# Chapter Unit - 2 : Light - Refraction Through Lens - Frank Solutions for Class 10 Physics ICSE

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## Chapter 2 - Light - Refraction Through Lens Excercise 115

Question 1
What do you mean by a lens?
Solution 1
A lens may be defined as a transparent refracting medium bounded by two curved surfaces which are generally spherical.
Question 2
State two main differences between a convex and concave lens.
Solution 2
Question 3
Draw neat diagram to show the
(i) Convergent action of a convex lens,
(ii) Divergent action of concave lens.
Solution 3
Question 4
Define the term focal length of a lens.
Solution 4
Focal length of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal lengths on either side of the lens and they are referred to as the first focal length and the second focal length of a lens:
(i) First focal length: The distance from the optical centre of the lens to its first focal point is called the first focal length (f1) of the lens.
(ii) Second focal length: The distance from the optical centre of the lens to its second focal point is called the second focal length (f2) of the lens.
Question 5
Can you guess the focal length of a plane mirror?
Solution 5
Focal length of a plane mirror is infinity.
Question 6
Write the SI unit of focal length of a concave lens.
Solution 6
The SI unit of focal length is 'metre'.
Question 7
Define the term principle axis of a lens.
Solution 7
Principal axis of a lens is the line joining the centres of curvatures of the two surfaces of the lens.
Question 8
What do you understand by focal plane of a lens?
Solution 8
Focal plane of a lens: Rays of light can pass through the lens in any direction and hence there will be two focal planes on either side of the lens and they are referred to as the first focal plane and the second focal plane of a lens:
(i) First focal plane: It is the plane passing through the first focal point and normal to the principal axis of the lens.
(ii) Second focal plane: It is the plane passing through the second focal point and normal to the principal axis of the lens.
Question 9
Show by a diagram the refraction of two incident rays parallel to the principle axis passing through a convex lens by treating it as a combination of a glass block and two triangular glass prisms.
Solution 9

## Chapter 2 - Light - Refraction Through Lens Excercise 116

Question 1
Show by a diagram, the refraction of two incident rays parallel to the principle axis passing through a concave lens by treating it as a combination of a glass block and two triangular glass prisms.
Solution 1
Question 2
State the condition for each of the following:
(i) A lens has both its focal lengths equal,
(ii) A ray passes undeviated through the lens.
Solution 2
(i) If the medium on both sides of the lens is same, the first and second focal lengths are equal.
(ii) A ray of light passing through the optical centre of the lens passes undeviated.
Question 3
Solution 3
Question 4
Solution 4
Question 5
An object of height 4.0 cm is placed at a distance 24 cm in front of a convex lens of focal length 8 cm. Draw a ray diagram to find the position of image. State the characteristics of the image.
Solution 5
Question 6
Solution 6
Question 7
Distinguish between a real and a virtual image.
Solution 7
Question 8
Solution 8

## Chapter 2 - Light - Refraction Through Lens Excercise 117

Question 1
A convex lens produces an image on a screen twice the size of the object. The distance between the object and screen is 45 cm. Find the distance of the lens from the object and its focal length by drawing a ray diagram.
Solution 1
Question 2
(i) Express power of a concave lens of focal length 50 cm with its sign.
(ii) The power of a lens is +2.0 D. Find its focal length and state what kind of lens it is.
Solution 2

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