Human Eye And Colourful World Short Answer Questions
Question 1. A star appears slightly higher than its actual position in the sky. Explain it.
Answer: It is due to atmospheric refraction. When the light of a star enters into the atmosphere, it is refracted continuously. The higher level of air acts as a rarer medium while the dense air near the surface of the earth acts as a denser medium. So, the atmosphere bends the starlight towards the normal.
As a result, the apparent position of stars is slightly different from its actual position. Thus the star appears slightly higher than its actual position in the sky.
Question 2. When we place a glass prism in the path of a narrow beam of white light, a spectrum is obtained. What happens when a second identical prism is placed in an inverted position to the first prism? Draw a labelled ray diagram to illustrate it.
The second prism recombines the spectrum obtained by the first prism. So from the other side of the second prism, n beam of white light is again obtained.
Question 3. With the help of scattering of light, explain the reason for the difference in colours ofthe sun as it appears during sunrise/sunset and noon.
Answer: The sun appears reddish at sunrise as well as at sunset and appears white at noon when it is overhead.
At sunrise/sunset, the blue light of shorter wavelength gets scattered away while passing through the thicker layer of air/large distance through air and red light of longer wavelength reaches our eye.
At noon, the sun is overhead and the light of the sun travels a relatively shorter distance to our eyes little blue and violet colours are scattered, and white light reaches our eyes.
Question 4. Why do some people use bifocal lenses?
Answer: Some people suffer from both myopia and hypermetropia. Such people require bi-focal lenses. The upper part ofthe bi-focal lens is a concave lens used for distant vision while its lower part is a convex lens used for reading purposes. These days, refractive defects are also corrected by using contact lenses or through surgical interventions.
Question 5. What is the “power of accommodation ofthe eye”? What happens to the image distance when the object being viewed is moved away from the eye?
Answer: The power of accommodation of the eye is the ability of the eye lens to adjust its focal length to see nearby and distant objects.
Image distance in the eye is fixed, being equal to the distance of the retina from the eye lens.
Question 6. Why do we observe the seven colours when white light passes through a glass prism? Which component of white light deviates the least?
Answer: When light passes through a prism offers different refractive indices to the different wavelengths of light. Thus, each colour suffers a different deviation. The red light deviates the least.
Question 7. Calculate the maximum power of accommodation of a person having normal vision.
A person needs to use glasses to read a newspaper. Identify the defect in her vision and the type oflens she would need to correct it.
Sometimes when we enter into a dark room from bright sunlight we are unable to see objects. Why?
Answer: D = 25 cm, Now, P = 1/f- D = 100/D = 100/25 = 4 D, thus for a person having normal vision, the power of accommodation is about 4 dioptre.
Long-sightedness, a convex lens.
The pupil regulates and controls the amount of light entering the eye. In bright sunlight, the size ofthe pupil is small and when we enter a dark room it takes some time for the pupil to expand in size due to dim light.
Question 8. A person can see up to 3 metres only. Prescribe a lens for him so that he can see up to 12 metres.
Answer: Clearly, the lens used in this case should be such that the rays proceeding from a point distant 12 m from the eye should, after refraction, appear to come from a point distant 3 m from the eye. Here, u =- 12 m, v =- 3 m, /= ?
Using lens formula, \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)
⇒ \(\begin{aligned}
& =-\frac{1}{3}+\frac{1}{12} \\
& =-\frac{1}{4} \quad \text { or } f=-4 \mathrm{~m}
\end{aligned}\)
Thus, a concave lens of a focal length of 4 m should be used.
Question 9. A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.
- Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE.
- Write the name and cause ofthe phenomenon observed.
- Where else in nature is this phenomenon observed?
- Based on this observation, state the conclusion which can be drawn about the constituents of white light.
Answer: The diagram is shown below
The phenomenon is called the dispersion of light. It is because ofthe reason that different wavelengths oflight travel at different speeds in the glass prism. In a rainbow. White light consists of seven different wavelengths viz. violet, indigo, blue, green, yellow, orange and red.
