. What makes an object visible?
A. An object reflects light that falls on it, this reflected light, when received by our eyes
enables us to see things.
2. How does a small source of light casts a sharp shadow of an opaque object?
A. Light always or seems to travel in straight line. Therefore, a small source of a light casts
a sharp shadow of an opaque object points to this straight-line path of light, usually
indicated as a ray of light.
3. What is diffraction?
A. If an opaque object on the path of light becomes very small, light has a tendency to bend
around it and not walk in a straight line. This is known as diffraction of light.
4. What is a mirror?
A. A highly polished surface is called mirror.
5. Give a property of mirror.
A. Mirror reflects most of the light falling on it.
6. Write the laws of reflection.
A. Laws of reflection
1. The angle of incidence is equal to the angle of reflection, and
2. The incidence ray, the normal to the mirror at the point of incidence and the reflected
ray all lie in the same plane.
7. What are spherical mirrors?
A. The mirrors whose reflecting surfaces are spherical are called spherical mirrors.
8. What is a concave mirror?
A. A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the
centre of the sphere, is called the concave mirror.
9. What is a convex mirror?
A. A spherical mirror, whose reflecting surface is curved outwards, is called a convex
mirror.
10. What is pole? How is it represented?
A. The centre of the reflecting surface of a spherical mirror is a point called the pole. The
pole is usually represented by P.
11. What is centre of curvature?
A. The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a
centre. This point is called centre of curvature of spherical mirror usually representing by C.
12. Is centre of curvature a part of mirror?
A. No.
13. Where is the centre of curvature of concave and convex located?
A. The centre of curvature of concave mirror lies in front of it while centre of curvature of
convex mirror lies behind it.
14. What is radius of curvature?
A. The radius of the sphere of which the reflecting surface of a spherical mirror forms a
part, it is called the radius of curvature usually represented by R.
15. What is angle made by principal axis at pole?
A. 90o
16. What is principal axis?
A. A straight line passing through the pole and the centre of curvature of a spherical mirror
is called principal axis.
17. What is focal length?
A. The distance between the pole and the principal focus of a spherical mirror is called the
focal length usually represent by f.
18. What is principal focus?
A. The parallel rays to the principal after reflection passes or appears to pass through a
point called principal focus.
19. What is aperture?
A. The diameter of the reflecting surface of spherical mirror is called its aperture.
20. Write the relation between R and f?
A. R=2f (for the mirrors of small apertures).
21. On what factors do nature, position and size of image formed by a concave mirror
depend?
A. On the position of object.
22. A ray is parallel to principal axis. What happens to it after reflection?
A. The ray passes through focus.
23. A ray is passing through focus what happens to it after reflection?
A. The ray passes parallel to principal axis.
24. Why is a ray passing through centre of curvature moves back in the same path?
A. The incident ray falls on the mirror along the normal to the reflecting surface.
25. What happens after reflection if a ray incident obliquely to the principal axis?
A. Ray is reflected obliquely.
26. Write some uses of concave mirror and why are they are used so?
A. Uses of concave mirror
a) Concave mirrors are used in torches; searchlights and vehicles headlights to get
powerful parallel beams of light.
b)
Concave mirrors are used as shaving mirrors to see a larger image of the face.
c)
The dentists use concave mirrors to see large images of the teeth of patients.
d) Large concave mirrors are used to concentrate sunlight to produce heat in solar
furnaces.
27. Which mirrors are fitted in the walls of Agra Fort and why?
A. Convex mirror is fitted in a wall of Agra Fort to view the tomb distinctly and full length
of the image.
28. Write some uses of convex mirrors and why are they are used so?
A. Convex mirrors are used as rear-view mirrors in vehicles because they always give erect
and diminished images. Also they have a wider field which enables the driver to view much
larger area than would be possible by a plane mirror.
29. Write the new Cartesian sign conventions.
A. The new Cartesian sign conventions.
1. The object is always placed to the left of the mirror. This implies that the light from the
object falls on the mirror from the left-hand side.
2. All distances parallel to the principal axis are measured from the pole of the mirror.
3. All distances measured to the right of the origin are taken as positive while those
measured to the left of the origin are taken as negative.
4. Distances measured perpendicular to and above the principal axis are taken as positive.
5. Distances measured perpendicular to and below the principal axis are taken as negative.
30. Define mirror formula and write it.
1. The relationship between object distance (u), image distance (v) and focal length (f) is
called mirror formula.
1/v + 1/u = 1/f
31. What is magnification?
A. The ration of the height of the image to the height of the object. This is called
magnification.
M=h’/h
32. When is magnification negative and when is it positive?
A. Magnification is negative for real image and positive for virtual images.
33. What is refraction?
A. 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 refraction of light.
33. Why are coins kept in a vessel of water appear to be raised?
A. Due to refraction coins kept in a vessel of water appear to be raised.
34. Why are incident ray and refracted ray parallel when refraction takes place in glass
slab?
A. The opposite sides of the glass slab are equal and parallel. Therefore, incident ray is
equal to refracted ray.
35. Why does refraction take place?
A. Refraction is due to change in the speed of light as it enters from one transparent
medium to another.
36. Write the laws of refraction.
A. Laws of refraction
a) The incident ray, the refracted ray and the normal to the interface of two transparent
media at the point of incidence all lie in the same plane.
b) The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for
the light of a given colour and for a given pair of media.
37. What is snell’s law of refraction?
A. The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for
the light of a given colour and for a given pair of media.
Sin i/Sin r = constant
38. What is refractive index?
A. The extent of the change in direction that takes place in a given pair of media is
expressed in terms of the refractive index.
