PURPOSE: To illustrate why a planet does not "twinkle" like a star.

DESCRIPTION: A beam expander enlarges the laser beam to a few inches diameter on a distant wall or screen (photograph at right). When a heater is placed under the beam, convection currents are readily visible in the large spot of laser light. However, because the spot is large it does not move, or "twinkle." This is contrasted with the direct light from the laser, which moves because of the convection currents from the heater. A star twinkles because it is so small, but because of the finite size of planets as viewed from the earth, planets do not twinkle.

SUGGESTIONS:

REFERENCES: (PIRA unknown.) See also demonstration E2-47: TWINKLING STAR.

EQUIPMENT: Laser cart with beam expander and hot plate.

SETUP TIME: 5 min.