If you pay attention, you can see some pretty cool stuff that you might otherwise miss. Have you really looked at a soap bubble? Notice how you can see a bunch of different colors? What about that tiny drop of gasoline in a puddle at the gas station—see the rainbow of colors? Oh, there is that weird car too. It appears to have paint that changes colors. These optical effects are all classified as "thin film interference." You need several physics ideas to really appreciate this optical phenomenon—so let's get to it.

Light Is a Wave

Everything we see is due to visible light, the very narrow spectrum of electromagnetic waves that our eyes can detect. Since it's difficult to visualize the wave properties of light, however, let's consider another wave—a wave on a string. Imagine a string on the ground. If I continually shake one end, I will create a repeating disturbance that travels down the length of the string. For this wave, there are three important properties: speed, wavelength, and frequency.

Rhett Allain

If you watched one of the disturbance peaks move along the string, its velocity is the wave speed (v). A different way of looking at it is to count the number of peaks that pass a fixed spot in a certain amount of time; that's the frequency (f). And if you took a snapshot of the string and measured the distance from one peak or trough to the next, that's the wavelength (λ). These three variables are not completely independent. The product of the wavelength and the frequency will give you the wave speed.

The speed of light is set at about 3 x 108 meters per second. If it's visible light, it has a very tiny wavelength with a value between about 380 nanometers and 740 nanometers, where a nanometer is 10-9 meters. Yes, that is super small. Our human eyes interpret different wavelengths as different colors. A wavelength of 380 to 450 nm would appear violet and the longer wavelengths of 630 to 740 nm would be red.

Interference of Waves

Let's go back to the wave on a string. What happens when you have two different waves on the same string? Imagine that you make a single pulse on the string and it travels from left to right. At the same time, you make another wave pulse on the same string—but from the other side. These two pulses will move towards each other, but they don't collide. When they meet, these two waves will simply add together to make a single bigger pulse. After that, they will just continue along and pass through each other.