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

Answer: (B)

$x=-f$

When the radius of curvature and the focus of a spherical mirror is defined as $R$ and $f$, we know that,

$f=\frac{R}{2}$

According to the question, the paraxial rays are focused at a distance of $\frac{R}{2}$, say at the spherical mirror's focus.

Therefore the marginal rays will merge behind the mirror, and they shall be focused at a distance of $-\frac{R}{2}$ or say at $-f$.

So, the correct option is (B)