Light as a Wave

What is light, really?

For centuries, scientists argued about what light is. Isaac Newton thought light was a stream of tiny particles. Christiaan Huygens argued it was a wave. Experiments eventually showed that light behaves like a wave, though, as we’ll see, the full story is even stranger.

The wave model

In the wave model of light, light is a transverse wave made of changing electric and magnetic fields. In the 1860s, Scottish physicist James Clerk Maxwell showed mathematically that such waves would travel at exactly the speed of light, and concluded that light is an electromagnetic wave.

Unlike sound, light doesn’t need a medium. It can travel through the vacuum of space.

Properties of light waves

Visible light has wavelengths from about 400 nm (violet) to 700 nm (red). A nanometre is one millionth of a millimetre.

speed = frequency × wavelength, so shorter wavelengths have higher frequencies. Violet light has about twice the frequency of red light.

Evidence that light is a wave

Interference: Young’s double-slit experiment

In 1801, English scientist Thomas Young shone light through two narrow slits onto a screen. Instead of two bright lines, he saw a pattern of many bright and dark bands.

This is interference: where crests from the two slits meet crests, they add to make bright bands (constructive interference). Where crests meet troughs, they cancel to make dark bands (destructive interference). Only waves can do this, so the double-slit experiment was strong evidence for the wave model.

You can see light interference in:

• the rainbow colours on soap bubbles
• the shimmer of oil on a wet road
• the colours on the back of a CD or DVD

Diffraction

Light spreads slightly when passing through very small openings or around sharp edges. Look at a streetlight through a fine cloth or squint through your eyelashes, and you’ll see patterns caused by diffraction.

Refraction

Light bends when it moves from one material into another because its speed changes. The wave model explains this neatly: light slows down in glass and water.

Different colours slow down by slightly different amounts, so a prism separates white light into a spectrum. Rainbows form the same way in raindrops.

Polarization

Light waves normally vibrate in all directions perpendicular to their travel. A polarizing filter lets through only waves vibrating in one direction. Polarization only works for transverse waves, which is further evidence that light is a transverse wave.

Polarized sunglasses block glare reflected from water, snow and roads. 3D movie glasses also use polarization.

Light as particles too

In 1905, Albert Einstein showed that light also behaves like a stream of tiny packets of energy called photons. This explained the photoelectric effect, where light knocks electrons out of metal. Today, physicists accept wave–particle duality: light acts like a wave when it travels and spreads, and like particles when it interacts with matter.

Common mistakes

• Thinking light needs a medium.
• Thinking colour depends on brightness. Colour depends on wavelength; brightness depends on amplitude.
• Thinking the wave model is "wrong." It’s correct for many behaviours; it’s just not the whole story.

Try it yourself

1. Blow soap bubbles in sunlight and describe the colours. Which wave behaviour causes them?
2. Look at a light through two polarizing sunglasses lenses. Rotate one. What happens?
3. Shine a laser pointer (with an adult, never toward eyes) through a tiny gap between two razor blades held together. Describe the pattern.

Quick recap

• Light is an electromagnetic wave that can travel through a vacuum.
• Wavelength and frequency determine colour; amplitude determines brightness.
• Interference, diffraction, refraction and polarization show light behaves as a wave.
• Young’s double-slit experiment was key evidence.
• Light also behaves as particles called photons: wave–particle duality.