Focal Length

Q. 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]

1. $1.2\text{m}$
2. $0.4\text{m}$
3. $0.6\text{m}$
4. $2.4\text{m}$