its principal focus.
Answer: 2. At twice the focal length.
Question 18. A spherical mirror and a thin spherical lens have each a focal length of 15 cm. The mirror and the lens are likely to be
 Both concave.
 Both convex.
 The mirror is concave and the lens is convex.
 The mirror is convex, but the lens is concave.
Answer: 1. Both concave (as both concave mirror and concave lens have a negative focal length).
Question 19. No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
 Plane.
 Concave.
 Convex.
 Either plane or convex.
Answer: 3. The mirror is convex.
Question 20. Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
 A convex lens of focal length 50 cm.
 A concave lens of focal length 50 cm.
 A convex lens of focal length 5 cm.
 A concave lens of focal length 5 cm.
Answer: 3. A convex lens of short focal length (5 cm).
Question 21. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.
Answer:
The object should be placed between the focus F and the pole P of the concave mirror, i.e., between 0 and 15 cm from the mirror. Then the image will be virtual, erect, and larger than the object.
Question 22. Name the type of mirror used in the following situations:
 Headlights of a car
 Side/rearview mirror of a vehicle
 Solar furnace
 Support your answer with reason.
Answer:
 Concave mirrors are used as reflectors in the headlights of cars. When a bulb is located at the focus of the concave mirror, the light rays after reflection from the mirror travel over a large distance as a parallel beam of high intensity.
 A convex mirror is used as a side/rearview mirror of a vehicle because of the following reasons:
 A convex mirror always forms an erect, virtual, and diminished image of an object placed anywhere in front of it.
 A convex mirror has a wider field of view than a plane mirror of the same size.
 Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Question 23. Onehalf of a convex lens is covered with black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Answer:
As shown in the figure, when the lower half of the convex lens is covered with black paper, it still forms the complete image of the object as that with the uncovered lens. However, the intensity of the image is reduced when onehalf of the convex lens is covered with black paper.
Question 24. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size, and nature of the image formed.
Answer:
A converging lens means a convex lens. As the distances given in the question are large, so we choose a scale of 1:5, i.e., 1 cm represents 5 cm.
Therefore, on this scale, a 5 cm high object, an object distance of 25 cm, and a focal length of 10 cm can be represented by 1 cm high, 5 cm, and 2 cm lines respectively.
Now, we draw the ray diagram as follows:
 Draw a horizontal line to represent the principal axis of the convex lens.
 The Centre line is shown by DE.
 Mark two foci F and F” on two sides of the lens, each at a distance of 2 cm from the lens.
 Draw an arrow AB of height 1 cm on the left side of the lens at a distance of 5 cm from the lens.
 Draw a line AD parallel to the principal axis and then, allow it to pass straight through the focus (F) on the right side of the lens.
 Draw a line from A to C (center of the lens), which goes straight without deviation.
 Let the two lines starting from A meet at A’.
 Draw A’B’ perpendicular to the principal axis from A’.
 Now A’B’ represents the real, but inverted image of the object AB.
 Then, measure CB’ and A’B’. It is found that CB’ = 3.3 cm and A’B’ = 0.7 cm.
 Thus, the final position, nature, and size of the image A’ B’are:
 Position of image A’B’ = 3.3 cm x 5 = 16.5 cm from the lens of the opposite side.
 The nature of image A’ B’ is real and inverted.
 Height of image A’B’ = 0.7 cm x 5 = 3.5 cm, i.e., the image is smaller than the object.
Question 25. A concave lens of focal length 15 cm forms an image of 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer:
Focal length, f= 15 cm [If is ve for a concave lens]
Image distance, v = 10 cm
[Concave lens forms a virtual image on the same side as the object, so v is ve]
As \(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\)
∴ \(\frac{1}{u}=\frac{1}{v}\frac{1}{f}\)
⇒ \(\frac{1}{10}\frac{1}{15}\)
⇒ \(\frac{3+2}{30}\)
⇒ \(\frac{1}{30}\)
Object distance, u = −30 cm
Drawing the ray diagram: Using a scale of 1:5, we get u = 2 cm, f= 3 cm. We draw the ray diagram as follows:
 Draw the principal axis (a horizontal line).
