How does light exhibit traits of a wave?
The wave theory of light was proven in 1801 by English physicist Thomas Young, who designed and conducted the famous double-slit experiment.
The experiment was fairly simple and required a light source, a thin card with two holes cut side by side, and a screen.
The expected result of the experiment was the presence of two bright spots on the screen, corresponding to the two slits made in the thin card. However, the observed result showed something very different.
Instead of two bright spots, a bar code pattern of bright bands and dark bands was visible on the screen; this was in agreement with the idea that light traveled in the form of waves.
Each of the two slits created a separate wave front, where interference between the waves occured. The bright bands were formed on the screen when two wave crests overlapped and added together (constructive interference), while the dark bands were formed when crests and troughs overlapped and cancelled each other out (destructive interference).