Question

The image of a candle flame placed at a distance of $30cm$ from a spherical lens is formed on a screen placed on the other side of the lens at a distance of $60cm$ from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is $3cm$ , find the height of its image.

Answer 1

The lens formula is given as,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$\frac{1}{f}=\frac{1}{+60}-\frac{1}{-30}$

$f=20cm$

Magnification of convex lens is given as,

$\frac{v}{u}=\frac{h_i}{h_o}$

$\frac{60}{-30}=\frac{h_i}{3}$

$h_i=-6cm$

The height of image is $-6cm$.