 Draw a convex lens, keeping the principal center (C) on the principal axis.
 Mark points F and B on the left side of the lens at a distance of 15 cm and 30 cm. respectively.
 Join any point D (nearly at the top of the lens) and F by a dotted line.
 Draw a line AD, parallel to the principal axis.
 Draw a line A’B’ perpendicular to the principal axis from B’.
 Draw a line CA’ backward, so that it meets the line from D parallel to the principal axis at A.
 Draw a line AB, perpendicular (downwards) from A to meet the principal axis at B.
 The AB is the position of the object. Measure distance BC. It will be found to be equal to 6 cm.
The object is placed at a distance of 6 cm x 5 = 30 cm from the lens.
Question 26. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
Answer:
Object distance, u = 10 cm
Focal length, f= +15 cm [f is +ve for a convex mirror]
Image distance, v =?
As \(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)
∴ \(\frac{1}{v}=\frac{1}{f}\frac{1}{u}\)
⇒ \(\frac{1}{+15}\frac{1}{10}\)
⇒ \(\frac{2+3}{30}\)
⇒ \(\frac{1}{6}\)
Image distance, v = +6 cm
As u is +ve, so a virtual, erect image is formed at a distance of 6 cm behind the mirror.
Question 27. The magnification produced by a plane mirror is +1. What does this mean?
Answer:
As m = \(\frac{h^{\prime}}{h}=\frac{v}{u}\)
Given a plane mirror, m = +1, so
h’ h and v=u
 m = 1 indicates that the size of the image is the same as a positive time sign of m indicates that a virtual image is formed behind the mirror.
Question 28. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror with a radius of curvature of 30 cm. Find the position of the image, its nature, and size.
Answer:
Object size, h = +5 cm
Object distance, u = 20 cm
The radius of curvature, R= +3.0 cm [R is +ve for a convex mirror]
Focal length, \(f=\frac{R}{2}=+15 \mathrm{~cm}\)
From the mirror formula,
∴ \(\frac{1}{v}=\frac{1}{f}\frac{1}{u}\)
⇒ \(=\frac{1}{+15}\frac{1}{20}\)
⇒ \(\frac{4+3}{60}=\frac{7}{60}\)
Image distance, \(v=\frac{60}{7}=8.6 \mathrm{~cm}\)
Magnification, \(m=\frac{v}{u}=\frac{h^{\prime}}{h}\)
Image size, \(h^{\prime}=\frac{v h}{u}\)
⇒ \(\frac{8.6 \times 5}{20}\)
= 2.15 ≅ 2.2 cm
A virtual, erect image of a height of 2.2 cm is formed behind the mirror at a distance of 8.6 cm from the mirror.
Question 29. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of a focal length of 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Answer:
Object size, h = +7.0 cm
Object distance, u = −27 cm
Focal length, f = 18 cm
Image distance, v =?
Image size, h’ = ?
As \(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)
∴ \(\frac{1}{v}=\frac{1}{f}\frac{1}{u}\)
⇒ \(\frac{1}{18}\frac{1}{27}\)
⇒ \(\frac{3+2}{54}=\frac{1}{54}\)
Or V = – 54 cm
The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image.
Magnification, \(m=\frac{h^{\prime}}{h}=\frac{v}{u}\)
Image size, \(h^{\prime}=\frac{v h}{u}\)
⇒ \(\frac{(54) \times(+7)}{(27)}\)
= 14 cm
The image is real, inverted, and enlarged in size.
Question 30. Find the focal length of a lens of power 2.0 D. What type of lens is this?
Answer:
Here, P = 2.0 D
⇒ \(f=\frac{1}{P}\)
⇒ \(=\frac{1}{2.0 \mathrm{D}}\)
= − 0.5 m
The type of lens is concave.
