In this demonstration, the large dot is a source that sends out radially expanding light waves. The color of the dot indicates the color of the light it emits while at rest, which corresponds to the light’s wavelength. Use the first slider to choose the wavelength of the emitted light and the second slider to choose the velocity of the light source (don’t forget to see what happens when the light source remains at rest!). Use the third slider to see how the system evolves in time. You can also press start to watch the evolution unfold. Notice that the light waves bunch up in the direction of motion but spread out in the direction opposite to the motion. Correspondingly, note that the light’s color shifts towards the blue end of the spectrum in the direction of motion as its wavelength becomes shorter and its frequency becomes higher (referred to as “blueshift”). Similarly, its color shifts towards the red end of the spectrum in the direction opposite to the motion as its wavelength becomes longer and its frequency becomes lower (referred to as “redshift”).