CBSE Class 10 Physics The Human Eye and The Colorful World Worksheet Set C

Assertion and Reasoning Based Questions

Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.

Question. Assertion : The Sun appears red during sunrise or sunset.
Reason : The scattering of light is inversely proportional to its wavelength.
Answer : (a) 

Question. Assertion : The human eye is one of the most valuable and sensitive sense organs.
Reason : Eye enables us to see the wonderful world and the colours around us.
Answer : (a) 

Question. Assertion : The curvature of the eye lens can be modified to some extent by the ciliary muscles.
Reason : The ciliary muscles are used to modify the curvature of lens.
Answer : (a) 

Question. Assertion : Sometimes, the eye may gradually lose its power of accommodation.
Reason : The crystalline lens of people at old age becomes milky and cloudy.
Answer : (b)

Question. Assertion : Hypermetropia is also known as nearsightedness.
Reason : The image of a distant object is not formed in front of the iris.
Answer : (d) 

 

Short Answer Questions

Question. Explain why the planets do not twinkle.
Answer : The planets are much closer to the earth and thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point sized sources will average out to zero, thereby nullifying the twinkling effect. Hence, the planets do not twinkle.

Question. State the main cause of atmospheric refraction of light.With the help of a diagram, explain the phenomenon of twinkling of stars. [2017 (O.D.) C]
Answer : Refraction of light caused by the earth‘s atmosphere due to change in the refractive indices of different layers at atmosphere.
Twinkling of stars : The twinkling of a star is due to atmospheric refraction of starlight. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. Further, this apparent position of the star is not stationary, but keeps on changing slightly.

Question. Why does the sun appear white at noon and reddish at the sunrise/sunset ? Explain. 
Answer : Light from the sun overhead would travel relatively shorter distance. At noon, sun appears white as only little of the blue and violet colours are scattered. However, near the horizon most of the blue light and shorter wavelength are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelength which gives rise to the reddish appearance of sun.

Question. (a) What is scattering of light ? Explain how the colour of the scattered light depends on the size of the scattering particles.
(b) Explain the reddish appearance of the Sun at sunrise or sunset. Why does it not appear red at noon ? 
Answer :
(a) Scattering of light is the phenomenon in which a part of the incident light is dispersed in different directions.
Dependence of colour and scattered light on the size of particles :
(i) When the particles like dust and water droplets present in the atmosphere are large in size, the scattered light appears white.
(ii) When the particles are extremely minute in size, they will scatter blue light present in the white sunlight.
(b) The reddish appearance of the sun at sunrise and sunset is due to the scattering of blue colour present in the sunlight away from our line of sight and leaves behind mainly red colour of the direct sunlight which reaches human eye.
The reason for Sun not appearing red at the noon is that the light has to travel a relatively shorter distance through the atmosphere to reach us and therefore, only a litte of blue colour of the white light is scattered.

Question. Why is red colour selected for danger signal lights?
Answer : Red colour light has maximum wavelength, it does not scatter due to atmosphere and reaches our eyes, travels fast and hence used for danger signals.

Question. An old person finds it difficult to see nearby objects comfortably and distinctly without corrective eye glasses.
(a) What defect of vision is he suffering from ? What is it ?
(b) List two causes for the development of this defect.
(c) What kind of lens will be required to see clearly the nearby as well as distant objects ? Give reasons.
(d) How is the eye defect of old person differing from near-sightedness and far-sightedness ?
Answer :
(a) Presbyopia
(i) Presbyopia is a condition that occurs as part of normal ageing.
Due to loss of power of accommodation of the eye, with age, objects at a normal near working distance will appear blurry. The near point gradually recedes away. This defect of eye is called presbyopia.
(ii) Sometimes, a person may suffer from both myopia and hypermetropia.
(b) Presbyopia is caused due to :
(i) Weakening of ciliary muscles, and
(ii) Eye lens becomes less flexible and elastic,i.e., reducing ability of eye lens to change its curvature with the help of ciliary muscles.
(c) Bifocal lens will be required to see clearly nearby as well as the distant object. For myopic defect, upper part of bifocal lens consists of a concave lens used for distant vision and to correct hypermetropia, lower part of bifocal lens consists of a convex lens. It facilitates near vision.
(d) Presbyopia occurs when the natural lens in the eye loses its flexibility with ageing while farsightedness and near-sightedness are related to the change in shape of the eyeball and are caused by the genetic and the environmental factors.

