Question

A point source of light S is placed on the axis of a lens of focal length 20cm at a distance 25cm from lense. A screen is placed normal to the axis of lens at a distance x from it (treat all rays as paraxial). Identify the true or false statements.
a. As x is increased from zero , intensity continuously decreases.
b. As x is increased from zero, intensity first increases then decreases.
c. Intensity at center of screen for `x =90 cm` and `x=110 cm` is same.

Answer 1

For position of image using
`(1)/(v)-(1)/(u)=(1)/(f)rArr (1)/(v)=(1)/((-25))=(1)/(20)`
`rArr (1)/(v)=(1)/(20)-(1)/(25)=(1)/(100)rArr v=100cm`
The intensity of light will be maximum at image position i.e., at `x=100 cm`. The intensity of light increase from lense poisition and becomes maximum at `x=100 cm` then decreases. The intensity of light is same at equal distance from image poisition. Hence statements (b) and (c) are true.