What is a good experiment that shows light traveling in straight line until it hits a new medium?

A classic experiment that demonstrates how light travels in a straight line until it encounters a new medium is the experiment using a glass prism. Here's how to conduct this experiment:

Materials needed:
1. Glass prism (usually made of glass with a triangular shape)
2. Light source (a flashlight or laser pointer)
3. White screen or a blank wall

Procedure:
1. Set up the experiment in a darkened room to minimize outside light interference.
2. Place the glass prism on a table or any flat surface. The wider base of the prism should be facing upward.
3. Position the light source in a way that it shines a beam of light directly onto the glass prism.
4. Position the screen or wall on the other side of the prism, a few feet away, to capture the light that is refracted by the prism.

Observation:
1. Turn on the light source and observe the beam of light passing through the prism.
2. Notice that as the light enters the prism, it bends or refracts due to the change in the medium.
3. Observe the rainbow-like spectrum of colors produced as the different wavelengths of light refract at different angles.

Explanation:
This experiment demonstrates how light waves travel in a straight line until they encounter a different medium (such as air to glass). When light passes through the prism, its speed changes, causing the direction of the light to bend. This bending is called refraction. The phenomenon of refraction causes the different colors of light to separate, resulting in the visible spectrum. The straight path of the light beam, which then changes direction when it enters the glass medium is visible evidence of how light travels.