As the hot plate warms up, you’ll see the laser spot start to wiggle and dance, steered this way and that by convection currents in the air.
When air near the hot plate heats up, it becomes less dense. This density change causes the air to rise and mix with the surrounding cooler air in a process called convection. The optical properties of hot air are different from those of cool air. Light passes through hot air more quickly than it does through cool air. This slight change in speed causes the light to bend, or refract, as it passes through air of varying temperatures.
If you were viewing the laser spot from the vantage of the “eye” of the printed image, the laser spot would veer in and out of view, or twinkle.
This same twinkling occurs when you view a distant star. Located trillions upon trillions of miles away, a star appears to us as a single point of light, like the light from your laser. Starlight travels through the vacuum of space unimpeded and in a straight line until it enters Earth’s atmosphere. As it passes through regions of gas of varying temperature, the light refracts, bending this way and that in response to puffs of hot or cold air. You perceive this thermally driven jiggling as a flickering of the light—or the twinkling of the star.
Astronomers call this twinkling effect scintillation, and generally seek to avoid it when looking through telescopes, since it blurs and warps images. Many modern large observatories use adaptive optics to mitigate scintillation, relying on fast computers and adjustable reflecting surfaces to compensate for distortions caused by scintillation. But the best way to avoid distortions caused by our atmosphere is to leave Earth altogether: Orbiting telescopes, such as the Hubble Space Telescope, offer the best possible observational clarity.