Waves

All teachers may not wish to use all the ideas suggested. It is up to you to fit them to your own pupils' needs and the school facilities.

Characteristics of waves

LESSON 1

Reflection, refraction and diffraction of waves, including light and sound as examples of transverse and longitudinal waves

Discuss the everyday effects of the reflection, refraction and diffraction of light and sound.
Reflection: Mirrors, echoes in sound
See: Reflection
Refraction: prisms and colour, lenses, sounds heard over long distances
See: Refraction
Reflection of sound
Refraction of sound

Diffraction: colours on a CD, sound waves spreading through a door, sound waves spreading round an obstacle like someone's head

Explain what diffraction is – spreading of a wave round an obstacle or through a hole.
See: Diffraction

Demonstration experiment
Use a ripple tank to investigate and demonstrate reflection, refraction and diffraction of water waves. (See schoolphysics.co.uk for a set of diagrams of the results).
See: Waves in a ripple tank


Resources needed
Ripple tank that can be used to project the wave patterns on a screen Barriers and dropper Suspended vibrating motor Mains stroboscope disc Lamp and power supply

Refraction is difficult to demonstrate – the change in depth of the water only gives a small refraction effect.

Extend this with examples of the reflection, refraction and diffraction of sound and light

Background information
In both reflection and refraction the wavelength is unaltered. In refraction the wavelength is changed as the wave passes from one material to another (or in the case of water waves from one depth to another).

It is left for the individual teacher to decide whether it is better to make the pupils aware of the effects of reflection, refraction and diffraction by looking at water waves first or to see experiments with sound and light directly.

Analogies of the refraction of waves:

(a) a car freewheeling from tarmac onto sand at an angle
(b) a group of soldiers marching from tarmac onto sand at an angle
See: Teachers/OHT/Light/Refraction and soldiers
See: Teachers/Creative teaching ideas/Waves

Computer simulations of reflection, refraction and diffraction
Use a computer simulation of the three effects – see Java applets

The difference between longitudinal and transverse waves

Show a wave model if possible and also use selected examples of computer simulations

Demonstrate both types with the use of a slinky spring.

Background knowledge
Longitudinal waves – vibration ALONG wave direction – sound, P waves
Transverse waves – vibrations at RIGHT ANGLES to wave direction – light and all other electromagnetic waves, water waves, S waves
See: Waves

Follow up work
1. Write about ONE of the types of wave that they have studied.
2. Extension to cover the dependence of the width of the diffraction pattern on
(i) the wavelength
(ii) the width of slit or size of the obstacle

See: 14-16/Wave properties/Problems/Wave model of radiation 2