Question

A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations :

Distance of flame from the lens (cm)

distance of the screen from the lens(cm)

(i) 60 - 20

(ii) 40 - 24

(iii) 30 - 30

(iv) 24 - 40

(v) 15 - 70

(a) From the above table find the focal length of lens without using lens formula. (b) Which set of observations is incorrect and why ? (c) In which case the size of object and image will be same ? Give reason for your answer..

Answer 1

Answer:

Explanation:

you can see, case (iii) ; here both the object and the image distances are 30cm .but we know this is possible only case when object kept at 2f ,so, 2f = 30cm => f = 15cm

(b) use lens formula for finding incorrect observations.

1/v - 1/u = 1/f

see last observation (v)

u = -15

f = +15

then, 1/v = 1/15 - 1/15 = 0 = 1/∞ => v = ∞

but v = 70cm is given hence, observation (v) is wrong.

(c) obviously case 3 , in this case size of image is equal to size of object due to image distance distance is equal to object distance from pole.