Refraction and Total Internal Reflection

This is one of the subtopics of Light. I felt this part a little confusing as it was challenging to draw the total internal reflection and the refraction digrams.

Refraction of Light
When light strikes an opaque reflecting surface such as a irror, most of the light is reflected. Some light, however, is absorbed by the surface.
At the air-glass surface (or interface), light is partially reflected off the surface and partially transmitted through the medium. THe reflected light follows the laws of reflection studied earlier. The light that is transmitted through the medium as it travels from one optical medium (air) into another (glass). This bending effect of light is known as refraction.

The amount of refraction depends on the optical density of the medium. Here we define optical density to be a measure of the exten to which a substance transmit visible light. The higher the opticaldensity of a medium, the lower the tranmittance, thus the greater is the refraction of light in it. For example, glass is an optically denser medium than water. If a light ray were to enter glass from air and water from air respectively, keeping all other parameters constant, the ray would bend more in glass than in water.
However, that the bending only occurs at the interface. Whithin the same medium, light still travels in a straight line.

How will light behave when it enters a different optical medium?
When a light ray strikes perpendically to the surface of an opticalmedium e.g. glass, it passes straight through without refraction.
This is because when it travels from air into glass, its angle of incidence is zero, thus, its angle of refraction is also zero. Even though light is not bent, its speed stilldecreases when it enters the optically denser medium, glass.

The angle of refraction is always smaller than the angle of incidence. This means that when a light ray travels from an optically less dense medium into an optically denser medium at an angle, it is always refracted towards the normal as light travels slower in the optically denser medium.