Question 10. A person with a myopic eye cannot see objects beyond a distance of 1.5 m. What would be the power ofthe corrective lens used to restore proper vision?
Answer: The remedial lens should make the object at infinity appear at the far point.
- For an object at infinity, u =-∞
- Far point distance of the defective eye, v =- 150 cm
- By lens formula,
⇒ \(\begin{aligned}
\frac{1}{f} & =\frac{1}{v}-\frac{1}{u} \\
& =\frac{1}{-150}-\frac{1}{-\infty} \\
& =-\frac{1}{150}+0
\end{aligned}\)
⇒ \(=-\frac{1}{150}\)
or, f=-150cm.
Power, \(\mathrm{P}=\frac{1}{f(\text { in } \mathrm{m})}\)
A negative sign shows that the remedial lens is concave.
Question 11. Why do we have two eyes for vision and not just one?
Answer: Advantage of binocular vision: There are many advantages of having two eyes instead of one. These are as follows.
- It gives a wider horizontal field of view of about 180°.
- It helps to detect even fainter objects.
- It provides a three-dimensional effect of objects around us.
- The two eyes give relief to each other after every fraction of a second.
Question 12. A person is unable to read a book clearly when kept at a distance of 25 cm from his eye. Name the defect. How can it be corrected? Draw ray diagrams for defective eye-corrected eyes and explain them.
Answer: Hypermetropia or long-sightedness.
It is corrected by using a convex lens of suitable focal length. The diagrams are as shown below:
Defective eye: Rays from the object at N are focused beyond the retina i.e., near point goes away from the eye.
Corrected eye: Using a converging lens makes the image at the retina.
Question 13. Explain why the colour of the clear sky is blue.
Answer: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light.
These are more effective in scattering light of shorter wavelength at the blue end than light of longer wavelength at the red end. The red light has a wavelength about 1.8 times greater than that of blue light.
Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter the blue colour (shorter wavelength) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering.
Question 14. State the cause of dispersion, when white light enters a glass prism. Explain with a diagram.
Answer: Light is made up of different colours. Each colour travels at its speed inside a prism. Due to this, different light colours bend through different angles for the incident ray, as they pass through a prism. The red light bends the least while the violet the most.
Thus, the rays of each colour emerge along different paths and thus become distinct. It is the band of distinct colours that we see in a spectrum. The diagram is as shown.
A beam of white light falling on a glass prism gets split up into seven colours marked 7 as shown in the diagram:
The colours at positions marked 3 and 5 are similar to the colour ofthe sky and the colour of gold metal respectively. Is the above statement made by the Is student correct or incorrect Justify?
Which of the above-shown positions corresponds approximately to the colour of:
- A Brinjal
- Danger Signal
- Neel Which Is Applied To The Clothes
- Orange
Answer: As a result of dispersion produced by the prism, the white light is split up into seven colours. These colours as we know are represented by the letters of the word ‘VIBGYOR’.
Thus, blue is in the 3rd position and yellow is in the 5th position. These are the colours of the sky and the gold medal respectively. Thus, the statement is correct.
The positions of the colours ofthe given objects are represented by:
- brinjal ……….7
- Danger singal…….1
- Neel ……….6
- Organe……2
Question 15. A person wears glasses of power -2.5 D. Is the person far-sighted or near-sighted? What is the point of the person without glasses?
Answer: P =- 2.5 D.
Negative power shows that the lens is concave, so the person is near-sighted.
⇒ \(\begin{aligned}
f & =\frac{1}{\mathrm{P}} \\
& =\frac{1}{-2.5} \mathrm{~m}=-\frac{2}{5} \mathrm{~m} \\
& =-40 \mathrm{~cm} \\
u & =-\infty, v=?
\end{aligned}\)
From the lens formula,
⇒ \(\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{-40}+\frac{1}{-\infty}=\frac{1}{-40}-0=-\frac{1}{40}\)
v=-40 cm
Thus, the far point of the eye is 40 cm from the eye.
