Question

For a concave mirror, paraxial rays are focused at distance $\frac{R}{2}$ from pole and marginal rays are focused at distance $x$ from pole, then?(R is radius of curvature)

• A. $x=\frac{R}{2}$
• B. $x=-f$
• C. $x>\frac{R}{2}$
• D. $x<\frac{R}{2}$

Answer 1

The correct option is B $x=-f$

We know that focus is given by $f=\frac{R}{2}$

when parallel rays are focused at $\frac{R}{2}$ i.e right-hand side to the mirror then marginal rays are focused on the left-hand side of the mirror

$x=-f$