Question 31. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Answer:
Here, P = +1.5 D
⇒ \(f=\frac{1}{\mathrm{P}}\)
⇒ \(\frac{1}{+1.5 \mathrm{D}}\)
⇒ \(+\frac{10}{15} \mathrm{~m}\)
= +66.67 cm
As the focal length is positive, the prescribed lens is converging.
Question 32. Define absolute refractive index. Absolute refractive indices of medium ‘A’ and medium ‘B’ are ‘n’ and ‘n’ respectively, what is the refractive index of medium ‘B’ concerning medium ‘A’? How does the velocity of light vary with the change in the optical density of the media?
Answer:
The absolute refractive index of a medium is defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium. If c is the velocity of light in the vacuum and v in the medium, then n = c/v.
If n_{a} and n_{b} are the absolute refractive indices of the medium A and B respectively, then
n_{a} = \(\frac{c}{v_a} \text { and } n_b=\frac{c}{v_b}\)then
_{b}n_{a} = \(\frac{\text { Velocity of light in B }}{\text { Velocity of light in A }}=\frac{v_b}{v_a}=\frac{n_a}{n_b}\)
Question 33. What is the minimum number of rays required for locating the image formed by a concave mirror for an object? Draw a ray diagram to show the formation of a virtual image by a concave mirror.
Answer:
At least two rays are required for locating the image of an object formed by a concave mirror.
A concave mirror forms a virtual image of an object when it is placed between the focus and pole of the mirror as shown:
Question 34.
 Define the ine power of a lens and write its SI unit.
 A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the lens, the the if image size is equal to the object size? Also, find the power of the lens.
Answer:
 It is defined as the reciprocal of focal length lens cans to bend the rays of light falling on it. Its SI unit is dioptre.
 Given v = 50 cm, since the size of the image and that of the object are the same, therefore, the image is formed at 2f.
Hence, 2f = 50 or f= 25 cm = 0.25 m
Now, P = \(\frac{1}{f}=\frac{1}{0.25}=\frac{100}{25}\)= + 4 D
Question 35. Discuss the position and nature of the image formed by a concave mirror when the object is moved from infinity toward the pole of the mirror.
Answer:
As the object is moved from infinity towards the pole of a concave mirror the image shifts from the focus of the mirror to infinity.
When the object is at infinity, the image is formed at the principal focus or in the focal plane. As the object is shifted, further the image is formed between the principal focus centimeter of curvature, then at the centimeter of curvature, beyond the centimeter of curvature, at infi, city, and finally the image is formed behind the mirror. The size of the increasing.
Question 36. With the help of a ray diagram, show the formation of an image by a convex lens when the object is placed between F and 2F. Write two characteristics of the image so formed.
Answer: The image formed is real, I’ve, red and diminished.
Question 37.
 Define the line radius of curvature and focal length of a spherical mirror and show it on a figure.
 Write the relation between the radius of curvature and the focal length of a spherical mirror.
Answer:
 The radius of curvature is the radius of the hollow sphere of which the reflecting surface of the spherical mirror forms a part. Focal length is the distance from the pole of the mirror to the focal point F.
 Relation: \(f=\frac{\mathrm{R}}{2}\)
Question 38. An object of height 5 cm is placed perpendicular to the principal axis of a concave lens of focal length 10 cm. If the distance of the object from the opticenterntre of the lens is 20 cm, determine the position, an true, and size of the image formed using the lens formula.
Answer:
Given h+5 cm, f = – 10 cm, u = 20 cm, v = ?, h’ =?
⇒ \(\frac{1}{f}=\frac{1}{v}\frac{1}{u}\)
∴ \(\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{(10)}+\frac{1}{(20)}=\frac{21}{20}=\frac{3}{20}\)
∴ \(v=\frac{20}{3} \mathrm{~cm}\)
Nature: Virtual, erect.
