When a light beam is incident from air into a semi-circular glass block in a direction towards its centre, the light beam goes straight through towards that centre as it is incident along a radius of the block. All radii of the block are normals.

If the angle in the glass block is less than its critical angle (of about 42 degrees), the beam will be refracted into the air out of the block. However there is a partial internal reflection (p.i.r.) within the block. That faint partially internally reflected beam will go straight through towards the upper right hand side of the photo.

When the angle that the light is incident into the glass block becomes progressively bigger, there will come a time when the refracted beam in air at the straight lower straight edge of the block is refracted along the edge. This can been seen on the right hand picture. There is a slight amount of dispersion because the incident beam used was a white beam of light. The p.i.r. beam has now become slightly brighter than that in the above photo, with the ligth beam exiting towards the upper right hand side of the photo.

When the angle in the glass block is further increased to a value greater than its critical angle (of about 42 degrees), the whole beam is totally internal reflected back within the block. It emerges at the upper right hand side of the photo. No light comes out of the block along its lower straight edge. We say that total internal reflection (t.i.r.) has taken place. The amazing phenomenon is the the transparent straight edge of the glass behaves like a "mirror" reflecting the light back into the block.

There are two conditions for t.i.r. to take place:

1. The incident beam must go from a denser medium (glass) to a less dense medium (air) . We are referring the situation at around the centre of the circular glass block.

2. The angle of incidence (of about 45 degrees in this case) in the denser medium must be greater than the critical angle (of about 42 degress) of the denser medium.

Task: List all the applications and phenomena of total internal reflection.