Question

A student 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 the lens as under,
Position of candle=12.0 cm
Position of convex lens=50.0 cm
Position of screen=88.0 cm
a)What is the focal length of the convex lens?
b)Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?

Answer 1

$$\begin{gathered} \left(1\right)U\sin g\frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\ u=-38cm,v=38cm \\ \Rightarrow f=19cm, \\ \left(2\right)Nowu=-19cm,f=19cm \\ \Rightarrow v=\infty ,Soimagewillbeformedatinfinity. \end{gathered}$$