39. What is the speed of light in vaccum?
A. 3´108 m/s
40. What is meant by n21?
A. The refractive index of medium 2 with respect to medium 1.
n21 = Speed of light in medium 1
Speed of light in medium 2
41. What is meant by optically rarer medium?
A. The medium having less refractive index is called optically rarer medium.
42. What is meant by optically denser medium?
A. The medium having more refractive index is called optically denser medium.
43. What is a lens?
A. A transparent material bound by two surfaces of which one or both surfaces are
spherical forms a lens.
44. What is convex lens?
A. The lens whose surface is thicker at the centre than at edges is called convex lens.
45. Which are called converging lens?
A. Convex lens.
46. What is concave lens?
A. The lens whose surface is thicker at the edges than at centre is called convex lens.
47. Which lens is called diverging lens?
A. Concave lens.
48. How many centre of curvatures does a lens has?
A. Two.
49. What is principal axis?
A. The straight line passing through the two centre of curvatures of a lens is called its
principal axis.
50. What is optical centre?
A. The centre point of a lens is its optical centre.
51. What happens when a ray passes through optical centre of a lens?
A. A ray of light passing through the optical centre of a lens passes without suffering any
deviation.
52. What is aperture of lens?
A. The effective diameter of the circular outline of a spherical lens is called its aperture.
53. What is meant by thin lens with small aperture?
A. The lens whose aperture is much less than its radius of curvature. Such lens is called
thin lens with small aperture.
54. What is principal focus?
A. When the light rays are parallel to principal axis after refraction through lens the rays
either passes or appears to pass through a point called principal focus.
55. How many foci are there for a lens?
A. Two.
56. What is focal length of lens?
A. The distance of the principal focus from the optical centre of the lens is called its focal
length.
57. Give a method by which we can find the focal length of a lens or a mirror.
A.
Activity
q Hold the lens or mirror in a hand. Direct it towards the sun.
q Focus the light from the sun on a sheet of paper. Obtain a sharp bright image of the sun.
q Hold the paper and the lens of mirror in the same the position for a while. Measure the
distance between image and lens or mirror.
This gives an approximate value of focal length.
55. What is lens formula?
A. 1/v-1/u=1/f.
56. What is relation between v, u and m in lens?
A. m=v/u.
57. What is meant by the power of a lens?
OR
Define the power of a lens.
A. The ability of a lens to converge or diverge is called power of that lens.
OR
The power of a lens is defined as the reciprocal of its focal length.
58. What is the SI unit of power of a lens?
A. The SI unit of power of a lens is a ‘D’ (dioptre).
59. Define 1 dioptre.
A. 1 dioptre is the power of a lens whose focal length is 1m.
60. Which lenses have positive power and which have negative power?
A. Convex lens has positive power and Concave lens has negative power.
61. What is meant by +2.0 D?
A. The power of lens is 2 dioptres and it is convex lens.
62. Define power of combination of lens.
A. The algebraic sum of the individual lens when placed in contact is called power of
combination.
63. What is the nature, size and position of image formed by plane mirror?
A. Image formed by a plane mirror is always virtual and erect. The size of the image is
equal to that of the object. The image formed is as far behind the mirror as the object is in
front of it. Image is laterally inverted.
64. In which mirror centre of curvature lies behind it?
A. Convex mirror.
65. Why does paper burn when a concave mirror is directed towards the Sun?
A. The light from the sun is converged at a point, as a sharp, bright spot by the mirror. In
fact, this spot of light is the image of the sun on the sheet of paper. The heat produced due
to the concentration of sunlight ignites the paper.
66. Why does pencil appears to be displaced at the interface?
A. The light reaching you from the portion of the pencil inside water seems to come from a
different direction, compared to the part above water. This makes the pencil appear to be
displaced at interface.
67. What happens to light ray when it enters from rare to dense medium?
A. The light ray bends towards normal
68. Write the nature, size and position of image of concave mirror when object is at infinity,
C, C and between C, C and F, at F and between P and F.
A. Nature, size position of image formed by concave mirror.
Position of the
object
Position of the
image
Size of the
image
Nature of the
image
At infinity
At the F
Highly diminished Real and inverted
Beyond C
Between F and C
Diminished
Real and inverted
At C
At C
Same size
Real and inverted
Between C and F
Beyond C
Enlarged
Real and inverted
At F
At infinity
Highly-enlarged
Real and inverted
Between P and F
Behind the mirror
Enlarged
Virtual and erect
69. What is the nature, size and position of image when object is at infinity and between
infinity and pole?
A. When the object is at infinity the image is formed at the focus, which is highly
diminished and virtual and erect. When the object is between infinity and pole the image is
formed between P and F, which is highly diminished and virtual and erect.
70. What happens to light ray when it enters from dense to rare medium?
A. The light ray bends away from normal.
71. On what does the refractive index of a given media depend?
A. The value of the refractive index of a given media depends upon the speed of light in the
media.
72. What is meant by absolute refractive index of the medium?
A. When the refractive index of medium is considered with respect to vaccum. This is
called the absolute refractive index of the medium.
73. What is meant by the refractive index of a substance is 1.33?
A. It means that the ratio of the speed of light in air and the speed of light in substance is
equal to 1.33.
74. What is the refractive index of water and crown glass?
A. Water-1.33 and Crown glass-1.52.
75. Name the substance, which has maximum refractive index.
A. Diamond [2.42].
76. On what does the power of the lens depends?
A. Focal length.
77. Which substances use a lens system in their preparation?
A. Lens system is used in the design of camera lens and the objective of microscopes and
telescopes.
78. Does a lens system have only one lens?
A. No, a lens system consists of several lenses.
79. Why are lenses combined?
A. Lenses are combined to increase the magnification and sharpness of the image.