What is a Wave?
Tossing a pebble into a pond creates ripples that travel from where the pebble landed to the edge of the pond. The ripples are also called waves. Another way to make waves is with a long rope. If you swing your arm up and down while holding the rope (as in Figure 2.1), you can see waves start from your hand and disappear off the end; moving the rope faster makes more waves.
Waves have three important properties: frequency, amplitude, and wavelength.
Frequency
This is the number of waves that happen over time. Figure 2.2 compares two frequencies:
Figure 2.2. Wave Frequency.
Amplitude
This is the height of a wave. Figure 2.3 compares two amplitudes:
Figure 2.3. Wave Amplitude.
Wavelength
This is the distance between the top of one wave to the top of the next. Figure 2.4 compares two wavelengths:
Figure 2.4. Wave Length.
Experiment
You can play with frequency and amplitude using your voice. When you talk, your throat vibrates which causes waves of sound to travel through the air around you. (This is just like how waves travel across the pond, except instead of the wave travelling across water, it goes through air.) Try this:
- Place two fingers against your throat.
- Start humming a single note.
- Change the pitch of your hums (from ohhhh to eeeee).
- Change how loud you hum.
You can feel the different vibrations: slower (lower, less frequent) waves, and faster (higher, more frequent) waves. The louder you hum, the larger the amplitude; likewise quiet hums have smaller amplitudes. A loud hum gives off more energy (and needs more energy), which causes a stronger vibration in your arm.
Interference
When two waves interact with one another at the same space and time, they create an interference pattern. Figure 2.5 shows two waves interacting. Where they meet, the regular pattern of circles is disrupted because waves have constructive and destructive behaviour.
Constructive Waves
When the top (crest) of one wave meets the crest of another, they make a higher wave. Similarly, when the bottom (trough) of a wave meets the trough of another, they make a deeper trough. Figure 2.6 shows how constructive waves can merge together.
Figure 2.6. Constructive Waves.
Destructive Waves
When the trough of one wave meets the crest of another, or vice-versa, they cancel each other out. That is, the troughs are made more shallow and the crests are lowered. If the crests are as tall as the troughs are as deep, then the waves can cancel each other out completely. Figure 2.7 shows how destructive waves can merge together.
Figure 2.7. Destructive Waves.
Waves and Light
Before 1801, scientists thought that light travelled through space as tiny particles. In 1801, Thomas Young elegantly demonstrated his double slit experiment. The experiment showed that light also behaves like waves, because it interferes with itself. The role light plays with other particles is crucial to understanding how quantum entanglement works. And those particles are found within the realm of the atom ...
