Question

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:

• A. As x is increased from zero, intensity on the screen continuously decreases
• B. As x is increased from zero, intensity on the screen first increases then decreases
• C. Intensity at centre of screen for x=$90$cm and x= $100$cm is same
• D. Radius of bring circle obtained on screen is equal to $1$ cm for $x=200$ cm

Answer 1

The correct option is C Intensity at centre of screen for x=$90$cm and x= $100$cm is same

$\begin{array}{c}Given\\ u=-25cm\\ f=20cm\\ fromlensformula\\ \frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{v}-\frac{1}{-25}=\frac{1}{20}\\ \frac{1}{v}=\frac{1}{20}-\frac{1}{25}=\frac{5-4}{100}=\frac{1}{100}\\ v=100cminrigidoflens\\ Hence,optionCisthecorrectanswer.\end{array}$