⇒ \(h^{\prime}=\frac{v}{u} \times h=\left(\frac{20}{3} \times \frac{1}{20} \times 5\right) \mathrm{cm}=+\frac{5}{3} \mathrm{~cm}\)
Question 39. If the image formed by a mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a ray diagram to justify your answer. Where and why do we generally use this type of mirror?
Answer:
Convex mirror
Use: As rear view mirror in vehicles. Also in malls, hotels, and airports for security reasons.
Reason:
 Forms erect image
 Wider field of view.
Question 40. A convex lens has a focal length of 25 cm. Calculate the distance of the object from the lens if the image is to be formed on the opposite side of the lens at a distance of 75 cm from the lens. What will be the nature of the image?
Answer:
Given, f= + 25 cm, v = 75 cm, u =?
Using \(\frac{1}{v}\frac{1}{u}=\frac{1}{f}\) We have
⇒ \(\frac{1}{u}=\frac{1}{v}\frac{1}{f}=\frac{1}{75}\frac{1}{+25}=\frac{2}{75}\)
Therefore, u = 37.5cm
The image is real, inverted, and magnified.
Question 41. A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. Give the location of the image and its magnification.
Answer:
Given that O= 4.5 cm, u = − 12 cm, f= + 15 cm
Using the mirror formula \(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)
Or \(\frac{1}{v}=\frac{1}{f}\frac{1}{u}=\frac{1}{15}\frac{1}{12}=\frac{9}{60}[/latex
Therefore, v = 6.7 cm
The positive sign indicates that the image formed is on the other side of the mirror.
Also, [latex]m=\frac{1}{\mathrm{O}}=\frac{v}{u} \Rightarrow \frac{1}{4.5}=\frac{6.7}{12}\)
Therefore, I = \(\frac{6.7 \times 4.5}{12}=+2.5 \mathrm{~cm}\)
I = + 2.5 cm
The positive sign indicates that the hat image formed is erect and virtual.
Question 42. How will you decide whether a given piece of glass is a convex lens, concave, lens, or plane glass plate?
Answer:
Hold the given piece of glass over some printed matter.
 If the letters appear magnified, the given piece is a convex lens.
 If the letters appear diminished, the given piece is a concave lens.
 If the letters appear to be of the same size, then it is a plane glass piece.
Question 43. A student wants to project the image of a candle flame on a screen 60 cm in front of a mirror by keeping the flame at a distance of 15 cm from its pole.
 Write the type of mirror he should use.
 Find the linear magnification of the image produced.
 What is the distance between the object and its image?
 Draw a ray diagram to show the image formation in this case.
Answer:
 Concave mirror
 Linear magnification is given by
 \(m=\frac{v}{u}=\frac{60}{15}=4\)
 6015 = 45 cm
 The diagram is as shown:
Question 44. A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. Give the location the the of image and magnification. Describe what happens to the image as the needle is moved farther from the mirror.
Answer:
h1 = 4.5 cm, u = – 12 cm, f = 15 cm, v = ?
Using the equation \(\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\) we have
⇒ \(\frac{1}{15}=\frac{1}{v}+\frac{1}{12} \text { or } \frac{1}{15}+\frac{1}{12}=\frac{1}{v}\)
Solving for u, we have v = + 6.6 cm
Now, \(m=\frac{v}{u}=\frac{6.6}{12}\) = 0The imagemage will diminish.
Question 45. With the help a of ray diagram, show that angle of incidence is equal to the angle of reflection when a ray is incident on the concave mirror.
Answer:
In the ray diagram shown below, CB is the normal and ∠i = ∠r, AB is the incident ray and BD is the reflected ray.
Question 46.
 State Snell’s law of refraction.
 When a ray of traveling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a ray diagram to illustrate it.
Answer:
 The ratio the the of sine of the angle of incidence to the signoff the angle of refraction is constant for the light of the color and a given pair of the medium.
 The diagram is as shown:
Question 47. The power of a lens is 2.5 dioptre. What is the focal length and the type of lens?
