Waves

A transverse wave causes the particles in the medium (the substance that the wave travels through) to vibrate at right angles to the direction of the wave’s motion. A cork in water and the coils of a spring are examples of this. They move up and down as the wave passes.

Examples

• Waving/moving the coils of a spring will make the coils move up and down as the wave passes.
• They then return to their original position after the wave has passed.
• This shows that a wave moves without transferring matter because the spring coils do not travel along with the wave.
• A cork bobbing on the water shows how a transverse wave works.

Longitudinal waves

A longitudinal wave causes the medium’s particles to vibrate in the same direction as the wave’s motion. Examples of longitudinal waves are sound waves and pushing a spring in and out.

Wave speed

We can calculate wave speed by multiplying the frequency of the wave by its wavelength. The unit for wave speed is metres per second (m/s).

Frequency

The frequency of a wave is the number of times a point on the wave oscillates per second.

Wavefronts

• Water waves can be set up in a Ripple Tank, where a rod at one end of a tank of water creates a series of ripples.
• A bright light shone through the water onto a sheet of paper shows the ripples on the water very clearly as a series of parallel lines travelling along with constant speed.
• These parallel lines are the peaks of the ripples on the water. We call them wavefronts.

Sound is a longitudinal wave. Because of this, it can be:

• Absorbed
• Reflected
• Transmitted
• Refracted

• Sound waves are longitudinal waves. They can travel through solids by causing vibrations in the solid.
• Sound is produced by the vibration of particles in a medium (the substance that waves travel through).
• The vibrations mean that sound waves travel in a series of compressions (where the medium is squashed together) and rarefactions (where the medium is stretched apart).

• Two people stand a measured distance from a tall vertical wall. This distance should ideally be about 100m.
• The first person bangs two wooden blocks together to make a sharp sound, and repeats this every time the echo is heard.
• Starting counting from zero, the second person uses a stopwatch (timer) to measure the time taken for a number of claps – 50 or 100.
• In the time between two successive claps, the sound travels to the wall and back.
• The speed of sound can be calculated from the following relationship:
  • speed of sound = distance to wall × 2 × number of claps (N) ÷ time taken for N claps.

• The electromagnetic spectrum can be split into seven types of wave.
• In order of highest frequency to lowest the seven types are: gamma, X-ray, ultraviolet (UV), visible, infrared, microwave, and radio waves.
• As you move from gamma rays to radio waves, the wavelengths increase and the frequencies decrease.
• Gamma rays have the shortest wavelength and the highest frequency.
• Radio waves have the longest wavelength and the lowest frequency.
• Electromagnetic waves transfer energy from the source of the wave to an absorber of the wave.
• Gamma rays also carry the most amount of energy than any other wave in the electromagnetic spectrum.
• Wave energy increases with frequency.
• Wave energy decreases with wavelength.

Gamma rays

• Gamma rays are used for medical imaging and therapy, astronomy, sterilisation and food preservation.
• Gamma rays are extremely penetrating and damaging to living tissues and cells.

UV light

• Ultraviolet light is used in medical and forensic photography, air purification, disinfection and medical therapy.
• Ultraviolet light can also be used to detect fake bank notes.
• Exposure to too much ultraviolet light can cause skin burns, skin cancer and cataract formations in the eye.

X-rays

• Low-energy X-rays are used for medical and industrial imaging.
• High-energy X-rays are used to treat cancer.
• X-rays are also used for security purposes to detect weapons in airports (and other places).
• X-rays are highly ionising (can damage body cells), even in low doses.

Infra-red radiation

• Infra-red radiation is used in TV controls.
• Infra-red can also be used for security purposes, such as in intruder alarms by detecting body heat.
• Infra-red radiation can cause serious skin burns if emitted from high-intensity sources.

Microwaves

• We should always reduce any exposure to microwaves.
• As with X-rays, always have some sort of shielding between the source of microwaves and living tissue.
• An example of this is the protective shielding on microwave ovens.
• Microwaves are used for the purpose of satellite communications (transmitting signals between stations on Earth and satellites).
• Microwaves are also used to transmit signal from a nearby phone mast (transmitter) to a mobile phone.
• Microwaves are absorbed by water, heating up the water in the process. This makes microwaves useful for cooking food because food contains lots of water.
• Because humans are largely made up of water, exposure to microwaves could have a harmful effect.

Radio waves

• Radio waves are used for radio and TV communications.
• At high intensities, radio waves can cause internal heating of living tissue with potentially harmful effects.

Production of electromagnetic waves

• Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range.
• Gamma rays originate from changes in the nucleus of an atom.

• A wave’s speed can change when moving from one medium to another.
• If the wave crosses to the new medium at an angle (not 90 degrees), the change in the wave’s speed will cause the direction of the wave’s motion to change and the wave will appear to bend.
• This is called refraction.
• When waves meet some materials, the energy is absorbed by the material.
• For example, when light falls on a matt black surface, most of the energy is absorbed.
• Waves carry on travelling through a new material.
• This often leads to refraction.
• Reflection happens when a wave hits a flat surface (plane) and bounces off.

Angle of refraction

The angle of refraction is the angle between the refracted light ray and the normal. When light enters a more optically dense medium, it is refracted closer to the normal and the angle of refraction is smaller.