Question

What changes will occur to the focal length of a concave mirror when violet light is replaced by a light of red colour?

Answer 1

Hint: The focal length of the mirror is the distance from the centre of the mirror to the focus of the mirror. It doesn’t depend on the wavelength of the light incident on the mirror or the frequency of the light. Find out the focal length of the mirror and we can conclude the answer.

Complete answer:
The focal length of a mirror can be calculated by measuring the distance between the centre of the mirror and the focus of the mirror. It is also equal to half the radius of curvature of the concave mirror. As we know, the distance between the centre of the mirror and focus will not change for different wavelengths of light and is always constant.
Thus, there won’t occur any change in the focal length of the mirror when there is change in the frequency of the blight incident on the concave mirror.

Additional information:
As per thin lens approximation, the focal length of the mirror is not depending on the material of the mirror used. Therefore, there won’t be change in the focal length for any change in colour of light. The image formed will also be at the same distance and the colour of the image formed will also be the same as the colour of the light projected on the object.

Note:
The focal length of the mirror or any lens used will also not depend on the refractive index of the medium used. The object distance and the image distance will change when the refractive index of the medium is changed. The focal length only depends on the radius of curvature, but not on any other factor.