Answer:
Power of a lens is P = \(\frac{1}{f}\)
Since power = 2.5 dioptre
2.5 = \(\frac{1}{f}\)
or f= \(\frac{1}{f} \text { or } f=\frac{1}{2.5}\) m = 0.4 m = 40 cm
The focal length of the lens is 40 cm. Since the focal length is positive, therefore, the lens is a convex lens.
Question 48. The refractive index of dense flint glass is 1.65 and for all,ohol it is 1. concerning to air. What is the refractive index of dense flint to alcohol?
Answer:
Given \({ }_a \mu_g=1.65 \text { and }{ }_a \mu_{a l c}=1.36\)
Now, \({ }_{\text {alc }} \mu_g=\frac{{ }_a \mu_g}{{ }_a \mu_{\text {alc }}}=\frac{1.65}{1.36}=1.21\)
Question 49.
 Water a has a refractive of dex 1.33 and alcohol a has a refractive of dex 1.36. Which of the two mediums is optically denser? Ga’s reason’s your answer. Draw a ray diagram to show the path of a ray of light passing obliquely from water to alcohol.
 The absolute refractive index of diamond is 2.42 and the absolute refractive index of glass is 1.50. Find the refractive index of a diamond concerning glass.
Answer:
1. Alcohol is optically denser than water. This is because use greater the refractive index more dense the liquid/material.
The diagram is as shown:
2. Given _{a}n_{d} = 2.42, _{a}n_{g} = 1.50, _{g}n_{d} = ?
Using the expression
⇒ \({ }_g n_d=\frac{{ }_a n_d}{{ }_a n_g}=\frac{2.42}{1.50}=1.61\)
Question 50. Show that the relative refractive index of medium concerning medium 1 can be expressed as the ratio of their absolute refractive indices.
Answer:
If n_{1} and n_{2} are the absolute refractive indices of media 1 and 2 the and speeds of light in the two media, then
⇒ \(n_1=\frac{c}{v_1} \quad \text { or } \quad v_1=\frac{c}{n_1}\)
and \(n_2=\frac{c}{v_2} \quad \text { or } \quad v_2=\frac{c}{n_2}\)
But the relative refractive index of medium concerning to medium 1 is
⇒ \(n_{21}=\frac{v_1}{v_2}=\frac{c / n_1}{c / n_2}\)
⇒ \(n_{21}=\frac{n_2}{n_1}\)
Question 51. Name the spherical mirror used as:

 Shaving mirror
 Rearview mirror in vehicles
 Reflector in searchlights.
 Write any three differences between a real and virtual image.
Answer:

 Concave mirror
 Convex mirror
 Convex mirror
2. Points of difference between a real and virtual image:
Question 52.
 For the same angle of incidence, 45°, the angle of refraction in two transparent media; 1 and 2 is 20° and 30° respectively. Out of 1 and 2, which medium is optically denser and why?
 Light enters from the air to the diamond which has a refractive index of 2.42. Calculate the speed of light in a diamond, if the speed of light in air is 3.00 × 10^{8}m s^{1}. Answer:
 Medium 1 is optically denser as the angle of refraction is lesser in it, hence light bends more towards normal.
 n = speed of light in vacuum/speed of light in medium 2.42 = 3 x 10^{8}/v or v = 1.24 × 10^{8} ms^{1}
Question 53. What is meant by the power of a lens? Give its SI unit. When two or more lenses are placed in contact, what will be their combined power?
Answer:
The power of a lens is defined as the ability of the lens to bend the rays of light incident on it. i.e., P = 1/f
The SI unit of power is called a dioptre. If P_{1} and P_{2} are the powers, then P of the combination is given by P = P_{1} + P_{2}.
Question 54.
 What is meant by the refraction of light?
 If applying the Cartesian sign convention for spherical lenses the image distance obtained is negative. State the significance of the negative sign.
Answer:
 When light travels obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as the refraction of light.
 A negative sign of image distance means the image is virtual and erect and is formed on the same side of the object concerning the lens.