Question 16. List two causes leading to myopia of the eye. Draw ray diagrams to show the image formation in case of defective eye and correct eye.
Answer: The two main causes of myopia are:
- The eyeball is longer than normal.
- Decrease in the focal length of the eye lens
Question 17. A person needs a lens of power -2.5 dioptres for correcting his vision. Name the defect of vision he is suffering from. Which lens will he be using for the correction1? Also, find the focal length oflens.
Answer: The person is using a concave lens of power -2.5 D. The person is, therefore, suffering from myopia. He is using a concave lens for the correction of the eye. Since P =1If, therefore, f= 1/P = 1/2.5 = 0.4 m = 40 cm.
Question 18. Explain how a normal human eye can see distinctly the object placed at a distance as well as those placed at a nearer distance. What are the far point and near point of a normal human eye?
Answer: For seeing distant objects, the focal length ofthe eye lens should be large and for seeing near objects the focal length of the eye lens should be small.
The focal length of the eye lens can be adjusted with the help of ciliary muscles holding the lens in position. The far point and near point are at infinity and 25 cm respectively.
Question 19. Name the part of the eye where the image is formed by the eye lens. What is the nature ofthe image formed? How is this image sent to the brain?
Answer: The image of the object formed by the eye lens is at the retina of the eye. The image formed on the retina is real and inverted. The image is sent to the brain with the help of the optic nerve.
Question 20. An old person is unable to see nearby objects as well as distant objects, What defect of vision is he suffering from? What kind oflens will be required to see the nearby as well as distant objects? Give reasons.
Answer: Presbyopia.
He shall have to use both kinds of lenses, convex lens for long-sightedness and concave lens for short-sightedness.
Question 21. Why does the sun appear reddish early in the morning? Illustrate with the help of a labelled diagram.
Answer: It is due to the scattering of light. The light near the horizon passes thicker layers of air and a larger distance of the atmosphere; hence, most ofthe blue light is scattered away and longer wavelength reaches our eyes giving rise to the reddish colour of the sun.
Question 22. Why does it take some time to see the objects in a dim room when we enter the room from the bright sunlight outside?
Answer: In bright light, the size ofthe pupil is small to control the amount of light entering the eye. When we enter a dim room, it takes some time so that the pupil to expand and allow more light to enter and help to see things.
Question 23. Why does it take some time to see objects in a cinema hall when we just enter the hall from, bright sunlight? Explain in brief
Answer: The pupil regulates and controls the amount oflight entering the eye. In bright sunlight, the size of the pupil is small and when we enter the cinema hall takes some time for the pupil to expand in size due to dim light.
Question 24. Describe the formation of the rainbow in the sky.
Answer: A rainbow is always formed in a direction opposite to that of the sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion oflight and internal deflection, different colours reach the observer’s eye.
- What is meant by the least distance of distinct vision?
- How does the thickness ofthe eye lens change when we shift looking from a distant tree
to reading a book?
Answer: It is the minimum distance, to which normal eyes can see two similar objects distinctly.
The eye lens is comparatively thicker while reading a book.
Question 25. To correct distant vision, a person needs a lens of power -5.5 dioptres, and to correct near vision, he needs a lens of power +1.5 dioptres. What is the focal length of the lens required for correcting distant vision and near vision?
Answer: Power of distance viewing part of the lens,
P1 =-5.5 D
The focal length of this part
⇒ \(f_1=\frac{1}{P_1}=\frac{1}{-5.5} \mathrm{~m}\)
= -18.73 cm.
As the power of the near-vision part is measured relative to the main part of the lens of power – 5.5 D, so we use
P1 + P2 = P
or — 5.5 + P2 = + 1.5
or P2 = + 6.5 D
Focal length near-vision part
⇒ \(\begin{aligned}
f_2 & =\frac{1}{\mathrm{P}_2}=\frac{1}{+6.5} \mathrm{~m} \\
& =+15.4 \mathrm{~cm}
\end{aligned}\)
Question 26. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power ofthe lens required to correct the problem1
Answer: The remedial lens should make the objects at infinity appear at the far point.
