Light, Electromagnetic Spectrum and Refraction
The Electromagnetic Spectrum
The electromagnetic (EM) spectrum is a continuous family of transverse waves. All EM waves travel at the same speed in a vacuum — the speed of light, \(3 \times 10^8\) m/s — and can travel through empty space without needing a medium. All EM waves are transverse: their oscillations are perpendicular to the direction of travel.
The seven main groupings in order of decreasing wavelength (and increasing frequency) are:
radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.
A useful mnemonic: Red Monkeys In Vests Usually Xcel Gloriously. Because all EM waves travel at the same speed, waves with higher frequency must have shorter wavelength (from \(v = f\lambda\) with constant \(v\)).
All seven EM wave types are transverse, travel at \(3 \times 10^8\) m/s in a vacuum, transfer energy from source to observer, and obey \(v = f\lambda\). Order (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Uses and Dangers of EM Waves
Each region of the spectrum has properties that make it suited to specific applications, but higher-frequency waves are generally more dangerous because they carry more energy and are more ionising.
Radio waves are used for broadcasting (TV and radio), satellite transmission and long-distance communication. They can diffract around hills and buildings. Radio waves can be produced by oscillating electrical circuits and can also induce oscillations in aerial circuits when they are received.
Microwaves are used in satellite communication, mobile phone networks and cooking. In microwave ovens, the waves are absorbed by water molecules, causing them to vibrate and heat the food. Danger: internal heating of body cells — avoid prolonged direct exposure.
Infrared (IR) radiation is emitted by all warm objects. Uses include thermal imaging cameras, television remote controls, short-range optical fibre communications, cooking (grills, toasters) and security systems. Danger: skin burns from excessive exposure.
Visible light is the narrow band our eyes detect, from red (longest wavelength in this range) to violet (shortest). Uses: photography, illumination, optical fibre communication. The different colours of visible light correspond to different frequencies.
Ultraviolet (UV) radiation causes skin to tan and can damage surface cells, leading to sunburn and increasing the risk of skin cancer. UV is used in fluorescent lamps, security marking and detecting forged banknotes (which do not fluoresce like genuine notes). Danger: damage to surface cells and eyes.
X-rays pass through soft tissue but are absorbed by denser materials such as bone and metal. Uses: medical imaging (radiographs), airport security scanners, detecting internal flaws in metal structures. Danger: cell damage and increased mutation risk — medical staff use lead shielding and minimise exposure time.
Gamma rays have the shortest wavelength and highest frequency. They are emitted by radioactive nuclei and are highly penetrating. Uses: radiotherapy (killing cancer tumours), sterilising food and medical equipment, and as tracers in medical imaging. Danger: mutation or damage to cells in the body — lead or thick concrete shielding required.
Danger increases with frequency: radio and microwaves pose low risk at typical exposure levels; UV damages surface cells and eyes; X-rays and gamma rays can cause cell mutation, damage to DNA and cancer. Higher frequency = more energy per photon = greater ionising effect.
Refraction and Ray Diagrams
When light crosses a boundary between materials of different optical densities, it refracts (changes direction). The ray is described relative to the normal — a line perpendicular to the surface at the point of incidence.
The angle of incidence is measured between the incoming ray and the normal. The angle of refraction is measured between the refracted ray and the normal. When light enters a denser medium (e.g. air into glass), it slows down and bends towards the normal. When it exits into a less dense medium, it speeds up and bends away from the normal.
Place a rectangular glass block on plain paper. Shine a narrow ray of light at the flat face at an angle. Draw around the block and mark where the ray enters and exits. Remove the block and use a ruler to draw the complete ray path, including the refracted ray inside the glass. Measure the angle of incidence and angle of refraction at both surfaces using a protractor. Record how the ray bends towards the normal on entering the glass and away from the normal on leaving.
This experiment demonstrates: (a) refraction changes direction at a boundary; (b) the ray inside the glass travels in a straight line; (c) the emergent ray is parallel to the incident ray but displaced sideways. Control variables include: distance of the light source, use of the same glass block throughout.
There is no refraction equation on this course, so you never need a refractive index. You do need total internal reflection. When light travels from glass into air, it bends away from the normal. As the angle of incidence inside the glass is made larger, the refracted ray bends further and further until, at the critical angle, it travels along the boundary itself. Above the critical angle no light escapes at all — every ray is reflected back inside the glass. This is how optical fibres carry light around bends.
Infrared Emission and Absorption
All bodies emit infrared radiation. The hotter an object, the more infrared it emits and the shorter its peak emission wavelength. For a body to remain at a constant temperature, the average power it radiates must equal the average power it absorbs. If it absorbs more than it emits, it warms up; if it radiates more than it absorbs, it cools down.
The intensity and wavelength distribution of radiation emitted by a body depend on its surface temperature. This principle underpins our understanding of how the Earth’s temperature is regulated: the planet absorbs solar radiation and re-emits infrared. Greenhouse gases in the atmosphere absorb some of this outgoing infrared and re-emit it in all directions, reducing net heat loss — the greenhouse effect.
Fill a Leslie cube (a hollow metal cube with four different surface finishes: matt black, shiny black, matt white, shiny white) with hot water. Place an infrared detector at an equal distance from each face in turn and record the radiation emitted. Result: matt black emits the most infrared; shiny silver (or white) emits the least. This confirms that dark, matt surfaces are the best emitters and absorbers of infrared, while shiny surfaces are the poorest emitters (best reflectors).