Question

Explain how Thomas Young's double slit diffraction experiment gives direct evidence that an electron is both a particle and a wave?

Answer 1

Waves are classified as either mechanical or electromagnetic. Mechanical waves (like sound) require a medium to travel through. Electromagnetic waves (like light) do not require a medium. Instead they are created by accelerating electrical charges which create oscillating electric and magnetic fields that radiate outward. These fields are what make light and they travel best where there is nothing - a vacuum. That's how light can get from the Sun to the Earth.

Young's double-slit experiment showed that light exhibits interference which is a wave property. When a part of a wave with positive amplitude (size) meets a part of another wave with negative amplitude, they will cancel each other out as they pass through each other. This is known as destructive interference. The cancellation isn't permanent. As the waves move past each other, they will regain their original form. In another form of interference, two positive or negative waves may temporarily add to make an ever more powerful wave. This is called constructive interference.

Young's experiment took a narrow beam of light, and shined it through very close slits. When the light went through the slits, it spread out so that by the time it got to a screen, light from the left slit would overlap light from the right slit. Where the overlapping occurred, interference took place and alternating bands of light and dark were seen as constructive and destructive interference took place.

If light were made of particles and had no wave-like properties, the results of the experiment would have simply been brighter light where the light from the two sources met, and dimmer farther away.

Again, other experiments have shown that light can behave as a particle as well as wave, but if you test it for wave properties, it will show them.