Question 55. A studentfocused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations:
 From the above table, find the focal length of the lens without using the lens formula.
 Which set of observations is incorrect and why?
 In which case the size of the object and image will be the same? Give a reason for your answer.
Answer:
 From the table of observation, it is clear that for u = 30, v = 30 cm. This means that both u and v must be equal to twice the focal length of the convex lens.
 Focal length of the convex lens = \(=\frac{30}{2}\) = 15 cm
 The observation (v) is incorrect. For this observation, u = 15f, i.e., the object is at
the focus and the image must be formed at infinity and not 70 cm.  Since m = \(\frac{v}{u}\) the size of the image will be equal to the size of the object, if v = u i.e., for the observation (iii).
Question 56.
 What happens to a ray of light when it travels from one medium to another having equal refractive indices?
 State the cause of the refraction of light.
Answer:
 No refraction or bending would take place. The light will travel in a straight line.
 The refraction occurs due to a change in the speed of light as it enters from one medium to another.
Question 57.
 What should be the position of the object, when a concave mirror is to be used:
 As a shaving mirror, and
 In torches producing parallel beams of light?
 A man standing in front of a mirror finds his image having a very small head and legs of normal size. What type of mirror are used in designing such a mirror?
Answer:

 The object should be between the pole and the focus.
 At the focus.
 Small head – convex mirror
 Legs of normal size – plane mirror.
Question 58. How can you identify the three types of mirrors without touching them?
Answer:
We will look into the mirror by going close to it:
 If an image is of the same size and erect – –plane mirror.
 If an image is bigger and erect – a concave mirror.
 If an image is of a smaller size and erect – convex mirror.
Question 59.
 Why does a ray of light pass through the center of curvature of a concave mirror after reflection is reflected along the same path?
 Why are convex mirrors commonly used as rearview mirrors in vehicles?
 A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water. Name the phenomenon of light responsible for it.
Answer:
 Because the incident ray falls on the mirror along the normal to the reflecting surface.
 Because they always give an erect, though diminished image. Thus, convex mirrors enable the driver to view a much larger area.
 Refraction of light
Question 60. An object is placed at the following distances from a concave mirror of focal length 15 cm
 10 cm
 20 cm
 30 cm
 40 cm
Which position of the object will produce:
 Virtual image
 A diminished real image
 An enlarged real image
 An image of the same size
Answer:
 10 cm
 40 cm
 20 cm
 30 cm.
Question 61. A ray of light is incident obliquely on a glass slab. Draw a ray diagram showing the path of the light ray. Mark the angle of incidence, angle of refraction, angle of emergence, and lateral displacement of the ray. Give a formula to find the refractive index of the glass slab in terms of the angle of incidence and angle of refraction.
Answer:
The ray diagram is as shown:
The required formula is \({ }_a n_g=\frac{\sin i}{\sin r}\)
Question 62. Identify the device used as a spherical mirror or lens in the following cases, when the image formed is virtual and erect in each case.
 The object is placed between the device and its focus, image formed is enlarged and behind it.
 The object is placed between the focus and the device, image formed is enlarged and on the same side as that of the object.
 The object is placed between infinity and the device, image formed is diminished and between focus and optical center on the same side as that of the object.
 The object is placed between infinity and the device, and an image formed is diminished and between the pole and focus, behind it.
Answer:
 Concave mirror
 Convex lens
 Concave lens
 Convex mirror
Question 63. Why does a light ray incident on a rectangular glass slab immersed in any medium emerge parallel?
Answer:
The extent of bending of the ray of light at the opposite parallel faces of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray on a rectangular glass slab. However, the light ray is shifted sideward slightly.
When the glass slab is immersed in any medium the interface and glass medium are equal and opposite, so, the emergent ray will always be parallel to the incident ray.
Question 64. When dipped in water in a glass tumbler, a pencil appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like kerosene or turpentine? Support your answer with reason.
