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1. a. Equal

b. Mirror

c. Multiple reflections

d. 30°

e. 0°

2. a. Visual aids 

b. Blind spot 

c. Concave lens

d. Iris 

e. Braille

Assessment Zone

A. Choose the correct answer.

1. a 

2. a 

3. a

4. c 

5. d 

6. b 

7. c 

8. a 

9. d

B. Fill in the blanks.

1. Normal 

2. Retina 

3. Cataract 

4. Optic nerve

5. Sclera 

6. Rarer medium, Denser medium 

7. Regular

C. State True or false.

1. False 

2. True 

3. True 

4. False 

5. False

6. True 

7. False 

8. False 

9. True 

10. False

D. Match the following:

1. d 

2. e 

3. a 

4. b 

5. c

E. Answer the following questions in brief.

 1. When we see an object, its image persists at our retina for 1/16th of second, even after

the removal of the object. This is called persistence of vision.

2. Characteristics of image formed by a plane mirror:

  • Upright or inverted: The image formed in a plane mirror is always upright or erect

and is never inverted.

  • Size: When the image of an object is formed in a plane mirror, the size of the image

formed is the same as the size of the object.

  • Distance of the image: The image formed in a mirror is at the same distance as the

object is from the mirror.

  • Lateral inversion: The image formed in a plane mirror displays lateral inversion.

  • Virtual image: All plane mirrors produce virtual images.

  3. The reflection of light from any reflecting surface is governed by two simple laws.

    • The incident ray, the reflected ray and the normal at the point of incidence all lie in

the same plane.

  • The angle of incidence is equal to the angle of reflection, i.e., i = r.

  4. When two mirrors are placed facing each other such that they are parallel to each other

and an object is placed between the two, then the number of images formed are infinite.

This is the phenomenon of multiple reflections of light. The number of images formed

depends on the angle between the mirrors.

    To find the number of images we use a formula, n = 360°/θ – 1. Here, n is the number of

images and θ is the angle between the two mirrors.

  5. This ability of the lens of the eye to self-adjust its focal length is termed as the power

of accommodation. The human eye has the unique ability to quickly adjust itself to see

nearby and distant objects in quick succession.

6. a

Myopia Hypermetropia

In myopia, a person is not able to see

distant objects clearly.

In hypermetropia, a person is not able to

clearly see the nearby objects.

In this defect, lens is not able to produce

sharp image of the distant objects on the

retina instead the image is formed in front

of the retina.

In this defect, image is formed behind the

retina.

The reduction in the focal length of the

eye lens may be a possible cause.

The increase in focal length of the eye lens

may be a cause.

Another cause can be the elongation of eye

ball.

It may also be caused due to flattening of

the eyeball.

Myopia can be corrected by using concave

lens of suitable focal length.

Hypermetropia may be corrected using

convex lens of suitable focal length

  b. Refraction is the bending of light, frequently due to a change in the propagation

velocity of light. Dispersion results from different frequencies of light having

different velocities.

In practice, all materials that refract light will disperse it to some degree. All

dispersive materials will refract light.  

The key difference between the two is that dispersion is referring to the frequency

dependence (which is the ability to separate different colours of light) while

refraction is referring to the bending of light in a material.

7. To find the number of images we use a formula, n = 360°/θ – 1. Here, n is the number of

images and θ is the angle between the two mirrors.

a. 8 b. 7 c. 5

8. Eye lens functions for power of accommodation. When we look at a nearby object, the

ciliary muscles contract and make the eye lens bulge so that the focal length of the lens

decreases. When we see a faraway object, the ciliary muscles relax and flatten the lens

so that its focal length increases.

9. Iris controls the size of the pupil through which the light rays enter the eye ball. When

the light is too bright, the iris expands to reduce the size of the pupil.

10. The white light splits into a band of seven constituent colours. The separation of a beam

of light into its constituent colours is known as dispersion of light.

11. The main use of periscope is in submarines that remain underwater. While the vessel

is in the submerged state, the periscope is used to see things above the surface of the

water. This instrument may also be used to view things at a different level, for example,

above the heads of a crowd.

