Waves

Waves transfer energy from one place to another without transferring matter overall. In water waves, the water particles oscillate around fixed positions while the disturbance travels across the surface. Similarly, in sound waves, air particles vibrate back and forth while the sound energy moves through the air.

Waves are classified as transverse or longitudinal. In transverse waves, the oscillations are perpendicular to the direction of energy transfer, as in electromagnetic waves and ripples on water. In longitudinal waves, the oscillations are parallel to the direction of energy transfer and produce compressions and rarefactions, as in sound waves travelling through air.

The main properties of waves are amplitude, wavelength, frequency and period. Amplitude is the maximum displacement of a point on a wave from its undisturbed position. Wavelength, represented by $\lambda$, is the distance between equivalent points on consecutive waves, such as crest to crest or compression to compression.

Frequency is the number of complete waves passing a point each second and is measured in hertz, Hz. The period is the time taken for one complete wave and is measured in seconds. Frequency and period are related by $T=\frac{1}{f}$ and $f=\frac{1}{T}$.

Wave speed is the speed at which the wave disturbance and its energy travel. It is calculated using $v=f\lambda$, where $v$ is wave speed in m/s, $f$ is frequency in Hz and $\lambda$ is wavelength in metres. This equation can also be rearranged to $f=\frac{v}{\lambda}$ or $\lambda=\frac{v}{f}$.

The speed of sound can be measured by recording the distance travelled by a sound and the time taken, then using $\text{speed}=\frac{\text{distance}}{\text{time}}$. A typical GCSE value for the speed of sound in air is approximately $330\text{ m/s}$, although the exact value depends on conditions such as temperature.

Wave properties can be investigated using a ripple tank. Frequency may be measured by counting waves passing a point each second, wavelength can be measured between consecutive crests, and wave speed can then be calculated using $v=f\lambda$. Careful measurements should use several wavelengths and divide by the number measured to reduce percentage uncertainty.

When a wave reaches the boundary between two materials, it may be reflected, transmitted or absorbed. Reflection occurs when the wave returns into the original medium. Transmission occurs when the wave passes into the second medium. Absorption occurs when the wave's energy is transferred to the material, often increasing its internal energy.

Reflection follows the rule that the angle of incidence equals the angle of reflection. Ray diagrams should show the incident ray, reflected ray, normal and the angles measured from the normal. Reflection from a smooth surface is called specular reflection, while reflection from a rough surface is diffuse because the reflected rays travel in many directions.

Refraction occurs when a wave changes direction because its speed changes as it enters a different medium. The frequency normally remains unchanged at the boundary, so a change in wave speed causes the wavelength to change according to $v=f\lambda$. Wavefront diagrams and ray diagrams can be used to represent refraction.

Sound waves are longitudinal mechanical waves and therefore require a medium such as a solid, liquid or gas. Sound cannot travel through a vacuum. In the ear, sound waves cause the eardrum and other structures to vibrate, converting sound-wave disturbances into vibrations in solids. Normal human hearing is approximately from $20\text{ Hz}$ to $20\,000\text{ Hz}$, or $20\text{ kHz}$.

Ultrasound is sound with a frequency above the upper limit of human hearing. Ultrasound can be partially reflected at boundaries between different materials, allowing the time taken for echoes to return to be used to calculate distances. This principle is used in medical imaging, industrial testing and echo sounding for measuring water depth.

Seismic waves provide information about the internal structure of the Earth. P-waves are longitudinal and can travel through solids and liquids, whereas S-waves are transverse and cannot travel through liquids. Differences in their speed, transmission and reflection provide evidence about the structure and size of layers within the Earth.

Electromagnetic waves are transverse waves that transfer energy and can travel through a vacuum. In order of increasing frequency and decreasing wavelength, the electromagnetic spectrum is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All electromagnetic waves travel at the same speed in a vacuum.

Different parts of the electromagnetic spectrum have different applications because of their wavelengths, frequencies, interactions with matter and penetrating abilities. Radio waves are used for television and radio communication, microwaves for satellite communication and cooking, infrared for heaters and thermal imaging, visible light for fibre optics, ultraviolet for lamps and tanning, and X-rays and gamma rays for medical imaging or treatment. Ultraviolet, X-rays and gamma rays can be hazardous, with X-rays and gamma rays being ionising.

Lenses form images by refraction. A convex lens brings parallel rays towards a principal focus and can form real or virtual images depending on object position, while a concave lens produces virtual images. Magnification is calculated using $\text{magnification}=\frac{\text{image height}}{\text{object height}}$ and has no unit because it is a ratio.

Visible light consists of a range of wavelengths corresponding to different colours. The colour of an opaque object depends on which wavelengths it reflects and which it absorbs. A white object reflects most visible wavelengths, while a black object absorbs most visible wavelengths. Transparent materials transmit light, while translucent materials transmit light but scatter it.

All objects emit and absorb infrared radiation. Hotter objects emit infrared radiation at a greater rate. A perfect black body is an ideal absorber and emitter of radiation. If an object absorbs radiation faster than it emits radiation, its temperature rises; if it emits radiation faster than it absorbs, its temperature falls; and at constant temperature the rates of absorption and emission are equal.

The temperature of the Earth depends partly on the balance between incoming radiation from the Sun and outgoing radiation from the Earth. Changes in absorption, reflection and emission can alter this balance. Black-body radiation and infrared emission therefore help explain how energy transfer by radiation affects temperatures of objects and planets.