Ray box or lamp?

In many optics experiments, the apparatus we show is a free-standing lamp on a stand. This may be shielded by housing, which comes in both left and right configurations, to aid in bringing the lamps close. This has several advantages.

• It is easy for students to understand that the object is the glowing filament itself.
• Used with a multi-slit comb, this apparatus produces a broad fan of ray streaks, needed in some experiments.
• Two lamps can be brought side by side to create the top and bottom of an imaginary object. This enables a comparison of the positions of the two lamps in the image produced by a lens. With ray boxes it is difficult to get two lamps close enough together.
• The height of the lamp above the bench is adjustable. When used with slits, this enables the user to alter the length and brightness of the ray streaks it produces.

Ray boxes will produce good rays, but they tend to emphasize parallel incident rays. Some designs have a built-in lens, but this is not always wanted.

A colour filter, such as yellow or magenta, placed in front of the lamp will produce more exciting ray streaks than white rays. A colour filter also helps to identify what happens to light from the top and bottom of an object when the light then passes through a lens. You may be amazed at the difference in response from students.

NOTE: Whether you use a ray box or a simple lamp to produce ray streaks, it is essential that the filament is in the same plane as slits in the comb (i.e. is vertical).