Question

To determine the focal length of a concave mirror, a student focuses a classroom window, a distant tree and the sun on the screen with the help of a concave mirror. In which case will the student get more accurate value of focal length?

Answer 1

As SUN is at infinity    rays coming from it will seem to be parallel to principle focus at smaller angles  thus  image FORMATION will take place at focus characteristic of concave mirror

Answer 2

Hii mate,

● Answer -
The Sun

● Explaination -
- Focal length is the distance between centre of mirror/lens and its focus.
- When parallel rays of light from infinity fall on concave mirror, they converges at principal focus of mirror.
- Hence, to determine focal length of concave mirror, the most distant object should be focused.
- As from given options, sun is most far away it'll best object for accurate determination of focal length.


Hope this helped...