Question

Find the angle of incidence of ray for which after reflection, it passes through pole of concave mirror.

• A. ${30}^{\circ }$
• B. ${60}^{\circ }$
• C. ${90}^{\circ }$
• D. ${75}^{\circ }$

Answer 1

The correct option is B ${60}^{\circ }$

Paraxial rays are those rays where the angle of incidence satisfies the following conditions.

$cosi\approx 1sini=tani\approx \theta$

In case of non paraxial rays, those parallel to the principal axis need not converge at the focus.

Consider a non paraxial incident on a concave mirror at an angle $i$.


Here, angle $AOB=i\Rightarrow AB=OB$
If the reflected ray passes through the pole,
$AB=OB=OA=2R$
Which means the triangle $AOB$ has to be equilateral and the angle of incidence $i={60}^0$