Question. (a) Explain the following terms used in relation to defects in vision and correction provided by them :
(i) Myopia, (ii) Bifocal lenses, (iii) Far-sightedness.
(b) Why is the normal, eye unable to focus on an object placed within 10 cm from eye ?
Answer :
(a) (i) Myopia or short-sightedness is caused due to excessive curvature of the eye lens or elongation of the eyeball. The image forms in front of the retina. So, a concave lens is used to correct it.
(ii) Bifocal lens is used to correct presbyopic eye. The upper portion of a bifocal lens is concave and its lower portion is convex. The concave lens is used for viewing long distant objects and the convex lens is used for reading purposes. This problem arises with age.
(iii) Far-sightedness or hypermetropia is caused due to greater focal length of eye lens or eye ball becomes smaller. The image forms beyond the retina and can be corrected by using a convex lens.
(b) The least distance of distinct vision for normal eye is 25 cm. The focal length of the eye lens cannot by further decreased by ciliary muscles to form an image on the retina for the objects nearer than 25 cm. Therefore, the normal eye is unable to focus on an object placed within 10 cm from the eye.

Question. (a) What is myopia ?
(b) How can it be corrected ?
Answer :
(a) The defect of human eye in which a person can see the objects lying at short distances clearly but cannot see the far objects distinctly is called myopia or short-sightedness.
(b) A myopic eye can be corrected by using spectacles with concave lens of suitable power or focal length.

Question. (a) What is hypermetropia ?
(b) How can it be corrected ?
Answer :
(a) The defect of the human eye in which a person can see the objects lying at long distances clearly but cannot see the nearby objects distinctly is called hypermetropia or long-sightedness.
(b) To correct a hypermetropic eye, the person is allowed to wear spectacles with a convex lens of suitable power or focal length.

Question. When we increase the distance of an object from the eye, what happens to the image distance in the eye ?
Answer : For a normal eye, image distance in the eye is fixed, being equal to distance of retina from the eye lens. When we increase the distance of an object from the eye, the focal length of eye lens is changed on account of accommodation power of the eye, so as to keep the image distance constant.

Creating Based Questions

Question. Reena was diagnosed with a short-sight problem. She was advised to wear spectacles continuously which made her feel low about her beauty. What would you suggest Reena other than wearing spectacles ?
Answer : When Reena is diagnosed with short-sight problem, the better advice to be considered is that she can wear spectacles. But when she is not comfortable wearing spectacles, I would suggest her to fix lens which won’t degrade her looks and will make her feel confident.

Question. Babin was diagnosed with a long sight problem. He was advised to wear lens but he was worried that wearing lens should be given an utmost care in his busy schedule. What would you suggest Babin other than usage of lens ?
Answer : It was a big problem for Babin to take care of lens in his busy schedule due to this reason I suggested him to undergo laser operation which would correct all common vision problems like near sight, a long sight which reduce the dependency on the lens. The eye treatment using laser is safe and effective.

Question. What would be your suggestion to your grandfather who is undergoing the eye defects both hypermetropia and myopia ?
Answer : Myopia is the near-sightedness and hypermetropia is the far-sightedness. The person who is suffering from both eye defects is advised to wear spectacles with a bi-focal lens which consists of both concave and convex lens. The upper portion would be a concave lens which rectifies the distant vision and lower portion is a convex lens which rectifies the near vision.

Question. John has undergone all medication including wearing spectacles, lens and laser treatment. None of the treatment helped him to recover his eyesight. What would be your suggestion to overcome his problem in his eyesight ?
Answer : Though all the treatment for recovering from his eye problems did not help him out, I would suggest him to opt for eye transplantation which would get his eyesight back to normal. In transplantation of eye, the entire diseased eye cannot be replaced. Only the diseased cornea can be replaced by the donated cornea.

Question. In today’s digital world, every human spends around a minimum eight hours with the digital electronic items which make their eyes to strain more and causes many eye problems. Still they undergo many exercises for their eyes during their working hours what would be your medical suggestion to reduce the stress and strain caused in the eyes ?
Answer : In addition to the exercise for the strained eyes, my medical suggestion would be the usage of the anti-glare lens in their spectacles which reduces the impact of blue light emitted from digital items and artificial lighting. This would reduce the stress and strain caused. But still the exercise for eyes in repeated time interval should also be regularly followed.

