Question

The diameter of a reflector of a solar cooker is $2.4\text{m}$. What will be the focal length of the reflector at which the cooker can be placed for obtaining maximum heat? [2 marks]

• A. $1.2\text{m}$
• B. $0.4\text{m}$
• C. $0.6\text{m}$
• D. $2.4\text{m}$

Answer 1

The correct option is C $0.6\text{m}$

The object is at infinity (here the sun), all the rays that fall onto the mirror, get reflected onto a single point - the focal point of the reflector.
Given:
The diameter of the reflector, $D=2.4\text{m}$
$⟹$ Radius of curvature, $R=1.2\text{m}$ [1 mark]

The focal length is half the radius of curvature.
$f=\frac{R}{2}=0.6\text{m}$ [1 mark]