Whether you’re talking about light, sound, strings, or just jumping into a pool, it’s all the same concept here. Waves are a representation of the movement of particles while interference is the way in which nearby waves interact with each other. Before we can fully delve into the interactions, we have to first define the parts of a wave.
Waves move along straight paths from the energy source that supplied them. This energy source can be a star emitting light, a rock falling into a pool or you pushing air out of your lungs to speak. Each wave oscillates between two points that we call crest and trough. Depending on your orientation, we usually call the crests the highest point and the troughs the lowest point. We usually represent waves on graphs with positive crests, negative troughs and zero right in the middle. The distance between the crest and zero is called the amplitude and it is sometimes a good measure of how much energy was used to generate the wave. This is the case with waves that move through matter: like sound waves, ocean waves, and seismic waves in the ground.
Waves are also cyclical, meaning they repeat themselves. The length between repetitions is called the wavelength and the time it takes for a wave to repeat is called the frequency. Wavelength is a measure of distance and frequency is a measure of inverse time, so combining them gives you the speed (distance per time) of the wave. Wave speeds usually stay constant depending on what the wave is traveling through. With constant speed, as the wavelength increases the frequency decreases, and vice versa. When moving through air, your brain picks up differences in frequency as pitch. Okay, that was a fast crash course in waves!
Now, on to interference! When two waves interact, their amplitudes will add together to form a composite wave. If the crest of one wave overlaps with the crest of another, there will be an overall increase in amplitude. If the crest of one wave overlaps the trough of another, there will be a decrease in amplitude. When there is an increase, we call that constructive interference, and conversely, a decrease is called destructive interference.
Not all waves have the same wavelength, frequency and amplitude. This means composite waves will not always be complete constructive or destructive interference. Composite waves take on strange forms; however, regardless of the strangeness the composite wave will repeat just like each of the waves making it up. That is, until another wave is added to the mix.
Listen for the interference in your music, throw a few pebbles into a pond and watch the interference of the ripples. It’s all around you, you only have to take the time to see it.