A ray of light travels from an optically denser medium to an optically rarer medium. What will be the change in the frequency of light ?

The correct option is No Change.

When a ray of light passes from one medium to another (such as from an optically denser medium to an optically rarer medium), its speed and wavelength change due to the difference in optical density of the two media.

However, the frequency of a light wave depends solely on the source of light that produces it. It represents the number of wave crests emitted per second by the source. Since the source of light remains the same during refraction, the frequency remains constant.

We can also understand this relationship mathematically using the wave equation:

$v=f\times \lambda$

where:
$v$ is the speed of light in the medium,
$f$ is the frequency of the light wave, and
$\lambda$ is the wavelength of the light wave.

When light travels into a rarer medium, its speed $v$ increases, and its wavelength $\lambda$ increases proportionally such that the ratio $\frac{v}{\lambda }$, which gives the frequency $f$, stays unchanged.

Therefore, there will be no change in the frequency of light.