Now I've replaced the sodium lamp with a white light source and set the two arms of the interferometer to exactly the same length. Let me turn on the white light source and let's see what we see. So now I've saved you about 20 minutes in setting this whole thing up. Let's take a look. We see our fringes in there but now they're colored fringes. They have colored fringes on them because the white light source puts out multiple wavelengths of light. The two arms of the interferometer being the same length. Those wavelengths all interfere constructively or destructively together. But because they're different lengths you don't have to move that mirror very far before say the red wavelength is constructively interfering and the blue wavelength is destructively interfering so you see these color fringes on top of what you would expect to be just black and white lines. I'm going to reach in now and move the interferometer arm, the mirror on this arm, and we'll see what happens. And you see the colors become much more pastel in nature and eventually disappear entirely. You can imagine all these different wavelengths on top of each other and no two of them agree whether they're constructively or destructively interfering. Now if I rotate it back again the other way you see the fringes come back. We get to the position where we're at zero path length difference and then keep going and then they disappear on the other side. So the total range of motion is literally just a dozen wavelengths or so before the interference pattern disappears entirely. I'm just going to oscillate back and forth a little bit so you can see the pattern appearing and disappearing. You can actually count the individual wavelengths as they go by disappearing off the top or the bottom of the screen. So this is the elusive and can be difficult to find white light interference fringes.