Waves: Properties and Behaviour
What Are Waves?
Waves transfer energy from one place to another without transferring matter. The particles in the medium vibrate back and forth, but they do not travel with the wave.
There are two types of wave. In a transverse wave, the oscillations are perpendicular (at right angles) to the direction of energy transfer. Examples include light, water waves, and all electromagnetic waves.
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. Sound is the most common example. Longitudinal waves have areas of compression (particles squashed together) and rarefaction (particles spread apart).
All waves can be described using four properties: amplitude, wavelength, frequency, and wave speed. The wave equation links three of these: \(v = f \lambda\).
Wave Properties
Amplitude is the maximum displacement from the rest position. Larger amplitude means more energy. Wavelength (\(\lambda\)) is the distance from one point on a wave to the same point on the next wave (e.g. crest to crest).
Frequency is the number of complete waves passing a point per second, measured in hertz (Hz). The period is the time for one complete wave and is related to frequency by: \(T = \frac{1}{f}\).
The wave speed equation is:
$$v = f \lambda$$
Where \(v\) is wave speed in m/s, \(f\) is frequency in Hz, and \(\lambda\) is wavelength in metres.
A sound wave has a frequency of 440 Hz and a wavelength of 0.75 m. Calculate the wave speed.
\(v = f\lambda = 440 \times 0.75 = 330\) m/s
Reflection, Refraction and Absorption
When waves meet a boundary between different materials, they can be reflected, refracted, or absorbed. Often a combination of all three occurs.
Reflection occurs when a wave bounces off a surface. The law of reflection states that the angle of incidence equals the angle of reflection (both measured from the normal).
Refraction occurs when a wave changes speed as it passes from one medium to another. If the wave slows down, it bends towards the normal. If it speeds up, it bends away from the normal. The frequency stays the same; only the speed and wavelength change.
The required practical is to measure the frequency, wavelength and speed of waves in a ripple tank and of waves in a solid. In the ripple tank, a vibrating bar makes plane waves on shallow water and a lamp projects the pattern onto a screen. Read the frequency from the signal generator, measure across ten waves to find one wavelength, then use \(v = f\lambda\). For waves in a solid, a vibration generator sends waves along a stretched string; measure the wavelength on the string and read the frequency from the generator.
For measuring the speed of sound, use two microphones a known distance apart and a data logger. Calculate speed from \(v = \frac{d}{t}\).
Not examined on this course. Diffraction is not in the specification for Combined Science, or for Separate Physics. Read this section for interest only.
Diffraction
Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. The effect is greatest when the gap width is about the same as the wavelength. If the gap is much larger than the wavelength, very little diffraction occurs.
Sound waves have long wavelengths (up to several metres), so they diffract easily around doors and buildings — you can hear someone around a corner. Light has very short wavelengths (around 500 nm), so it only diffracts noticeably through extremely narrow slits.