Page 168
1. Define the principal focus of a concave mirror.
Answer:
It is a point on its principal axis to which all the light rays which are parallel and close to the axis converge after reflection from the concave mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer:
Focal length = 1/2 × Radius of curvature = 1/2 × 20 cm = 10 cm
3. Name a mirror that can give an erect and enlarged image of an object.
Answer:
Concave mirror.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer:
A convex mirror always produces an erect image of the objects. The image formed in a convex mirror is highly diminished or much smaller than the object, due to which a convex mirror gives a wide field of view of the traffic behind. A convex mirror enables the driver to view a larger area of the traffic behind them.
Page 171
1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
R = +32 cm and f = R/2 = +32/2 = +16 cm
2. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Answer:
Because the image is real, the magnification m must be negative.
m = -3, u = -10 cm
m = -v/u ⇒ -3 = -v/-10 ⇒ v = -30 cm
Thus, the image is located at a distance of 30 cm in front of the mirror on the same side as the object.
Page 176
1. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Answer:
The light ray bends towards the normal because it goes from a rarer medium (air) to a denser medium (water).
2. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 108 m/s.
Answer:
Refractive index of glass, ng = 1.50
Speed of light in glass = Speed of light in vacuum/Refractive index
= 3 × 108/1.50
= 2 × 108 m/s
3. Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.
Answer:
From Table 10.3, diamond has the highest refractive index (= 2.42), so it has the highest optical density.
Air has the lowest refractive index (= 1.0003), so it has the lowest optical density.
4. You are given kerosene, turpentine, and water. In which of these does the light travel fastest? Use the information given in Table 10.3.
Answer:
For kerosene, n = 1.44
For turpentine, n = 1.47
For water, n = 1.33
Because water has the lowest refractive index, light travels fastest in this optically rarer medium compared to kerosene and turpentine oil.
5. The refractive index of diamond is 2.42. What is the meaning of this statement?
Answer:
By saying that the refractive index of diamond is 2.42, we mean that the speed of light in diamond is lower by a factor of 2.42 relative to that in vacuum.
Page 184
1. Define 1 dioptre of power of a lens.
Answer:
One dioptre is the power of a lens whose focal length is 1 metre.
2. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
Answer:
Here, v = +50 cm
Because the real image is of the same size as the object, magnification m = -1
m = v/u ⇒ -1 = 50/u ⇒ u = -50 cm
Using the lens formula: 1/f = 1/v - 1/u
= 1/50 - 1/-50
= 1/50 + 1/50 = 2/50 = 1/25
f = 25 cm = 0.25 m
Power, P = 1/f = 1/0.25 = +4 D
3. Find the power of a concave lens of focal length 2 m.
Answer:
Because the focal length of a concave lens is negative, f = -2 m
Power, P = 1/f = 1/-2 = -0.5 D