# Question #ec9fc

May 22, 2016

As the speed of light is constant in vacuum the question is not correctly framed.

#### Explanation:

As per postulate of Special Theory of relativity the speed of light in vacuum is constant for all observers, irrespective of the motion of the light source. The constant has been set at $299792458 m {s}^{-} 1$ by definition. It is taken as $3 \times {10}^{8} m {s}^{-} 1$

The relationship connecting the speed $c$ of electromagnetic waves (light) in vacuum, their wavelength $\lambda$ and frequency $\nu$ is as follows

$c = \nu \lambda$ .........(1)

The energy $E$ of a light packet (photon) is given by the equation

$E = h \nu$, .......(2)
where $h$ is the Planck's constant and $= 6.626 \times {10}^{-} 34 J$

As the speed of light is constant in vacuum the question is not correctly framed.
We also see from above equation (1) that with change in wavelength associated with light, its frequency changes. Larger wavelength will have smaller frequency and vice-versa.

However, phase of light $v$ in a medium having refractive index $\mu$ can be different from this value of $c$ but it will always be less than $c$.

Discussions regarding Speed of lights of various colours in refractive media can be read here.