Q37 of 38 Page 84

Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of –50 cm. What is the nature of the lens and its power used by each of them?

Power is defined as the degree of divergence of convergence achieved by the lens and it is reciprocal of the focal length.


The unit of Power is Dioptre.


Focal length = 50 cm


In meter, focal length is : 0.5 m


Power = 1/f = 1/0.5


Power of the lens = +2D


The lens will be a convex lens


Focal length = -50 cm


In meter, focal length will be = -0.5 m


Power = 1/-0.5 = -2D


In this case, the lens will be a concave lens.


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34

Draw ray diagrams showing the image formation by a convex mirror when an object is placed

(a) at infinity


(b) at finite distance from the mirror

 35

The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?

 36

Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to 1/3rd of its size. At what distance the object has been placed from the mirror? What is the nature of the image and the mirror?

 38

A student focused the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle screen and the lens as under Position of candle = 12.0 cm Position of convex lens = 50.0 cm Position of the screen = 88.0 cm

(i) What is the focal length of the convex lens?


(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?


(iii) What will be the nature of the image formed if he further shifts the candle towards the lens?


(iv) Draw a ray diagram to show the formation of the image in case


(iii) as said above.