This article introduces the topic of optical interference and phase.
The topic of the previous article was diffraction—the phenomenon that causes waves to bend around corners. Another behavior easily observed with water is the general ability of waves to interact with one another, creating complicated patterns on the surface. That mutual influence among waves is called “interference.” The phenomenon also occurs with light waves and is called optical interference.
When waves cross paths, they add together. The result of the addition depends on whether the waves are in “phase”— the relative difference between the peaks of two waves. Phase is measured like an angle, from 0° to 360°.1 As the phase between two waves increases, the peaks eventually realign so that the waves are in phase again, just like how something winds up facing the original direction after making a 360° turn.
There are two extreme cases of optical interference—when the peaks of one wave perfectly align with the peaks of another wave and when the peaks of one wave align with the “troughs” of another wave. In the first case, the phase difference is 0°. The two waves reinforce each other, creating a wave that has twice the amplitude. That reinforcement is called constructive interference. By contrast, in the second case, the phase difference is 180°. The waves cancel each other out. That cancellation is called destructive interference.
Why does optical interference matter?
Mostly we don’t see the effects of interference. Daylight has a broad spectrum of wavelengths, and the waves are traveling in different directions. Interference does occur among the waves, but the waves are rarely in or out of phase for very long or for very far.2 There are instances, however, where interference is visible. Or rather NOT visible. The next article will explain a phenomenon most of us are familiar with—anti-reflection coatings on eyeglasses.
Collecting this information, we can create some “very least you need to know” definitions for optical interference.
Phase is the relative difference between the peaks of two waves. In instances where the peaks overlap, the waves are said to be “in phase.” Phase is measured as an angle from 0° to 360°.
Interference is the tendency of waves to interact with themselves, enhancing or canceling out one another depending on the phase difference. When applied to light it is known as optical interference.
Constructive Interference occurs when light waves interact to create a combined wave with a larger amplitude than the original waves.
Destructive Interference occurs when light waves interact to create a combined wave with a smaller amplitude than the original waves.
Total Destructive interference occurs when waves completely cancel one another out.
1. Angles can also be measured in radians, from 0 to 2π.↩
2. For the record, this is called “temporal coherence” and “spatial coherence,” but that falls outside of the domain of “the very least you need to know about optics,” at least for now.↩