Answer:
A pencil partly immersed in water in a glass tumbler, appears to be displaced at the interface of air and water. The light reaching out from the portion of the pencil inside the water seems to come from a different direction, compared to the part above the water due to the refraction of light.
The pencil appears to be bent to different extents, if instead of water, liquids like kerosene or turpentine are used as their refractive indices are different which in turn produces deviation from incident ray by different extents.
Question 65. How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium concerning another in terms of speed of light in these two media.
Answer:
Refractive index, μ \(=\frac{\text { Speed of light in vacuum, } c}{\text { Speed of light in medium, } v}\)
For medium 1, refractive index of first medium, μ1 \(=\frac{c}{v_1}\)
For medium 2, refractive index of second medium, μ2 \(\mu_2=\frac{c}{v_2}\)
∴ \(\mu_{21}=\frac{\mu_2}{\mu_1}=\frac{\frac{c}{v_2}}{\frac{c}{v_1}}=\frac{v_1}{v_2}\)
Question 66. The refractive index of diamond concerning glass is 1.6 and the absolute refractive index of glass is 1.5. Find out the absolute refractive index of a diamond.
Answer:
The refractive index of diamond concerning glass, and \(=\frac{v_g}{v}\) = 1.6, absolute refractive
Index of glass, ng= \(\frac{c}{v_g},\) and absolute refractive index of diamond, nd = \(\frac{c}{v_d}\)
Therefore, nd= \(\frac{v_g}{v_d} \times \frac{c}{v_g}\) = 1.6 x 1.5= 2.40
Question 67. A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Answer:
The statement is correct if the object is placed within 20 cm of the lens in the first case and between 20 cm and 40 cm in the second case.
Question 68. Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus on the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
Answer:
Sudha should move the screen towards the lens to obtain a clear image of the building. The approximate focal length of this lens is 15 cm.
Question 69. How are the power and focal length of a lens related? You are provided with two lenses of focal lengths of 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light?
Answer:
P= \(\frac{1}{f}\) P∞ \(\frac{1}{f}\) Power of a lens is inversely proportional to its focal length therefore lens has a focal length of 20 cm will provide more convergence.
Question 70. Under what condition in an arrangement of two plane mirrors, incident ray, and reflected ray will always be parallel to each other, whatever may be the angle of incidence? Show the same with the help of a diagram.
Answer:
When two plane mirrors are placed at a right angle to each other then the incident and reflected rays will always be parallel to each other.
Question 71. Draw a ray diagram showing the path of rays of light when they enter with oblique incidence
 From air into water,
 From water into air.
Answer:
When a ray of light enters with oblique incidence from air into water, then it goes from an optical rarer medium to an optical denser medium, and the velocity of light decreases which in turn bends the incident light towards the normal. Also i > r.
When a ray of light enters with oblique incidence from water into air, then it goes from an optical denser medium to an optical rarer medium, and the velocity of light increases which in turn bends the incident light away from the normal. Also I < r.
Question 72. At a small town fair, Akshay took his friend and showed him a mirror in which his image showed his upper half very fat and his lower body very thin. Akshay’s friend got upset but Akshay explained to him by showing him a similar image in the mirror.
 Name two mirrors used in this fair shop.
 Name the mirror in which the size of the image is small.
 What values of Akshay are reflected?
Answer:
 Concave and convex mirror
 Convex mirror
 Akshay showed compassion and empathy.
LightReflection And Refraction Multiple Choice Question And Answers
Question 1. Which of the following can make a parallel beam of light when light from a point source is incident on it?
 Concave mirror as well as convex lens
 Convex mirror as well as concave lens
 Two plane mirrors placed at 90° to each other
 Concave mirror as well as concave lens
Answer: 1. Concave mirror as well as convex lens
Question 2. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is
 30 cm
 20 cm
 40 cm
 60 cm
Answer: 2. 20 cm
Question 3. Under which of the following conditions a concave mirror can form an image larger than the actual object?
