Optics Disk

What it shows:

All of the concepts summarized by the above keywords can be clearly and quantitatively demonstrated with this piece of apparatus.

How it works:

A light source 1 rotates around the circumference of a large white disk 2 with degree graduations around the entire perimeter. The collimated beam of light grazes the surface of the disk, creating a highly visible pencil of light, so that ray tracing is easily accomplished. A horizontal mirror, positioned at the center of the disk, is used for the law of reflection, and a semi-circular piece of plastic 3 works for all the refraction phenomena.

optics disk

The light source may be moved and locked in position anywhere around the disk. For simplicity, not all the degree numbers have been indicated in the diagram above. We reckon angles of incidence, refraction, etc., measured from the normal.

By swinging the light source around into the bottom hemisphere, one can set up the situation of light traveling from a medium of high index of refraction (plastic, n=1.54) to a medium of lower refractive index (air). In the lower hemisphere the incident ray is always normal to the semicircular plastic and so there is no confusing refraction at this first interface. As the angle of incidence is increased, one can see very nicely how the partial internal reflection increases in intensity to finally 100% when the critical angle is reached (about 42°) and continues with total internal reflection for larger angles of incidence.

Setting it up:

The optics disk is vertically supported by a 1/2" (1.27 cm) diameter aluminum rod and its wooden base and power supply 4 are an integral unit. It's best to set it on one of the low carts, off to the side, so that the view of the blackboard will not be blocked. With a long extension cord, it can be wheeled to center stage at show time and then pushed off into the wings when finished.


The light rays are very visible. This is ray tracing made simple; whether you use it qualitatively or quantitatively, it works well.

1 We have discarded the original (rather feeble) light source and replaced it with a lamp housing from the Klinger Blackboard Optics Kit (Klinger Educational Products part no. KO4101) which, in turn, we have modified to accept a 50 watt Halogen automobile lamp. The lamp operates off a 12 VAC transformer.
2 Manufactured by Griffin & George Limited, the metal disk measures 60 cm in diameter.
3 KEP part no. KO4107 measures 20 cm in diameter and is 5 cm thick. We have put black tape around the edges to make it more visible against the white background of the optics disk.
4 12 volt, 6 amp filament transformer

Optics DiskOptics Disk