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 in the

following table:

Distance of the flame from the lens

(cm)

Distance of screen from the lens

(cm)

60 20

40 24

30 30

24 40

12 70

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

(b) Which set of observation is incorrect and why?​

Answer 1

Answer:

focal length =15cm

the last observation {12,70} is incorrect{explanation below}

Explanation:

a)

when image and object are at same position means that it is the centre of curvature .

c=30cm

f=c\2

f=30\2 = 15cm

b)

in all the observations ,by the lens formula focal length should come 15 BUT in last observation(12,70) the focal length is not coming 15 [{(be sure with sign convention)}]