 When the object is kept at a distance equal to its radius of curvature
 When an object is kept at a distance less than its focal length
 When an object is placed between the focus and center of curvature
 When an object is kept at a distance greater than its radius of curvature
Answer: 3. When an object is placed between the focus and center of curvature
Question 4. The figure shows a ray of light as it travels from medium A to medium B. Refractive index of medium B relative to medium A is
 √3/√2
 √2/√3
 1/√2
 √2
Answer: 1. √3/√2
Question 5. A light ray enters from medium A to medium B as shown in the figure. The refractive index of medium B relative to A will be
 Greater than unity
 Less than unity
 Equal to unity
 Zero
Answer: 2. Less than unity
Question 6. Beams of light are incident through holes A and B and emerge out of box through the holes C and D respectively as shown in the figure. Which of the following could be inside the box?
 A rectangular glass slab
 A convex lens
 A concave lens
 A prism
Answer: 1. A rectangular glass slab
Question 7. A beam of light is incident through the holes on side A and emerges out of the holes on the other face of the box as shown in the figure. Which of the following could be inside the box?
 Concave lens
 Rectangular glass slab
 Prism
 Convex lens
Answer: 4. Convex lens
Question 8. Which of the following statements is true?
 A convex lens has 4 dioptre power having a focal length of 0.25 m
 A convex lens has 4 dioptre power having a focal length of 0.25 m
 A concave lens has 4 dioptre power having a focal length of 0.25 m
 A concave lens has 4 dioptre power having a focal length of 0.25 m
Answer: 1. A convex lens has 4 dioptre power having a focal length of 0.25 m
Question 9. Magnification produced by a rearview mirror fitted in vehicles
 Is less than one
 Is there more than one
 Is equal to one
 Can be more than or less than one depending upon the position of the object in front of it
Answer: 1. Is less than one
Question 10. Rays from the Sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that the size of its image is equal to the size of the object?
 15 cm in front of the mirror
 30 cm in front of the mirror
 Between 15 cm and 30 cm in front of the mirror
 More than 30 cm in front of the mirror
Answer: 2. 30 cm in front of the mirror
Question 11. A fulllength image of a distant tall building can be seen by using
 A concave mirror
 A convex mirror
 A plane mirror
 Both concave as well as plane mirror
Answer: 2. A convex mirror
Question 12. In torches, searchlights, and headlights of vehicles the bulb is placed
 Between the pole and the focus of the reflector
 Very near to the focus of the reflector
 Between the focus and center of curvature of the reflector
 At the center of the curvature of the reflector
Answer: 2. Very near to the focus of the reflector
Question 13. The laws of reflection hold good for
 Plane mirror only
 Concave mirror only
 Convex mirror only
 All mirrors irrespective of their shape
Answer: 4. All mirrors irrespective of their shape
Question 14. The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as A, B, C, and D in the figure. Which one of them is correct?
 1
 2
 3
 4
Answer: 2. 2
Question 15. You are given water, mustard oil, glycerine, and kerosene. In which of these media a ray of light incident obliquely at the same angle would bend the most?
 Kerosene
 Water
 Mustard oil
 Glycerine
Answer: 4. Glycerine
Question 16. Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in the figure?
 1
 2
 3
 4
Answer: 4. 4
Question 17. Which of the following ray diagrams is correct for the ray of light incident on a lens shown in the figure?
 1
 2
 3
 4
Answer: 1. 1
Question 18. A child is standing in front of a magic mirror. She finds the image of her head bigger, the middle portion of her body of the same size, and that of the legs smaller. The following is the order of combinations for the magic mirror from the top.
 Plane, convex, and concave
 Convex, concave, and plane
 Concave, plane, and convex
 Convex, plane, and concave
Answer: 3. Concave, plane, and convex
Question 19. In which of the following, the image of an object placed at infinity will be highly diminished and pointsized?
 Concave mirror only
 Convex mirror only
 Convex lens only
 Concave mirror, convex mirror, concave lens, and convex lens
Answer: 4. Concave mirror, convex mirror, concave lens, and convex lens