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Types of waves
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Which type of wave looks like a coiled spring, with some sections pressed close together while others are stretched out?
Maria and Sofia are packing for a summer holiday at the beach. I can’t wait to soak my feet in the sea, and feel the waves. Waves are so much fun to swim in! You know, there are other types of waves, beside the waves in the sea. What?!
You just packed a slinky toy Sofia, pick it up and let’s do an experiment! Hold one end of the slinky each. Now, Maria, hold it tightly, and Sofia, start moving your end of the toy up and down. Wow, it looks almost like a sea wave! When Sofia jerks the slinky, it moves upwards, and forms a U-shape.
Then that U shape seems to travel towards Maria like a water wave. But if you look closely at the slinky, the individual turns of the coil are not actually moving towards Maria. Instead, each of them only moves up and down, at right angles to the motion of the wave. We call this type of wave a transverse wave. Now, let’s try something slightly different.
Put the slinky on the table, but keep holding it at each end. Then, Maria, move your arm back and forth. Notice what happens to the slinky toy. There are bits of it that are closer together and bits that become more stretched out. This pattern repeats all the way to Sofia’s end of the toy.
You’ve just created another type of wave! But it looks nothing like a sea wave! Not all waves look like sea waves or transverse waves. This movement causes the toy to have areas of higher pressure — where the slinky looks more compressed, and areas of lower pressure — where the slinky looks more stretched out. Look at an individual turn of the coil!
It doesn’t move up and down like before. Instead, it moves in the same direction to the motion — backwards and forwards. This type of wave is a longitudinal wave. Sound is another example of a longitudinal wave. All waves are produced by a disturbance which causes energy to spread from the place of disturbance.
For example, when you drop a stone in the sea, it creates a disturbance at that point, which then spreads outwards as a wave. Or the wind can push the surface of the sea around in ways that create waves. But remember: while it might look like things are being pushed forward, backwards, up or down, that is actually not the case. Instead, things just move up or down like in a transverse wave, or back and forth like in a longitudinal wave. When the disturbance ends, they return to their starting position — like the coil turns on a slinky.
Some waves, like the ones Maria and Sofia just demonstrated, need something to travel through — a medium — such as liquid, gas or a solid. We call these mechanical waves. But there are also waves that don’t need a medium to travel through — they can travel through a vacuum. These waves are disturbances in electrical and magnetic fields — we call them electromagnetic waves. Light is an example of an electromagnetic wave, and that’s how light can travel from the Sun through vacuum to reach the Earth!
All electromagnetic waves are also transverse waves. Other electromagnetic waves are X-rays, often used in medicine, and microwaves, used in a kitchen microwave! Okay Sofia, let’s finish packing. I can’t wait for the sea waves. And the sun waves too!