Waves in matter Study Guide

Waves are disturbances that transfer energy from one point to another without the net transfer of matter. All waves, regardless of their type (e.g., sound waves, light waves, water waves), share fundamental properties that describe their behaviour. These properties are velocity (or wave speed), frequency, and wavelength. The relationship between these three quantities is described by the wave equation, which is a foundational concept in physics: velocity = frequency × wavelength.

Velocity (v) is the speed at which a wave travels through a medium. It is measured in metres per second (m/s). Frequency (f) is the number of complete wave cycles (oscillations) that pass a given point per second. It is measured in Hertz (Hz), where 1 Hz means one cycle per second. Wavelength (λ, lambda) is the spatial period of the wave, which is the distance over which the wave's shape repeats. It is the distance between two consecutive identical points on a wave, such as two crests or two troughs. Wavelength is measured in metres (m).

This equation allows us to calculate any one of these quantities if the other two are known. For example, if you know a wave's speed and its frequency, you can determine its wavelength. Conversely, knowing the wavelength and speed allows you to calculate the frequency. This relationship is crucial for understanding how waves behave and interact in various phenomena, from the propagation of light to the transmission of sound.

The colour of an opaque object depends on how it interacts with the light that falls on it. When white light, which contains all colours of the visible spectrum, hits an object, some colours are absorbed, some are transmitted (if the object is transparent or translucent), and some are reflected. The colour we perceive an opaque object to be is the colour of light that it reflects most strongly. For example, a red apple appears red because when white light illuminates it, the apple absorbs most of the other colours (like blue and green) but reflects red light into our eyes. Similarly, a green leaf absorbs most colours except green, which it reflects. If an object appears black, it means it absorbs nearly all colours of the visible spectrum and reflects very little light. Conversely, a white object reflects nearly all colours of the visible spectrum equally. Transparent or translucent objects appear the colour of the light they transmit. A red filter, for instance, transmits red light while absorbing other colours. The absence or presence of specific colours of light, combined with the material's properties of absorption, transmission, and reflection, dictates the final perceived colour. This interaction is fundamental to understanding how we see the world in colour.