HOTS Question

Question. Give one use of the following properties of light:
(i) Scattering of light
(ii)Persistence of vision
(iii) Power of accommodation
(iv) Refraction of light
(v) Reflection of light
Answer : (i) Due to scattering of light we can see the different colours of sky and rainbow formation.
(ii) Used in cinematography.
(iii) Eye can see both nearby and far off objects from same point.
(iv) We can see through lenses, eye defects can be corrected, we can see under water.
(v) Our eyes can see object only due to reflection of light by the object into our eyes.

Question. In presbyopia, we use bi-focal lens with upper portion concave lens and lower portion convex lens. Why is the arrangement so?
Answer : The upper portion is concave lens so that our eyes can see the distant objects. When the rays come parallel from infinity it will pass through the upper part of spectacles.
Whereas for the lower part consists of convex lens which facilitates our eye to see the near objects.

 

 

Key concepts and terms:

1) Eye: The natural optical device using which man could see objects around him. It forms an inverted, real image on a light sensitive surface called the retina.

2) Power of Accommodation: The maximum variation in power of the lens so that the far-off and nearby objects are viewed is called power of accommodation.

3) Near point of Eye: The minimum distance between the object and the eye (lens) so that a clear image is formed on the retina. This distance is 25 cm.

4) Far point of Eye: The farthest point up to which the eye can see objects clearly is called the far point of eye. It is infinity (normal eye).

5) Persistence of vision: The time for which the impression or sensation continues in the eye is called the persistence of vision. It is about 1/16th of the second.

6) Myopia or Short-sightedness: Inability of the eye in viewing long distance objects. It is caused due to i) excessive curvature in cornea or ii) elongation of eye- ball. It is corrected by using concave lens.

7) Hypermetropia or Long – sightedness: The inability of the eye in viewing the nearby objects. It is caused due to i) Greater focal length of lens ii) Eye ball becoming smaller. It is corrected by using convex lens.

8) Presbyopia: In human eye, with age, the near point recedes and the far point get reduced. The eye becomes both myopic and hypermetric. This is caused due to i) weakening of ciliary muscles and ii) reducing ability of the lens to change the curvature. It can be corrected by using bi-focal lens.

 

I) Red light is used for danger signal as (1mk)

   a) It has higher wave length

   b) It can travel long distance

   c) It scatters least

   d) It scatters the longest

II) Acts as lens in the eye (1mk)

III) Colour blindness is caused by (1mk)

   A: Vitamin A deficiency

   B: Defective rod cells

Then the correct answer is / are

  i) A only ii) B only iii) A & B Both iv) None of these.

4. A person with a myopia eye cannot see objects beyond a distance of 1.5m. What would be the power of corrective lens? Which type of lens is used? (2 mks)

5. What do you understand by myopia? Write two causes of it? 

6. What do you mean by far point and near point of eye? 

7. What is presbyopia? State the causes of it and How is it corrected? 

8. Explain: 1) Why does sky look blue on a clear day 

                2) Twinkling of stars.

9. What is Hypermetropia? State two causes of Hypermetropia with help of ray diagrams show : 

i) The eye defect hypermetropia.

ii) Correction of Hypermetropic by using a lens

 

1. Quiz :

1) Name the place where image is formed in the eye?

2) Name the muscular diaphragm that controls the size of the pupil?

3) What is the cause of dispersion of light?

4) Give the cause of cataract of eye?

5) Which colour has got more wave length?

6) How many colours evolve when white light disperses?

7) What is the reason for the different deviation?

8) Who discovered that white light consists of seven colours?

9) What makes bees respond to ultraviolet light?

2. Home Assignment :

1) Name the photographic film equivalent of our eye?

2) Why does a glass slab not disperse while light?

3) Why do we not perceive the depth of a lake?

4) Name two causes of myopia.

5) Name the liquids that keep our eye soft.

6) Does the speed of light vary with medium?

7) With Diagram illustrate the defective and corrective hypermetric eye ?

8) The far point of a near sighted person is 1.5 m. Find the nature and power of lens.

9) What causes rainbow formation?

10) What is mirage?

3. Project Work :

1. To understand the dispersion of light with help of activity? (Hint: Materials, An irregularly shaped glass, white screen)

2. List, observe, reason and explain three cases of nature where dispersion happens.

3. (Hint : i) Sun rise and sun set

             ii) Formation of Rainbow

            iii) Twinkling of stars)

4. Draw a labeled diagram of Human eye and explain the functions of Retina, Cornea, Iris, Pupil, Rods and Cons)

5. Seminar : (Students will be divided into groups & they will present papers on the topic)

Topic - Problems of Vision:

4) Means to overcome

5) Corrective measure