Question

A ray parallel to the principal axis is incident at \(30^\circ\) from the normal on a concave mirror having radius of curvature \(R\). The point on the principal axis where rays are focused is \(O\) such that \(PQ\) is:



 

• A. \(\dfrac{R}{2}\)
• B. \(\dfrac{R}{\sqrt{3}}\)
• C. \(\dfrac{2\sqrt{R}-R}{\sqrt{2}}\)
• D. \(R\left(1-\dfrac{1}{\sqrt{3}}\right)\)

Hint:

For oblique incidence on spherical mirrors, focal length depends on angle: \[ f_{\theta} = \frac{R}{2}\cos\theta \] Use geometry instead of paraxial approximation.

Answer 1

The Correct Option is D