Electromagnetic waves

Electromagnetic (EM) waves are transverse waves that transfer energy from a source to an absorber (for example, from the Sun to the Earth, or from a microwave oven to food). Unlike sound, EM waves don't need particles to travel — they can travel through a vacuum (empty space), and in a vacuum all electromagnetic waves travel at the same speed (the speed of light).

Being transverse, an EM wave is really two oscillations travelling together at right angles to each other: an oscillating electric field and an oscillating magnetic field, both perpendicular to each other and to the direction the wave travels. Picture two sine waves overlapping in perpendicular planes, moving forward in step — that's what makes it electro-magnetic.

The electromagnetic spectrum is a continuous range of wavelengths, grouped by convention into seven main types. In order of decreasing wavelength and increasing frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.

Radio waves can be produced by, and can themselves produce, oscillations (alternating currents) in electrical circuits: an electrical circuit with an oscillating current emits radio waves at the same frequency, and radio waves absorbed by a circuit induce an alternating current in it at the wave's frequency.

Different substances absorb, transmit, refract, or reflect different parts of the electromagnetic spectrum differently, because the substance's particles interact differently with different wavelengths/frequencies. For example, glass transmits (and refracts) visible light, but absorbs most ultraviolet radiation and reflects most infrared radiation.

Refraction: when a wave crosses a boundary between two materials of different density at an angle, it changes speed, which changes its direction (unless it hits the boundary exactly along the normal). Going into a denser material, the wave slows down and bends towards the normal; going into a less dense material, it speeds up and bends away from the normal.

Dispersion (HT only): different wavelengths of light refract by different amounts when entering a denser material — shorter wavelengths (e.g. blue light) slow down and bend more than longer wavelengths (e.g. red light). This is why white light splits into a spectrum of colours when passed through a glass prism: each colour refracts by a slightly different amount and they spread apart.

Most electromagnetic radiation is generated or absorbed by changes in the arrangement of electrons within atoms.

• •When an electron moves to a higher energy level (further from the nucleus), the atom has absorbed electromagnetic radiation.
• •When an electron falls to a lower energy level (closer to the nucleus), the atom emits electromagnetic radiation.
• •If an electron absorbs enough energy to leave the atom completely, the atom becomes a positive ion — this is ionisation.

UV, X-rays, and gamma rays can all be hazardous to human tissue — the severity depends on the type of radiation and the size of the dose received. UV radiation causes premature skin ageing and increases the risk of skin cancer (sun cream reduces over-exposure). X-rays and gamma rays are ionising radiation and can cause gene mutations that lead to cancer, so exposure — for example during medical imaging — is kept to the minimum needed.

A lens forms an image by refracting light. Light passing through the exact centre of a lens does not change direction. Each lens has focal points on either side, where parallel rays of light converge (or appear to diverge from).

A real image is one where light rays actually converge and could be captured on a screen placed at that point. A virtual image is one where light rays only appear to diverge from a point (as with a reflection in a mirror or the enlarged image seen through a magnifying glass) — no screen placed there would show anything, because the light doesn't actually pass through that point.

Each colour in the visible spectrum corresponds to its own narrow band of wavelength and frequency: violet/blue light has the shortest wavelength and highest frequency, red light has the longest wavelength and lowest frequency. White light (such as sunlight) is a mixture of all the visible wavelengths together.

A colour filter works by absorbing every wavelength of light except the one(s) corresponding to its own colour, which it transmits.

Objects that let light through are either transparent (allowing most light through with little scattering, so objects behind can be seen clearly) or translucent (scattering most of the light, so only some passes through and objects behind appear blurred).