We all know what water waves are. Physicists also say that light is a wave, and even matter sometimes behave like a wave. In our Wavelabor, the slider controls the wavelength and the phase speed. You can switch between two light sources or use them together.
If you use both light sources together, you get something which is called interference. Please notice that some spots of the picture are not changing. You can see this best if you use the “rainbow” color theme: there are some green lines that stay green. But because a wave only exist if it changes, in these spots the wave disappears. So, sometimes light and light leads to darkness!