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# Types of wave

## Progressive waves

A wave which travels through space is called a progressive (or travelling) wave (e.g. light waves, ocean waves).

• A progressive wave causes a net transfer of energy between two points in space.

• This is because each oscillation has a fixed total energy which is transferred as the wave moves in space.

An ocean wave which collides with a seawall transfers the energy of the water particles to the wall.

• There is no net transfer of mass by a progressive mechanical wave as there is no permanent displacement of the particles of the wave.

A standing wave (such as a wave on a guitar string) does not move through space. It vibrates in place. Standing waves are not progressive waves.

A progressive wave travelling from left to right causes the oscillation of a particle at a particular position.

## Mechanical waves

A mechanical wave is a series of oscillations in a physical medium such as air or water.

Sound waves, ocean waves and earth waves (in an earthquake) are mechanical waves

Mechanical waves require the interaction of the particles of a physical medium to transfer energy. They cannot travel through a vacuum.

Sound does not travel through vacuum. An explosion in space will generate no sound.

Sound waves are mechanical waves.

## Electromagnetic waves

Electromagnetic waves are generated by oscillations in the electric and magnetic fields of charged particles. Unlike mechanical waves, electromagnetic waves do not require a medium. They can move in a vacuum.

Microwaves, x-rays, visible light and radio waves are all electromagnetic waves.

The speed at which electromagnetic waves move depends on the medium. In a vacuum, they move at the speed of light (approximately 300,000,000 m/s).

The variation of the electric ($\vecphy{E}$) field and the magnetic ($\vecphy{B}$) field of an electromagnetic wave moving from left to right.

## Transverse and longitudinal waves

Waves can be categorised as transverse or longitudinal, depending on the direction of vibration (oscillation) of the wave.

A transverse wave is a wave in which the direction of vibration of the wave elements is perpendicular to the direction of motion of the wave.

Ocean waves: the ocean level rises and falls (vertically) but the wave travels horizontally.

A longitudinal wave is a wave in which the direction of vibration is parallel to the direction of motion of the wave.

Sound waves: the air molecules vibrate in the same direction as the motion of the wave from a speaker.

View looking down on the table. Top: a transverse wave. Bottom: a a longitudinal wave.

## Waves on a rope

A rope that is swung upwards and downwards creates a series of upwards-and-downwards movements along the entire length of the rope. These are collectively called a wave.

The energy that is given to one end of the rope by moving it upwards and downwards is transferred through the length of the rope to the other end.

However, each section of the rope still has the same mass (no mass was transferred to the other end along with the energy).

Two girls holding a rope. If the rope is swung with greater energy, a larger number of upwards-and-downwards repetitions are generated.

## Waves in a ripple tank

The behaviour of waves is often studied using a ripple tank.

A ripple tank is a glass container filled with water. To create different types of water waves, a vibrating dipper or bar is placed within the tank.

These create ripples in the water. Each ripple of a water wave in a ripple tank is a wavefront.

A vibrating dipper creates circular wavefronts in the ripple tank while the vibrating bar creates linear wavefronts.

Linear wavefronts could be used to model a straight beam of light while circular wavefronts could be used to model light spreading out from a lamp.

A vibrating dipper in a ripple tank creates circular wavefronts. The wavefronts spread out in all directions away from the source of the disturbance.

## Waves on a slinky spring

To show examples of different waves, a "slinky" spring can be fixed at one end of a horizontal table.

A slinky spring (a type of spring which is long and easily stretched).

Moving the free end of the spring back-and-forth in a direction perpendicular to its length produces transverse waves along the spring.

Moving the free end of the spring back-and-forth in a direction parallel to its length produces longitudinal waves along the spring.

View looking down on the table. Top: a slinky being used to make a transverse wave. Bottom: a slinky being used to make a longitudinal wave.