A student has obtained the image of a distant object with a concave mirror to determine its focal length. If he has selected a well illuminated red building as an object, which of the following correctly describes the features of the image formed?
A. Virtual, inverted, diminished image in red shade
B. Real, erect, diminished image in pink shade
C. Real, inverted, diminished image in red shade
D. Virtual, erect, enlarged image in red shade

Hint: The student has obtained the image of an object with concave mirror to determine its focal length it means image must have been formed at focus of the mirror, for distance objects image is formed at focus in concave mirror so image must be real, inverted, diminished and in red shade.

Complete answer:
This is a classical way to find the focal length of a concave mirror. We know that for a concave mirror when incident rays are parallel to the principal axis then image is formed at focus. And for distant objects almost all incident rays coming to the mirror are parallel to the principal axis so the image must be formed at infinity. Also size of image should be highly diminished because all the incident rays are concentrated at focus after reflection it means image is a single point or highly diminished. Also all the real images formed by lens are always inverted so image must be inverted and colour of image should be same as of the object

Hence out of those given options, option-C(real, inverted, diminished image in red shade) is correct.

Note:For objects at larger distances image formed is always at focus for concave mirror because incident rays are parallel to principal axis for distant objects and it is real, inverted, and size diminished so nature of image formed must be real, inverted, diminished image in red shade.