Teaching Notes

Encourage the students to think about the action of the grating. Ask them to contrast the effect on white light with the effect on red light. You might set up a prism for comparison. Make a spectrum of sunlight and ask the students to compare the effect of the prism and the grating on white light (both produce a continuous spectrum). Shine the laser into the prism so the students can see that the prism only bends the light (it does not change the color). When the students calculate the wavelength of light in Step 7, they must measure the angle between the line of sight to the spectrum lamp and the line of sight to a bright line. Look at the drawing on page 325. The distance x is the distance from the partner’s finger to the spectrum tube. The distance L is the distance from the spectroscope to the partner’s finger.

If possible, group the students into pairs for the spectroscope activity.

SAFETY PRECAUTION: Warn students not to look into the laser beam. Have each group build a backstop of cardboard, propped up with a stack of books, to absorb the beam.

Interference is a difficult topic. Having already learned that “light goes in straight lines,” they must confront a phenomena in which it does not. You can help them by asking if light always goes in straight lines. After they answer, ask for examples in which light does and does not go in straight lines (the laser beam itself, and the beam going through the diffraction grating). Students may suggest refraction, but you can reply that refraction involves a change in medium and is therefore a separate effect.