Ray streaks and ray diagrams: some cautions
Teaching Guidance for 14-16
The ray box is a useful and convenient piece of apparatus for demonstrating optical phenomena, but be aware of its limitations. When using ray boxes, you need to make sure that students do not make invalid assumptions. Take care to bring out the correct physics. Here are some suggestions.
The pattern of light emerging from the comb (or slits) in front of a ray box (or simple lamp) is often treated as if it consists of discrete rays, but it does not. What you see on the paper fixed to the bench are streaks of light, each emanating from a range of points along the extended object of the lamp filament. A succession of beams constitutes a line across the surface on the bench. This is more correctly described as a
shadow of the comb (or slits) cast onto the paper. (A point source too would produce ray streaks, though the intensity of light is likely to be reduced.) Note that the height of the lamp filament affects the length of these streaks.
If you make home-made slits which, deliberately, are not perfectly parallel or straight, they can produce wavy
rays. The waviness of those
rays might help students think about what is said in the previous paragraph: it is all a matter of shadows.
Be ready, therefore, to clarify any misunderstanding that might take place.
Misconceptions can also arise from the accepted practice of using only two or three ray streaks in experiments, or rays in corresponding ray diagrams.
The image of an extended object (typically a light source) will be formed from a flux of light passing through the optical system, not just two or three rays. The lens focuses
cones of light. Limiting the explanation to two or three rays makes drawing easy, and in many cases correctly predicts the outcome. But without cautions from the teacher, this can mislead and confuse students. For example, many students think that a small opaque disc placed in front of a lens will produce a
hole in the image, whereas in practice it simply reduces its brightness. The Newtonian reflecting telescope is a good illustration of an obstruction doing this.
For this reason it is often better to use a multiple slit (or often 5-slit) shutter in front of the ray box rather than the 3-slit shutter normally provided.
This Guidance note was inspired by an article in the journal Physics Education by Prof Laurence Viennot, University Denis Diderot, Paris. [ Physics Education Vol 41 (2006}, 400-408]