Double Award - Science for GCSE

Chapter 30

The properties of waves

1.	Light and sound Light and sound are wave motions. Light consists of vibrating electromagnetic waves. Sound is produced when materials vibrate. When light rays and sound waves hit materials and surfaces, three things can happen. – The light and sound can be transmitted (e.g. light and sound through air). – The light and sound can be absorbed (e.g. light by black surfaces, sound by soft furnishings in a room). – The light and sound can be reflected (e.g. light by mirrors, sound by large, flat surfaces causing echoes).
2.	Longitudinal waves and transverse waves Sound waves are longitudinal waves – the material vibrates to and fro in the same direction as the wave. Ultrasounds are sound waves with very high frequencies of vibration. Humans cannot hear ultrasounds. Light waves and other electromagnetic waves are transverse waves. The electromagnetic vibrations are at right angles to the direction of the wave.
3.	Images in mirrors are: the same size as the object; the same distance behind the mirror as the object is in front; virtual (light does not really go to them); laterally inverted.
4.	Measuring waves Wavelength is the distance from start to finish of one complete wave. Amplitude is the maximum displacement of the vibrating material from its undisturbed (normal) position. As amplitude increases, the energy carried by the wave increases. For example, think of bigger waves at sea and louder sounds. Frequency is the number of wave cycles per second. High pitched notes have a greater frequency than low pitched notes. The wave equation
5.	Reflection and refraction of waves
6.	Total internal reflection When light passes from a more dense to a less dense material (e.g. from glass or water into air), the angle of refraction is greater than the angle of incidence. When the angle of incidence reaches 42° for glass to air or 49° for water to air, the refracted ray travels along the surface of the denser material. The angle of refraction is 90°. When this position is reached, the angle of incidence is called the critical angle. When the angle of incidence is greater than the critical angle, there is no refracted ray. All the light is reflected internally. This is called total internal reflection. It has important uses in bicycle reflectors, periscopes and optical fibres.