∴ For an object at infinity u =-∞
Far point distance of the defective eye, v =- 80 cm
By lens formula, \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{-80}-\frac{1}{-\infty}=-\frac{1}{80}+0=-\frac{1}{80}\)
or f=-80cm
Power, \(\mathrm{P}=\frac{100}{f(\mathrm{in} \mathrm{cm})}\)
⇒ \(\begin{aligned}
& P=\frac{100}{-80} \\
& P=-1.25 D
\end{aligned}\)
A negative sign shows that the remedial lens is concave.
Question 27. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point ofthe normal eye is 25 cm.
Answer: The object placed at 25 cm from the correcting lens must produce a virtual image at 1 m or 100 cm.
u = -25 cm, v =- 100 cm
By lens formula \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{-100}-\frac{1}{-25}=-\frac{1}{100}+\frac{1}{25}=+\frac{3}{100}\)
or \(f=+\frac{100}{3} \mathrm{~cm}=+\frac{1}{3} \mathrm{~m}\)
Power, \(\mathrm{P}=\frac{1}{f}=+\frac{3}{1}=+3 \mathrm{D}\)
Question 28. Why is a normal eye not able to see the objects placed closer than 25 cm?
Answer: At a distance of less than 25 cm, the ciliary muscles cannot bulge the eye lens anymore, the object cannot be focussed on the retina and it appears blurred to the eye, as shown in the figure.
Object O within 25 cm from the eye is not focused on the retina and is seen blurred.
Question 29. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: Due to its property of accommodation, the eye lens of a normal eye forms images of objects at various distances on the same retina. So the image distance in the eye remains the same.
Question 30. Why do stars twinkle?
Answer: The apparent position of a star is slightly different from the actual position due to the refraction of starlight by the atmosphere. Due to the variation in the atmospheric conditions, the amount oflight from a particular star changes randomly with time. This gives rise to a twinkling effect of the star.
Question 31. Explain why the planets do not twinkle.
Answer: As the planets are much closer to the Earth, the amount oflight received from them is much greater and the fluctuations caused in the amount of light due to atmospheric refraction are negligible as compared to the amount oflight received from them.
Question 32. Why does the sun appear reddish early in the morning?
Answer: Early in the morning, the sun is near the horizon. Sunlight reaches us after covering a large thickness ofthe atmosphere.
So shorter waves of blue region are almost completely scattered away by the air molecules. Red waves of longer wavelength are least scattered and reach our eyes. The sun appears red.
Question 33. Why does the sky appear dark instead of blue to an astronaut?
Answer: The atmosphere is quite thin at very high altitudes. There is almost no scattering of sunlight. So the sky appears dark to an astronaut.
Question 34. What is meant by the power of accommodation of the eye?
Answer: The power of accommodation of the eye is the maximum variation of its power for focusing on near and far (distant) objects. for a normal eye, the power of accommodation is about four diopters.
Question 35. A student sitting at the back of the classroom cannot read the letters written on the blackboard. What advice will a doctor give to her? Draw a ray diagram for the correction ofthis defect.
Answer: The student is suffering from myopia or near-sightedness. In myopia, one can see nearby objects clearly but cannot see distant objects distinctly. The doctor advises a concave lens of suitable power to bring the image back into the retina.
The ray diagram for the correction of this defect is shown below
Question 36. A person needs a lens of power -4.5D for correction of her vision.
- What kind of defect in vision is she suffering from?
- What is the focal length ofthe corrective lens?
- What is the nature ofthe corrective lens?
Answer: As the power of the lens is negative, she must be suffering from myopia.
Power, P =- 4.50, focal length, f=?
