P7.1 – General Wave Properties

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1. Demonstrate understanding that wave motion transfers energy without transferring matter in the direction of wave travel.

A wave transfers energy from one place to another without transferring matter in the process. For example, when a pebble is dropped into a pond, circular ripples move outward on the surface of the water. Any object on the surface of the water (e.g. a leaf) would only bob up and down, not moved from its position (because water waves are transverse waves- explained below).

2. Describe what is meant by wave motion as illustrated by vibration in ropes and springs and by experiments using water waves.

Wave motion is the transfer of energy from one point to another. We can demonstrate this by hanging an object on a stretched string and then sending a pulse down the string (by moving it up and down quickly like a wave). When the pulse meets the suspended object, the object bobs up and down for a moment, as shown below.

 

3. State the meaning of and use the terms speed, frequency, wavelength and amplitude.

The amplitude of a wave is its maximum distance from the equilibrium.

The wavelength of a wave is the distance between a point on one wave and the same point on the next wave i.e. one wave’s length. This can be from one crest to the next crest or from one trough to the next trough. It is symbolised by the Greek alphabet lambda, λ.

The frequency of a wave is the number of waves produced each second. It is denoted by the letter f and is measured in hertz (Hz).

 

4. Recall and use the equation v = f λ

The speed of a wave (v) is its frequency (f) multiplied by its wavelength(λ).

v = f λ

 

5. Distinguish between transverse and longitudinal waves and give suitable examples.

For transverse waves, the displacement of the medium is perpendicular to the direction of propagation (pulsing) of the wave i.e. the displacement and propagation are at right angles. A ripple on a pond and a wave on a string are examples of this.

In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave i.e. the displacement and propagation are in the same direction. Sound waves are an example.

 

 

6. Identify how a wave can be reflected off a plane barrier and can change direction as its speed changes.

When a wave hits a smooth plane barrier, it is reflected so that its angle of incidence equals the angle of reflection (see P8.1 – Reflection of Light).

When a wave hits a rough plane barrier, it is mostly refracted; the wave bends and travels through the plane barrier instead of reflecting (see P8.2 – Refraction of Light). However, the wave’s speed, direction and wavelength will change. Note that the frequency of the wave will remain constant.

 

7. Interpret reflection and refraction using wave theory.

When waves reflect off a plane barrier (like a mirror) there is no travelling through a different medium, so the density of the medium is the same, which means that the speed and wavelength of the waves remain the same. The angle between the incident (original) wave and the normal (the perpendicular bisector to the barrier) will be equal to the angle between the reflected wave and the normal.

A wave refracts because the medium it is now travelling through has a different density, which causes its speed to change. When this speed changes, the wavelength changes and consequently its direction changes too.
For example: when waves hit water, which has an uneven surface, it will refract. The water is denser than air (water molecules are more closely packed together than air molecules), so the speed of the wave decreases, and so does its direction.

 

 

Notes submitted by Lintha

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