Waves in matter Study Guide

When a sound wave travels from one medium to another, its speed (velocity) changes. This change in speed is a fundamental aspect of wave behaviour and occurs because the properties of the new medium affect how sound energy propagates. For example, sound travels faster in solids than in liquids, and faster in liquids than in gases, due to the different densities and elasticities of these states of matter. When the velocity of a sound wave changes, one of its other key properties must also change to maintain a consistent relationship. The frequency of a sound wave is determined by its source and generally remains constant as the wave passes from one medium to another. This is because the rate at which the source vibrates does not change, and this rate dictates the frequency of the wave. Consequently, if the velocity changes and the frequency remains constant, the wavelength must adjust. The relationship between these properties is given by the wave equation: velocity (v) = frequency (f) × wavelength (λ). Therefore, if the velocity increases, the wavelength must also increase to keep the frequency constant. Conversely, if the velocity decreases, the wavelength must decrease. Understanding this inter-relationship is crucial for explaining phenomena like refraction, where waves bend as they enter a new medium at an angle, precisely because their speed and wavelength change while frequency remains the same. This concept is applicable to all types of waves, not just sound.

When we talk about waves, like light or sound, two fundamental properties help us describe them: wavelength and frequency. Imagine a ripple moving across water. The wavelength is the distance between two consecutive identical points on the wave, such as two crests or two troughs. It's usually measured in metres (m). This spatial measurement tells us how 'stretched out' or 'compressed' a wave is. A long wavelength means the crests are far apart, while a short wavelength means they are close together.

Frequency, on the other hand, tells us how often a part of the wave passes a fixed point. Using our water ripple example, if you stood still and counted how many crests passed you in one second, that would be the frequency. Frequency is measured in Hertz (Hz), where one Hertz means one wave cycle per second. So, a high frequency means many waves pass by every second, while a low frequency means fewer waves pass by.

Wavelength and frequency are inversely related. This means if a wave has a long wavelength, it will have a low frequency, and if it has a short wavelength, it will have a high frequency. This relationship is crucial because it helps us understand how different types of waves behave and interact with matter. For example, in the electromagnetic spectrum, radio waves have very long wavelengths and low frequencies, while gamma rays have extremely short wavelengths and very high frequencies.