Question

a student focused the image of a candle flame on a white screen using a convex lens he noted the position of candle convex lens and screen is 12cm ,50cm ,88 cm find the focal length?

Answer 1

Object distance, u= 50-12=-38 cm (Minus sign for object position)
Image distance, v= 88-50=38 cm

Using lens formula, we get
1/f=1/v-1/u=1/38-(-1/38)=2/38
Therefore, f=19cm

Answer 2

Answer:

u = 50-12 = 38 cm ( - )

v = 88 - 50 = 38 cm

f = ?

1/f = 1/v - 1/u

1/f = 1/38 +1/38

1/f = 1+1/38

1/f = 2/38

f = 38/2 = 19 cm