Question

need help can anyone help

Answer 1

Firstly write the question a bit correctly, it was difficult to understand. The question should be:

Q-The image of a candle placed at a distance of 30cm from a spherical lens is formed on a screen placed at a distance of 60cm from the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 2.4cm, find the height of the image.

Now the solution:

$u = - 30cm \\ v = 60cm$
The lens is a convex lens as the image is formed on a screen which means it is real. Only Convex lenses can form real images.


From lens formula,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ \frac{1}{60} - ( - \frac{1}{30} ) = \frac{1}{f} \\ \frac{1}{60} + \frac{1}{30} = \frac{1}{f} \\ 20cm = f$
Therefore the focal length is 20cm.

Now,
magnification=
$\frac{i}{o} = \frac{v}{u}$
where i and o are heights of image and object respectively.

$\frac{i}{2.4} = - \frac{60}{ 30} \\ i = - 4.8cm$

This means that the image is inverted and has height 4.8cm.