⇒ \(\mathrm{P}=\frac{1}{f} \Rightarrow f=\frac{1}{\mathrm{P}}=\frac{1}{-4.5}=-0.222 \mathrm{~m}=-22.2 \mathrm{~cm}\)
The nature ofthe corrective lens is concave or divergent.
Question 37. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out ofthe second prism as white light? Draw the diagram.
Answer: A narrow beam of white light incident on one prism emerges out of the identical prism placed in an inverted position to the first prism close to the first prism as shown in the figure given below.
Question 38. Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours ofthe spectrum obtained.
Answer:
Question 39. Why do we see a rainbow in the sky only after rainfall?
Answer: We see a rainbow in the sky only after rainfall due to the dispersion of sunlight by tiny water droplets, present in the atmosphere due to rainfall. The water droplets act as a prism.
They refract and disperse the incident sunlight then reflect it internally and finally refract it again when it comes out of a raindrop hence due to the dispersion of light and internal reflection, rainbow formation takes place during rainfall.
Question 40. Why is the colour ofthe clear sky blue?
Answer: The blue colour of the clear sky is due to Rayleigh’s scattering of sunlight. The molecules of air and other fine particles in the atmosphere have sizes smaller than the wavelength of visible light.
These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
Question 41. What is the difference in colours of the sun observed during sunrise-sunset and noon? Explain each.
Answer: During sunrise/sunset, the sun looks reddish because, at this stage, rays from the sun have to travel a much larger part of the atmosphere. The red colour having the largest wavelength is scattered the least.
At noon, the sun is nearly overhead. The sunlight has to pass through a smaller portion of the earth’s atmosphere. the scattering is much less and hence the sun looks White.
Question 42. A child sitting in a classroom is not able to read the writing on the blackboard.
- Name the type of defect from which his eye is suffering.
- With the help of a ray diagram show how this defect can be remedied.
Answer: The child’s eye is suffering from the defect of myopia.
The defect can be removed by using a concave lens as shown in the figure below:
Question 43. Why do we have two eyes instead of one?
Answer: Two eyes are better than one eye because of the following reasons:
- The field of view with two eyes is more than with one eye.
- Two eyes give three three-dimensional pictures of an object whereas one eye gives only two dimensional picture of an object.
Human Eye And Colourful World Multiple Choice Questions
Question 1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to
- Presbyopia.
- Accommodation.
- Near-Sightedness.
- Far-Sightedness.
Answer: 2. The ability of the eye to focus objects at different distances is called accommodation.
Question 2. The human eye forms the image of an object in its
- Cornea.
- Iris.
- Pupil.
- Retina.
Answer: 4. The human forms the image of an object at its retina.
Question 3. The least distance of distinct vision for a young adult with normal vision is about
- 25 m.
- 2.5 cm.
- 25 cm.
- 2.5 m.
Answer: 3. The least distance of distinct vision for a normal eye is 25 cm.
Question 4. The change in focal length of an eye lens is caused by the action of the
- Pupil.
- Retina.
- Ciliary Muscles.
- Iris.
Answer: 3. The change in the focal length of the eye is due to the action of ciliary muscles.
Question 5. A person cannot see distinct objects kept beyond 2m. This defect can be corrected by using a lens of power
- + 0.5D
- – 0.5 D
- + 0.2 D
- – 0.2 D
Answer: 2. – 0.5 D
Question 6. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook. Which of the following statements is correct?
- The near point of his eyes has receded away
- The near point of his eyes has come closer to him
- The far point of his eyes has come closer to him
- The far point of his eyes has receded away
Answer: 1. The near point of his eyes has receded away
Question 7. A prism ABC (with BC as the base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in the figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
- 1
- 2
- 3
- 4
Answer: 2. 1
Question 8. At noon the sun appears white as
- Light Is Least Scattered
- All The Colours of white Light Are Scattered Away
- Blue Colour Is Scattered The Most
- Red Colour Is Scattered The Most
Answer: 1. Light Is Least Scattered
Question 9. Which of the following phenomena of light are involved in the formation of the rainbow?
