# The Double Slit Experiment and the Wave Nature of Light

From the Lecture Series: The Evidence for Modern Physics: How We Know What We Know

## In quantum mechanics, when multiple waves exist in the same place at the same time, they interact. The term scientists use is interfere. This interference can be either positive or negative. Applying the same, if light is a wave, we should be able to observe something similar. But, do we? Does it conclusively settle the question whether light is a particle or a wave?

### Adding and Canceling of Waves

We can clearly observe this interference at the beach. Waves come in toward the shore and sometimes cross. When the peak of two waves cross, the result is that the water level is lifted unusually high. When a peak of one wave crosses the trough of another, the two cancel each other out and the result is that the level of water doesn’t change at all. Interestingly, if waves hit each other randomly, with neither peaks hitting peaks, nor peaks hitting troughs, something in between happens, with the two waves somewhat enhancing each other or cancelling each other out.

This adding and cancelling thing works for all waves. But does it work for light, too?

### Thomas Young’s Double Slit Experiments

In 1801, British scientist Thomas Young set up an experiment that, once and for all, showed that light was a wave. A modern version of Young’s experiment can be performed by taking a laser and pointing it at two very narrow slits cut in an aluminum foil. To work, the two slits have to be very close to one another. What we then see is that, coming out of the two slits, is not one, but many beams of light. On a distant wall, a series of bright dots are visible, separated by dark spaces.

This is exactly the same phenomenon as the interference of water waves. The bright spots are caused by the waves from one slit, enhancing the waves from the other slit, and the dark spots are caused when the waves cancel each other out. By covering up one of the slits, it can be further proved that it is caused due to the interference of waves.

Young’s original paper was published in 1803. He didn’t have lasers or even the ability to easily make such slits. He even had to work really had to show that light was a wave. But prove it, he did. Thomas Young’s double slit experiments and others that followed showed quite clearly that light was a wave. The arguments of the 18th century that, whether light was a particle or a wave, were over. Or were they?

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### Heinrich Hertz

In the mid-1800s, scientists invented an electric eye whereby, by shining light on certain material, they could cause electricity to flow in a circuit. The same kinds of techniques nowadays are used for cameras and other electronics to detect how bright the environment is.

But, it was in 1887 that the plot thickened. German physicist, Heinrich Hertz, did a series of very interesting experiments. He basically took two metal electrodes, separated by a small distance, and placed both of them in a vacuum surrounded by a glass container. He then charged the electrodes up with a very high voltage and as expected, nothing happened. But things changed when he shined a light on the electrodes. By doing so, he could cause a spark to occur. However, when he started changing both—the color and brightness of the light—a mystery unfolded.

When Hertz shined blue light, he got a spark just as with purple and green. However, yellow didn’t. Neither did orange or red. Hence, bluish colors caused a spark, but reddish didn’t.

Moreover, the brightness didn’t matter, or at least it didn’t matter in odd ways. If he increased the brightness of the bluish colors, it caused much bigger sparks and caused more electricity to flow. Hertz knew that because, by this time, he had added some extra equipment to measure the current flow. But when he took the red, yellow, and orange light and absolutely maxed out how bright they were, he got nothing. No sparks and no electricity flow.

### Brightness of Light Proportional to Its Amplitude

This clearly didn’t make sense if light was a wave. For waves, the amount of energy carried by the wave is determined by the amplitude of the wave, whereby, amplitude refers to the height. This makes complete sense for water waves. If we’re standing in the lake, little ripples have nearly no effect as they pass over us. Three or four feet high big waves, can knock us off our feet. And, of course 30 or 40 feet tall tidal waves are completely deadly and can destroy buildings and scour clean a shoreline. For water waves, like many things, size matters.

Additionally, the brightness of a light is also proportional to its amplitude under the wave theory of light. A dim blue beam could well have much less energy than a super bright red beam. Yet, with Hertz’s experiments, there was simply nothing he could do to make a spark occur with red light.

### Inconsistent with the Wave Model of Light

Considering that this was in 1887, the electron hadn’t been discovered yet. The discovery of radioactivity was about to occur and, in 1897, physicist J. J. Thomson discovered the electron. Once the electron had been discovered, it was possible to look at Hertz’s experiments differently.

A spark was caused when electrons were knocked out of atoms. And, under that explanatory framework, it seemed that red, orange, and yellow light simply didn’t have the power to knock electrons out of atoms, while the bluer colors had no problem although nobody knew why. However, this was a bit of an antithesis as this observation was completely and totally inconsistent with the wave model of light.

### Common Questions about the Double Slit Experiment and the Wave Nature of Light

Q: What did Thomas Young’s experiment set up to prove?

Thomas Young set up an experiment that, once and for all, showed that light was a wave.

Q: When did scientists invent an electric eye?

In the mid-1800s, scientists invented an electric eye whereby, by shining light on certain material, they could cause electricity to flow in a circuit.

Q: What determines the amount of energy carried by a wave?

For waves, the amount of energy carried by the wave is determined by the amplitude of the wave, whereby, amplitude refers to the height.