What does the double slit experiment tell us about light?

Imagine an opaque barrier (no light can get through) with two parallel slits in it, onto which we shine a coherent light source (light of only one wavelength and in phase - a laser, for example, can produce this). There is a screen behind the barrier and slits.

 

What will the pattern on the screen look like?

 

If light was made up of tiny particles, like spraying tiny pellets towards the slits, we would expect that the particles which went towards a slit would fly straight through and land on the screen, forming two bright lines on the screen - one behind each slit. The photoelectric effect tells us that light acts with a particle nature, so this is what we should expect.

 

However this is not what we see with light!

 

With light, the pattern that emerges on the screen is a series of bright and dark bands, alternating across the screen (not just two bright bands). This is known as an interference pattern. This pattern is exactly what we would see if we had passed a water wave through the slits - strips where the wave was high and strips where the water was still. This results from the interference of waves.

This demonstrates that light has a wavelike nature, which seems to be contrary to what the photoelectric effect demonstrated. 

 

So which one is wrong?

 

Neither!

 

Amazingly, light sometimes acts like a wave and sometimes like particles - sometimes we have to think of it like waves to explain what happens and sometimes we have to think of it as made of particles.

 

This unusual property is called the wave-particle duality of light.