Question

A studnt focussed the image of a candle flame on white screen using a convex lens .He noted down the position of candle , screen and lens as under. position of candle =26cm , position of lens =50 cm and position of screen =74 cm (1). what is the f of the lens (2). where will the image formed if he shifts the candle towards the lens at a position of 38 cm and draw a ray diagram in (2) case

Answer 1

u = 50-26 = 24cm
v= 74-50=24cm
by mirror formula,
1/f=1/24+1/24=2/24=1/12
i) f=12cm
ii) u = 50-38=12cm
candle is at f because now image of object is eq. to the f
image is formed at infinity
you can draw ray diag