Sangeetha Pulapaka

You can understand this better with an experiment

1.A rectangular aquarium

2.A few drops of milk (or some powdered milk) to add to the aquarium water to make the beam visible

3.A light source with a well-defined beam

Fill the aquarium with water.. Hold your laser or flashlight up to the side of the aquarium so the light is shining into the water. Add the milk one drop at a time, stirring after each drop, until you can see the light beam passing through the water. If you use powdered milk, add a pinch at a time.

Direct the light beam upward through the water so that it hits the surface of the water from underneath. You can shine the beam into the water through the transparent bottom of the aquarium, or in through a side wall. The beam will be more visible if you can dim the room lights.

Point the beam so that it hits the surface of the water at just about a right angle. In the aquarium, you may be able to see both the reflected beam, which bounces back into the water, and the refracted beam, which comes out of the water and into the air. (Dust in the air helps you see the refracted beam. You can add chalk dust to the air. You can also search for the beam and track it with a piece of paper.)

Notice that most of the beam leaves the water and only a faint beam is reflected back down into the water

Slowly change the angle at which the beam of light hits the surface of the water.

Notice that the beam reflected into the water grows brighter as the beam transmitted into the air becomes dimmer. Also notice that the transmitted beam is bent, or refracted.

Experiment until you find the angle at which the transmitted beam completely disappears. At this angle, called the critical angle, all the light is reflected back into the water.

What is going on?

In general, when a beam of light (the incident beam) hits the interface between two transparent materials, such as air and water, part of the beam is reflected and part of it continues through the interface and on into the other material. The light beam is bent, or refracted, as it passes from one material into the next

The farther the beam is from perpendicular when it hits the surface, the more strongly it is bent. If the light is moving from a material with a low speed of light into a material with a higher speed of light (for example, from water into air), the bending is toward the surface. At some angle, the bending will be so strong that the refracted beam will be directed right along the surface; that is, none of it will get out into the air.

Beyond that angle (the critical angle), all the light is reflected back into the water, so the reflected beam is as bright as the incident beam. This phenomenon is called total internal reflection, because very nearly 100 percent of the beam is reflected, which is better than the very best mirror surfaces.

The critical angle for water is measured between the beam and a line perpendicular to the surface, and is 49 degrees.

Vivekanand Vellanki

When light goes from an optically denser medium to an optically lighter medium, refraction happens as shown in the below image.

The important thing to note is that \angle r\ >\ \angle i in this case. That is, the angle of refraction is greater than the angle of incidence.

The natural question to ask is what happens if \angle i is increased until \angle r becomes 90. Clearly, r becomes 90 before becomes 90. The interesting thing is that when \angle r=90, the refracted ray actually goes along the edge of the surface. The angle of incidence, i, when \angle r=90 is called critical angle.

Now, what happens if i > critical angle? Clearly, the light ray cannot exit out of the medium since that would make r < 90. At this point, the light ray cannot exit the medium. It has to stay inside the medium and now acts like a reflected ray of light following laws of reflection.

When this happens, this is called total internal reflection. That is, the light doesn't exit the medium, but stays inside the medium as though the boundary acts like a mirror. See the below image to understand this:

Arturo Torres

I don't really know but I think this means something that shows drawings, and what do you think can show from the inside.