To understand how interference fringes form on film, you need to know a little bit about light. Light is part of the electromagnetic spectrum -- it's made of high-frequency electrical and magnetic waves. These waves are fairly complex, but you can imagine them as similar to waves on water. They have peaks and troughs, and they travel in a straight line until they encounter an obstacle. Obstacles can absorb or reflect light, and most objects do some of both. Reflections from completely smooth surfaces are specular, or mirror-like, while reflections from rough surfaces are diffuse, or scattered.
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The wavelength of light is the distance from one peak of the wave to the next. This relates to the wave's frequency, or the number of waves that pass a point in a given period of time. The frequency of light determines its color and is measured in cycles per second, or Hertz (Hz). Colors at the red end of the spectrum have lower frequencies, while colors at the violet end of the spectrum have higher frequencies. Light's amplitude, or the height of the waves, corresponds to its intensity.
White light, like sunlight, contains all of the different frequencies of light traveling in all directions, including ones that are beyond the visible spectrum. Although this light allows you to see everything around you, it's relatively chaotic. It contains lots of different wavelengths traveling in lots of different directions. Even waves of the same wavelength can be in a different phase, or alignment between the peaks and troughs.
Laser light, on the other hand, is orderly. Lasers produce monochromatic light -- it has one wavelength and one color. The light that emerges from a laser is also coherent. All of the peaks and troughs of the waves are lined up, or in phase. The waves line up spatially, or across the wave of the beam, as well as temporally, or along the length of the beam. You can check out How Lasers Work to see precisely how a laser does this.
In the next section we'll look at light reflection and redundancy.