Electromagnetic waves: a single family that carries energy without matter

1. Every electromagnetic wave is transverse, so its oscillations are at $90^{\circ }$ to the direction in which the wave travels and carries energy.
2. The wave moves energy, not matter: nothing physical is thrown from the source to the absorber, only energy arrives.
3. When light from a lamp lands on your hand, the lamp is the source, your hand is the absorber, and energy is transferred from lamp to hand by the wave.
4. Electromagnetic waves can travel through a vacuum, so they need no particles and no medium, which is why sunlight crosses empty space to reach Earth.

The electromagnetic spectrum: one continuous band, split by wavelength and frequency

1. The spectrum is continuous: the wavelengths and frequencies form an unbroken run with no gaps between the named groups.
2. The seven named groups are useful labelled slices of this one band, not seven separate kinds of wave.
3. All electromagnetic waves travel at the same speed through a vacuum, and at very nearly the same speed through air, so speed does not tell the groups apart.
4. What separates the groups is their wavelength and their frequency.
5. Wavelength is the distance from one point on a wave to the same point on the next wave.
6. Frequency is the number of complete waves passing a point each second, measured in hertz, $Hz$.
7. Wavelength and frequency are linked: as wavelength decreases, frequency increases.

The order of the spectrum: radio to gamma, long to short

1. From long wavelength to short wavelength (which is also low frequency to high frequency), the groups run:
   1. radio waves
   2. microwaves
   3. infrared
   4. visible light (red through to violet)
   5. ultraviolet
   6. X-rays
   7. gamma rays
2. Visible light is itself a small band of colours running from red at the long-wavelength end to violet at the short-wavelength end.
3. The colours of visible light, from longest to shortest wavelength, are red, orange, yellow, green, blue, indigo and violet; a common way to remember the order is the mnemonic VIBGYOR (violet, indigo, blue, green, yellow, orange, red), which lists them the other way, from shortest to longest wavelength.
4. Red light has a longer wavelength and lower frequency than violet light.
5. Violet light has a shorter wavelength and higher frequency than red light.

Our eyes see only a sliver of the spectrum

1. Human eyes detect only visible light, which is a very small part of the full electromagnetic spectrum.
2. We cannot see radio waves, microwaves, infrared, ultraviolet, X-rays or gamma rays, yet all of these are still electromagnetic waves in the same family.
3. This is why detectors, cameras and instruments are needed to reveal the parts of the spectrum our eyes miss.

Everyday energy transfer by electromagnetic waves

1. Visible light and infrared from the Sun transfer energy across space to warm and light the Earth.
2. Infrared from an electric heater transfers energy to your skin and clothes, which absorb it and warm up.
3. Visible light from a lamp transfers energy to any surface that absorbs it.
4. Microwaves transfer energy to the water in food, which absorbs them and heats up.