Ray Diagrams
Light, Sound and Waves

Fine beams and shadows

Teaching Guidance for 11-14 Supporting Physics Teaching

Sources of fine beams

The devices used to cast narrow beams of light across bench tops are often referred to as ray boxes. That cannot be helpful, of course, since rays are not physical, but conceptual. You will not find a ray on the bench top unless you have constructed a diagram using one.

These devices are often used to produce two-dimensional physical depictions of ray diagrams on the bench top. If you look down on the pattern the light beams form on the bench top, then what you'll see closely resembles what is drawn to account for the action of the optical system (lens, mirror, or prism). As such, these are very useful devices to trace beams of light through two dimensional systems (the specialised lenses, prisms, and glass blocks that you'll find in laboratories).

If you think about the geometry for long enough you'll realise that the beams that these produce are not strictly two dimensional, as the light that is reflected from the bench farther away from the source must have started off higher up, farther from the surface of the bench. So if the slots in the front of the beam were wiggly, the resulting beams across the bench top – would be also be wiggly and without any optical devices at all.

So do use them, but we'd suggest using them in conjunction with real three-dimensional optical systems, and making explicit that they're an aid to linking the ray diagrams to useful optical devices (such as the real three-dimensional lenses, in your eyes, that make real images).

So not ray boxes. Instead you might introduce them like this:

Teacher Tip: Here's a special light source, that helps us focus on just some of the beams passing through a lens [adapt as appropriate], helping to show you how the diagrams we draw with rays explain the action of a lens.

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