Question

while performing an experiment using a convex lens, lens and a screem is placed in front of each other such that an inverted and focused image is formed on the screen. What changes will take place in the image when we move screen towards or away from the lens ? and why ?

Answer 1

The focal length $f$ of the lens is related to the object distance $u$ and the image distance (screen distance) $v$ as follows,
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
Also, the magnification is related to these quantities as,
$M=-\frac{v}{u}=\frac{h_i}{h_o}$
where $h_i\text{and}{h}_o$ are the height of the image and the object respectively.

Thus, if the distance of the screen is increased or decreased but the object distance then the image may not be focused on the screen and sharp image cannot be obtained on the screen. The image thus formed will be blurred. So, in order to focus the image the distance of the abject as well as the distance of the screen must be gradually adjusted. So, by doing so the focal length as well as the magnification of the lens can be calculated by using above equations.