Why does light bend when it travels through different materials?
In short: When light moves from air into glass, it doesn't just change speed; it changes direction. This phenomenon, known as refraction, occurs because the phase velocity of light shifts as it encounters new media. This simple shift is responsible for everything from the rainbows in the sky to the way a stick looks bent in water.
At its core, refraction is driven by a change in speed. When an electromagnetic wave, such as light, crosses the boundary between two transparent media, its velocity changes. This change in speed causes the wave to bend, provided it enters the new medium at an angle. For instance, light slows down when transitioning from air into glass. This change in velocity is not merely a simple slowing; it is the result of a complex interaction where the incoming electromagnetic wave causes electrons within the medium to oscillate. These oscillating electrons produce a secondary electromagnetic wave, and the resulting superposition of the original and secondary waves creates a new phase velocity that differs from the speed of light in a vacuum.
This mechanism allows us to manipulate light for various purposes. When white sunlight enters a glass prism, it undergoes refraction that separates its constituent wavelengths. Because each wavelength travels at a different speed within the glass, each bends at a unique angle. This dispersion causes the colors to emerge in a consistent, predictable sequence: red, orange, yellow, green, blue, indigo, and violet. This is the fundamental process that creates a rainbow.
Refraction is not limited to visible light. Sound waves are also subject to this effect. For example, the speed of sound is greater in warm air than in cold air. At night, when air near the surface of a lake is cooled, sound traveling upward can be refracted back down by the warmer layers of air above. This can result in voices or music being heard much farther across the water at night than during the daytime. Similarly, ocean waves refract as they approach a beach; the part of the wave further from the shore moves faster than the part closer to it, causing the wave to swing until it moves perpendicular to the shoreline.
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Sources
- Summary by Intellect · source: Refraction of light | Physics | Khan Academy
- Also: Refraction | Definition, Examples, & Facts | Britannica
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