PAGE NO- 142
1. Define the principal focus of a concave mirror.
Answer:
The principal focus of a concave mirror is the point on its principal axis where all light rays, parallel to the axis, meet after being reflected from the mirror.
2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer:
Given: Radius of curvature (R) = 20 cm
We know that,
( R = 2f )
So,
( f = R/2 = 20/2 = 10 )
Therefore, the focal length of the mirror is 10 cm.
3. Name the mirror that can give an erect and enlarged image of an object.
Answer:
A concave mirror can give an erect and enlarged image of an object.
4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer:
A convex mirror gives a wider field of view, allowing the driver to see more area behind the vehicle. It always forms erect, virtual, and diminished images, making it ideal for rear-view mirrors.
Page NO-143
1. Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
Given: ( )
We know ( R = 2f )
So, ( f = R/2 = 32/2 = 16 m} )
Hence, the focal length is 16 cm.
2. A concave mirror produces three times magnified real image of an object placed 10 cm in front of it. Find the image position.
Answer:
Hence, the image is formed 30 cm in front of the mirror and is inverted.
PAGE NO- 150
1. A ray of light travelling in air enters obliquely into water. Does it bend towards or away from the normal? Why?
Answer:
It bends towards the normal because light slows down when it moves from a rarer medium (air) to a denser medium (water).
2. Light enters from air to glass with refractive index 1.50. What is the speed of light in glass?
Answer:
Refractive indexHence, the speed of light in glass is 2 × 10⁸ m/s.
3. Find the medium with highest and lowest optical density.
Answer:
From the table:
- Lowest optical density: Air (n = 1.0003)
- Highest optical density: Diamond (n = 2.42)
The higher the refractive index, the greater the optical density.
4. You are given kerosene, turpentine and water. In which does light travel fastest?
Answer:
Light travels fastest in water because its refractive index (1.33) is lower than that of kerosene (1.44) and turpentine (1.47).
5. The refractive index of diamond is 2.42. What does it mean?
Answer:
It means that light travels 2.42 times slower in diamond than in air or vacuum.
In other words, speed of light in diamond = times its speed in air.
PAGE NO – 158
1. Define 1 dioptre of power of a lens.
Answer:
One dioptre is the power of a lens with a focal length of 1 metre.
2. A convex lens forms a real and inverted image of a needle at 50 cm. Where is the needle placed if image is same size? Find the power.
Answer:
When image size = object size → object is at 2F.
So, object distance = image distance = 50 cm.
Hence, focal length .
Power, ( P = 1/f = 1/0.25 = +4D ).
3. Find the power of a concave lens of focal length 2 m.
Answer:
( P = 1/f = 1/(-2) = -0.5D )
So, the power is –0.5 dioptres.
EXERCISE
1. Which one of the following materials cannot be used to make a lens?
(a) Water (b) Glass (c) Plastic (d) Clay
Answer: (d) Clay, because light cannot pass through it.
2. The image formed by a concave mirror is virtual, erect and larger. Where is the object placed?
Answer: (d) Between the pole and the principal focus.
3. Where should an object be placed in front of a convex lens to get a real image of the same size?
Answer: (b) At twice the focal length (2F).
4. A spherical mirror and a thin spherical lens have focal length –15 cm. What are they?
Answer: (a) Both concave.
5. No matter how far you stand, your image is erect. The mirror is likely to be—
Answer: (d) Either plane or convex.
6. Which lens is used to read small letters in a dictionary?
Answer: (c) Convex lens of focal length 5 cm.
7. We wish to get an erect image using a concave mirror of focal length 15 cm.
Answer:
Object distance: 0 to 15 cm from the pole.
Nature: Virtual, erect, and enlarged image.
8. Name the type of mirror used in these cases:
(a) Headlights of a car → Concave mirror (focuses light into a beam)
(b) Rear-view mirror → Convex mirror (gives a wide view)
(c) Solar furnace → Concave mirror (concentrates sunlight at focus)
9. Half of a convex lens is covered with paper. Will it form a complete image?
Answer:
Yes, a complete image will form, but it will be less bright because fewer light rays pass through.
10. An object 5 cm tall is placed 25 cm from a convex lens (f = 10 cm).
Answer:
Using lens formula,
Image distance The image is real, inverted, and 3.3 cm tall, formed on the other side of the lens.
11. A concave lens of focal length 15 cm forms an image 10 cm away.
Answer:
Using lens formula, object distanHence, object is 30 cm in front of the lens.
12. An object 10 cm from a convex mirror (f = +15 cm).
Answer:
Image distance Hence, the image is virtual, erect, and diminished.
13. The magnification of a plane mirror is +1. What does it mean?
Answer:
It means the image is virtual, erect, and of the same size as the object.
14. An object 5 cm high is placed 20 cm in front of a convex mirror (R = 30 cm).
Answer:
Focal length
Image distance Hence, image is virtual, erect, and smaller in size.
15. An object 7 cm tall is placed 27 cm from a concave mirror (f = 18 cm).
Answer:
Image distance Image is real, inverted, and smaller, formed on the same side as the object.
16. Find the focal length of a lens of power –2.0 D.
Answer:
It is a concave lens because focal length is negative.
17. A doctor prescribes a lens of power +1.5 D. Find focal length and type.
Answer:
It is a convex (converging) lens because power is positive.