Light travels fastest through a vacuum, it slows down when moving through a material medium.
->Progress is impeded by the electronic charges in atoms and molecules. But, in metals where they have freely moving delocalised electrons, it stops the light wave completely.
In materials like water, glass and plastics the light waves are absorbed and slowed down.
The ratio of speed of light in vacuum(before entering medium) to its speed in the medium is called the refractive index.
-> N = C (speed before enter) / V (after enter)
When the angle of refraction is at 90°, the angle of incidence is then called the critical angle.
But when the angle of incidence is greater than the critical angle, light cannot pass through its surface, it's all reflected.
