Reflection & Refraction (C10)

Rohit 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 lens as under:
Position of candle = 26.0 cm
Position of convex lens = 50.0 cm
Position of screen = 74.0 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 38 cm?
iii) Draw a ray diagram to show the formation of the image in case (ii) as said above?

Answer

i) u = 50 - 26 = 24 cm

v = 74 - 50 = 24 cm

2f = 24 cm

f = 24/2 = 12 cm

ii) u = 50 - 38 = 12 cm

i.e. Candle is at f

So, image is formed at infinity.

iii)