A point source of light $S$ is placed on the axis of a lens of focal length $20 cm$ as shown. A screen is placed normal to the axis of lens at a distance $x$ from it. Treat all rays as paraxial.

Radius of bright circle obtained on screen is equal to $1 cm$ for $x=200 cm$

As $x$ is increased from zero intensity continuously decreases

Intensity at centre of screen for $x=90 cm$ and $x=110 cm$ is same

As $x$ is increased from zero intensity first increases then decreases