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Investigate experimentally the critical angle and the conditions under which total internal reflection occurs within a semi-circular glass block

1. On a sheet of white paper, draw around a semi-circular glass block.
2. Remove the glass block. Locate the centre of the side AB, and, using a protractor, draw a normal. Label the normal, O.
3. Replace the glass block carefully on its outline.
4. Connect a ray box to a low voltage power supply. Direct ray of light along a radius of the block towards O. The angle of incidence at O should be about 15^o.
5. Observe the refracted ray – away from the normal into air. Note that there is also a weak reflected ray inside the glass block.
6. Slowly increase the angle of incidence, until the angle of refraction is 90^o. The angle of incidence in the glass block is the critical angle C. Note that there is still a reflected ray inside the glass block but it is stronger now.
7. Mark the position of the incident ray with two pencil Xs.
8. Remove the glass block. Use a ruler and a pencil to join the Xs to O.
9. Use a protractor to measure the critical angle C.

The critical angle in glass is approximately 42^o.