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)
1. 60	20
2. 40	24
3. 30	30
4. 24	40
5. 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 the object and image will be same? Give reason for your answer.

Answer 1

a) From the observations, it is clear that for u = 30, v = 30 cm. This means this value must be equal to twice the focal length of the convex lens. 

∴ Focal length of convex lens = 30/2 = 15 cm 

b) The observation (5) is not correct because if u = 15 cm i.e., the object is kept at focus so the image should be at infinity and not at 70 cm. 

c) For twice the focal length we know size of object = size of image. So when object is kept at 30 cm the size of object and image are same.