Transverse and Longitudinal Waves

Transverse and Longitudinal Waves

A wave is made up of oscillations that travel from one place to another. These oscillations can either be perpendicular to the wave velocity, or parallel to it. We shall familiarise ourselves with a few examples, and then look at the directions of the oscillations.

If you shake a rope at one end, you make a wave pattern travel along the rope. If you shout, your sound is a pressure wave that travels through the air. If you switch on the light, the brightness from the lamp is an oscillating electromagnetic (electric and magnetic) field.

The thing that oscillates are very different in these examples. In the rope, it is the solid bits in the rope that move up and down. In sound, it is the air molecules that come together and move apart. In light, the electric and magnetic fields change strength and directions repeatedly.

When we talk about the wave, we are interested in the oscillations in displacements in the rope, the oscillations in pressures in the sound, and the oscillations in the electromagnetic fields in the light. We are interested in how the oscillations travel from one place to another.

Each point in a rope wave moves up and down. The different points along the rope reach the top at different times. Just as one point starts to move down, a point further on moves to the top. This makes it look as if the peak of the wave is moving along the rope. The same idea carries over to sound and light. The pressure at a point in air increases and decreases as a sound wave travels over it. Just as the pressure at this point starts to go down, the pressure in front reaches the top. This makes it look like the pressure wave is travelling outwards. Likewise for the electric and magnetic fields in light wave.

Waves in which oscillations are perpendicular to the wave velocity are called transverse waves. Examples are rope and light waves. Waves in which oscillations are parallel to the wave velocity are called longitudinal waves. Examples are spring and sound waves.

The rope wave is obvious, since we can see it with our eyes. If you shake a rope, the wave travel horizontally along the rope, while each point on the rope moves up and down vertically. It is sound and light waves that are less easy to understand, because we cannot see them.

Sound waves are generated by vibrating objects. When you sing, the voice box in your throat vibrates. It pushes at the air molecules repeatedly. Air molecules are tiny particles separated by empty space, or vacuum. When some get pushed in one direction, they move outwards and hit other molecules. After the collision, those other molecules move further out and hit more molecules. The original molecules would bounce back. The collision also produces the high pressure region, which travels outwards as molecules further away collide. So the high pressure region moves in the direction of the oscillation of molecules. Sound wave is thus longitudinal.

Light wave is tricky. It depends on two properties of electricity and magnetism. One is that a current through a wire generates a magnetic field around it. This happens in electromagnets. The other is that moving a magnet into a coil of wire induces a voltage in the wire. This is called Faraday's law, and is used to generate electricity. If there is current and voltage in the wires, there are electric fields in the wires also. From this, we learn that an electric field in a wire can generate a magnetic field around it. We also learn that a magnetic field can generate an electric field around it.

Notice that one field can generate the other field, and the new field would be around the old field. So the new field would not be along the same direction as the old field, but would be beside it. This is exactly what happens in light wave. Since the new field is always beside the old field, it just means that the fields are moving outward in a direction that perpendicular to the fields themselves. That is to say, light is a transverse wave.

How do we know that light wave is electromagnetic? One evidence is that it has the same speed as radio wave, which is generated by oscillating currents, that is electric fields, in an antenna.

The Physics Notes