Ch. No. 6. Wonders of Light Part – I
Q1. Fill in the blanks
1. A ray of light parallel to principal axis after reflection from concave mirror passes through ________. (Focus)
2. The focal length of ____________ lens is positive. (convex)
3. The image of an object is formed behind __________ in hypermetropia. (retina)
4. An optical device used by watch repairers is _____________. (simple microscope)
5. The power of spectacle for myopic eye is _____________. (negative)
Q2. Rewrite the following table so as to match second and third column with first column.
| Column I | Column II | Column III |
| 1. Myopia | Old age problem | Convex lens |
| 2. Hypermetropia | Near – sightedness | Bifocal lens |
| 3. Presbyopia | Long - sightedness | Concave lens |
| 1. Myopia | Near sightedness | Concave lens |
| 2. Hypermetropia | Long sightedness | Convex lens |
| 3. Presbyopia | Old age problem | Bi-focal lens |
Q3. Answer the following
1. State the different positions of source of light with respect to concave mirror-
a. In torches
b. Projector lamps
c. Flood lights.
Ans.
i. In torches: - The source of light is placed at the focus.
ii. Projector lamps: - The object is placed at the center of curvature.
iii. Flood lights: - The source of light is placed just beyond the centre of curvature.
2. What is ray diagram? What are the rules used for drawing ray diagrams for the formation of image by spherical mirror?
Ans. A ray diagram is a specialized pictorial representation used to trace the path of light rays.
The rules used for drawing ray diagrams are as follows.
i. If the incident ray is parallel to the principal axis, then reflected ray passes through the focus.
ii. If the incident ray passes through the focus then the reflected ray becomes parallel to the principal axis.
iii. If the incident ray passes through the centre of the curvature, the reflected ray traces the same path.
3. What is meant by power of accommodation of eye?
Ans.
i. The ability of the lens of adjusting focal length is known as power of accommodation.
ii. The process of focusing the eye at different distances is called accommodation.
4. State the function of iris and ciliary muscles in human eye.
Ans.
I. Iris in human eye controls and regulates the amount of light entering the eye by contracting and dialating the pupil.
II. Ciliary muscles adjust the focal length of eye lens by contracting and relaxing.
5. If you are given a part of hollow spherical glass, how will you convert it into concave mirror?
Ans. The inner side or concave side of the hollow spherical glass will be polished to get a concave mirror.
Q4. Distinguish between.
1. Myopia and Hypermetropia
| Myopia | Hypermetropia |
| 1. In this defect human eye can see nearby objects distinctly but is unable to see distant objects clearly. 2. Image of distant object is formed in front of retina. 3. Eye ball is lengthened or lens is curved. 4. This defect is corrected using concave lens. | 1. In this defect human eye can see distant objects distinctly but is unable to see nearby objects clearly. 2. Image of nearby objects falls behind retina. 3. Eye ball is shortened or lens is flattened. 4. This defect is corrected using convex lens. |
2. Convex mirror and concave mirror.
| Convex mirror | Concave mirror |
| 1. It is called as diverging mirror. 2. The light is reflected from outer surface. 3. Centre of curvature and focus lies behind the mirror. 4. According to sign convention, focal length is positive. | 1. It is called as converging mirror. 2. The light is reflected from inner surface. 3. Centre of curvature and focus lies in front of the mirror. 4. According to sign convention, focal length is negative. |
Q5. Give reasons:
1. You cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.
Ans.
i. If the object is too close, the eye lens cannot curve enough to focus the image on to the retina.
ii. Therefore, while watching a movie from a very short distance a blurred image is formed causing strain to the eyes.
iii. Hence, one cannot enjoy watching a movie from a very short distance from the screen in a cinema hall.
2. A simple microscope is used by watch repairers.
Ans.
i. A magnification of about 20 times is obtained by the convex lens of small focal length used in a simple microscope.
ii. Therefore, it is used by watch repairers to see the minute parts of the watch clearly without causing any strain to the eyes.
3. A concave lens is used to correct myopia.
Ans.
i. In myopia the image of distant object is formed in front of retina.
ii. A concave lens causes light rays to diverge before they strike the lens of the eye so that the image is formed on the retina.
iii. Hence, a concave lens is used to correct myopia.
4. In old age bifocal lens is necessary for some persons.
Ans.
i. In old age, sometimes person suffers both myopia and hypermetropia and then the person needs bifocal lens.
ii. In bifocal lens, upper part is concave lens to correct myopia and lower part is convex lens to correct hypermetropia.
5. Concave mirrors are used in solar devices.
Ans.
i. Concave mirrors are used in solar devices to collect heat and radiations.
ii. Heat radiations from the sun coming from infinity are brought to focus by concave mirror in its focal plane.
Q6. Draw well labeled ray diagrams for image formation by convex lens when
1. Object is at 2F1.
Position of image: -At 2F2
Nature of image: - Real, Inverted and same size as that of the object.
2. Object between F1 and 2F1
Position of image: - Beyond 2F2
Nature of image: - Real, inverted and magnified.
3. Object between focus F1 and optical center O.
Position of image: - On the same side of the lens as the object.
Nature of image: - Virtual, erect and magnified.
Q7. Draw ray diagram for concave mirror when
1. Object at centre of curvature.
2. Object at focus.
3. Object between centre of curvature and focus.
Q8. Given below is a diagram showing a defect of human eye.
Study it and answer the following questions:
1. Name the defect shown in figure.
2. Give two possible reasons for this defect of eye in human being.
3. Name the type of lens used to correct the eye defect.
4. Draw a labeled diagram to show how the defect is rectified by using the lens.
Ans.
1. The defect of vision is hypermetropia.
2. The two possible reasons of hypermetropia are:
a. Weak action of ciliary muscles cause low converging power of eye lens.
b. The distance between eye lens and retina decreases on account of either shortening of eyeball or flattening of lens.
3. The defect is corrected using convex lens.
4. The convex lens converges the light rays and image is now formed on the retina.
Q9. Fill in the blanks for convex lens.
| | 0.2 | … | 0.1 |
| P(D) | … | 2 | … |
Ans.
| | 0.2 | 0.5 | 0.1 |
| P(D) | 5 | 2 | 10 |
| | … | 5 | 10 |
| | - 30 | - 20 | …. |
| M | -2 | … | -0.5 |
Ans.
| | 15 | 5 | 10 |
| | - 30 | - 20 | 5 |
| M | -2 | -4 | -0.5 |