As light is an electromagnetic wave that causes sensation of sight.
Every electromagnetic wave has electrical and magnetic components in it which are perpendicular to the direction of propagation of wave.
The electrical and magnetic components are mutually perpendicular to each other in an electromagnetic wave.
The speed of light in vacuum is c=3\times 10^{8}\ m.s^{-1}, which decreases when it passes through a matter (precisely a transparent medium) which allows the passage of light through it. When the light passes through some transparent matter then its speed decreases as compared to the speed of light in vacuum.
