Thomas Young conducted his famous double slit experiment to show that light is a wave.
He used light from a single source and allowed it to fall on a screen with two very small holes or slits. This is why we call it the double slit experiment. He placed a second screen in front the double slit and looked at the light that fell on it. What he saw was a series of bright and dark bands of light, almost like dots, instead of two single bands of light.
If light was a stream of particles, they should travel straight through the slits and fall on the screen producing two straight lines.
But this did not happen. Instead, there was a series of dots produced on the viewing screen. This suggested that some sort of interaction was taking place with the light.
If light was a wave, then the pattern set up on the viewing screen could easily be explained by interference. Interference is a phenomena displayed by all waves.
The wavefront of all waves consist of crests and troughs in a series. When it passes through a small slit, the wavefront spreads out wider. This is called diffraction.
Each slit on the double slit screen causes diffraction in the light as it passes through it.
The crests from each diffracted wave interact with each other, adding together to create a bright band. This is constructive interference.
When the crests from one wave interacts with a trough from the other wave, it cancels out to create a dark band. This is destructive interference.
The result is a series of light and dark bands on the screen in front the double slit.
The image below shows an interference pattern set up by a light bulb after passing a double slit. Find all the points of constructive interference along a straight line and trace it all the way to the screen. There is a bright band at this point on the screen. (The bright band in this image is the white area.) Next, find the all the points of destructive interference along a straight line and trace it to the screen. There is a dark band at this point (yellow light in this image).
Images taken from