Non-Twinkling Planet

Purpose

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

Equipment

Laser cart with beam expander and hot plate.

Setup Time

5 min.

Images

e2-48

Description

A beam expander enlarges the laser beam to about one inch diameter on a distant wall or screen. When a heater is placed under the beam, convection currents are shadowed in the large spot of laser light. However, because the spot is large it does not move, or "twinkle." This is contrasted with the light directly 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.