12. Kaleidoscope and periscope.

F. Answer the following questions in detail.

 1. Refer to fig 16.12 on page no. 236

2. Ways to take care of eyes:

  • Wash and clean your eyes with cold water at least twice daily.

    • Avoid reading in too dim or too bright light.

    • Do not read in moving vehicle as it strains the eyes.

• Avoid looking directly at too bright objects such as the Sun and wear sunglasses to

protect eyes from harmful sunrays.

  • Rest your eyes frequently while working on computers or reading books. Reading

books and looking at computers for longer durations may harm your eyes.

3. a. When an object is seen through prism it appears coloured because as light passes

through a prism, the light is scattered and it displays a rainbow (dispersion of light).

What we see as colour is just light, which bounces off objects and into our eyes.

b. The amount of light entering the eye through pupil is controlled by iris. But this

adjustment takes some time. Our eyes cannot see everything clearly when we enter

a dark room from a bright light. The reason for this is that in bright light the size of

pupil of our eye is small. When we enter the dark room, due to small size of pupil,

very little light enters our eyes. But after some time, the pupil expands and we

become able to see things as more light enter our eyes now.

    c. It is very useful to us that all surfaces do not give regular reflection because diffused

reflection of light enables us to see objects around us.

  4. Refer to fig 16.3 & 16.4 on page no. 230

 If a beam of light is incident on a smooth and highly polished surface, the rays of light

remain parallel even after reflection as they strike the surface at the same angle of

incidence. Thus, their angles of reflection will also remain same. Such type of reflection

in which the parallel rays of light remain parallel after reflection is called regular

reflection. Plane mirrors, polished metals, etc. show parallel reflection.

 If a beam of parallel rays of light falls on a rough surface, the rays of light do not

remain parallel after reflection from the surface. The rays of light get scattered in

different directions, as they strike the surface at different angles of incidence. Such

type of reflection in which the parallel rays of incident light do not remain parallel after

reflection is called irregular or diffused reflection

5. a. Function of pupil: The whole job of the iris and pupil is to control the amount of light

that gets into the eye. It is called a pupillary reflex. You have probably noticed that a

person’s pupils are smaller in bright light and bigger in low light.

b. Function of iris: In humans and most mammals and birds, the iris is a thin,

circular structure in the eye, responsible for controlling the diameter and size of

the pupil and thus the amount of light reaching the retina. Eye colour is defined by that

of the iris.

c. Function of optic nerve: The optic nerve is located in the back of the eye. It is also

called the second cranial nerve or cranial nerve II. It is the second of several pairs of

cranial nerves. The job of the optic nerve is to transfer visual information from the

retina to the vision centers of the brain via electrical impulses.

d. Function of retina: The retina is a light-sensitive layer at the back of the eye that covers

about 65 percent of its interior surface. Photosensitive cells called rods and cones in the

retina convert incident light energy into signals that are carried to the brain by the optic

nerve.

  6. Using the law, the angle of incidence is equal to the angle of reflection, i.e., i = r

    Given, angle of incidence = 40°

    Therefore, angle of reflection = 40°

    So, angle between incident ray and reflected ray would be = i + r = 40° + 40° = 80°.

  7. Using the law, the angle of incidence is equal to the angle of reflection, i.e., i = r

    If, angle between incident ray and reflected ray is= 70°.

Therefore, angle of incidence = angle of reflection = 70°/2 =35°

8. Refer to fig. 16.11 on page no. 236.

9. A distorting mirror, funhouse mirror or carnival mirror is a popular attraction

at carnivals and fairs. Instead of a normal plane mirror that reflects a perfect mirror

image, distorting mirrors are curved mirrors, often using convex and concave sections to

achieve the distorted effect. If you look in a circular concave mirror, your image will be

magnified. Or a convex mirror will show an image smaller than the object.

Based on the above, funny mirrors are mirrors with a combination of flat, concave and/

or convex surfaces, thus showing distorted images.

10. To increase number and intricacies of designs increase the number of items using

in kaleidoscope to make designs. Use variety of items in it having different shapes

and size. Use small items for making variety of designs. Another way is increase the

number of mirror which we use to make kaleidoscope. This will increase the number of

reflections which will result in increase in number of intricacies.