Pascal's principle, also called Pascal's law, in fluid (gas or liquid) mechanics, statement that, in a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and to the walls of the container.
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.