- Reflection, Refraction And Dispersion
- Refraction, Dispersion And Total Internal Reflection
- Refraction, Dispersion And Internal Reflection
- Dispersion, scattering and total internal reflection
Answer: 3. Refraction, Dispersion And Internal Reflection
Question 10. The twinkling of stars is due to atmospheric
- Dispersion Of light By Water Droplets
- Refraction Of Light By Different Layers Ofvarying Refractive Indices
- Scattering Of light By Dust Particles
- Internal Reflection of Light By Clouds
Answer: 2. Refraction Of light By Different Layers Ofvarying Refractive Indices
Question 11. The clear sky appears blue because
- Blue Light Gets Absorbed In The Atmosphere
- Ultraviolet Radiations Are Absorbed In The Atmosphere
- Violet And Blue Lights Get Scattered More Than Lights Of All Other Colours By The
Atmosphere - Light all other Colours Is Scattered More Than The Violet And Blue Colourlights By The Atmosphere
Answer: 3. Violet And Blue Lights Get Scattered More Than Lights Of All Other Colours By The Atmosphere
Question 12. Which of the following statements is correct regarding the-propagation oflight of different colours of white light in the air?
- Red Light Moves the Fastest
- Blue Light Moves Faster Than Green Light
- All The Colours of white Light Move at the Same Speed
- Yellow Light Moves With The Mean Speed As That Ofthe Red And The Violet Light
Answer: 3. All The Colours of white Light Move With The Same Speed
Question 13. The danger signals installed at the top of tall buildings are red. These can be easily seen from a distance because among all other colours, the red light
- Is Scattered The Most By Smoke Or Fog
- Is Scattered The Least By Smoke Or Fog
- Is Absorbed The Most By Smoke Or Fog
- Moves Fastest In Air
Answer: 2. Is Scattered The Least By Smoke Or Fog
Question 14. Which of the following phenomena Contributes Significantly To The Reddish Appearance Of The Sun At Sunrise Or Sunset?
- Dispersion Of light
- Total Internal Reflection Of light
- Scattering Of light
- Reflection Of Light From The Earth
Answer: 2. Total Internal Reflection Oflight
Question 15. The Bluish Colour Of Water In Deep Sea Is Due To
- The Presence Of algae And other plants Found In Water
- Reflection Ofsky In Water
- Scattering Oflight
- Absorption Oflight By The Sea
Answer: 3. Scattering Of light
Question 16. When Light Rays Enter The Eye, Most of the refraction Occurs At The
- Crystalline Lens
- Outer Surface Of the Cornea
- Iris
- Pupil
Answer: 2. Outer Surface Of the Cornea
Question 17. The Focal Length Of The Eye Lens Increases When Eye Muscles
- Are Relaxed And Lens Becomes Thinner
- Contract And Lens Becomes Thicker
- Are Relaxed And Lens Becomes Thicker
- Contract And Lens Becomes Thinner
Answer: 1. Are Relaxed And Lens Becomes Thinner
Question 18. Which of the following Statements Is Correct?
- A Person With Myopia Can See Distant Objects Clearly
- A Person With Hypermetropia Can See Nearby Objects Clearly
- A Person With Myopia Can See Nearby Objects Clearly
- A Person With Hypermetropia Cannot See Distant Objects Clearly
Answer: 3. A Person With Myopia Can See Nearby Objects Clearly
Question 19. Tarun’s father is an eye surgeon. He persuaded his father to put a camp in his society to educate people about eye donation. Rohan made a banner. One donation can give eyevision to two blind persons.
- Name the part of the eye that is used, during eye transplant.
- Name the defect that can be corrected by this transplant.
- What value of Tarun is reflected?
- The defect is caused by to cornea.
Answer: 1. Cornea.
- A defect is caused by to cornea.
- Tarun shows sympathy, compassion and empathy in his behaviour.