A light source (object) and a screen are placed 100 cm aparton an optical bench.

Question

A light source (object) and a screen are placed 100 cm aparton an optical bench. A sharp image of the light source is obtainedon the screen when a lens is positioned 20 cm from the screen. Whatis the focal length of the lens?
In the above problem, the light source is 1 cm in height. What isthe height of the image? A light source (object) and a screen are placed 100 cm aparton an optical bench. A sharp image of the light source is obtainedon the screen when a lens is positioned 20 cm from the screen. Whatis the focal length of the lens?

Explanation / Answer

separation of the object and screen D = 100 cm object distance u = 80 cm image distance v = 100 -80 = 20 cm from the relation ( 1 / v ) - ( 1/ u ) = ( 1/ f ) from this focal length of the lens   f = uv / ( u -v )                                                    = 26.666 cm we know   x / y = u / v from this height of the image   y = x v / u                                               = 1 * 20 / 60                                               = 0.3333 cm

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