Sunlight – What Is It?

The Sun, the star at the heart of our solar system, continuously emits an enormous amount of energy in the form of electromagnetic radiation. This travels outwards from the Sun, in all directions, as electromagnetic waves: fluctuations of electric and magnetic fields that travel through space in a broadly similar manner to waves on the surface of water, ripples along a rope, or sound through the air (or other materials). All of these other types of waves require matter in order for them to propagate, or travel, however; electromagnetic waves do not, they can travel through a vacuum and are absorbed to a greater or lesser extent by different types of matter.

Electromagnetic radiation is classified according to its wavelength, the distance between adjacent peaks or troughs of the waves:

  • Radio waves – longer than about 30 cm. Used for broadcast radio and TV transmissions. Modern FM radio and TV stations lie at the shorter end of this range
  • Microwaves – about 1 mm to 30 cm. Microwave ovens use a specific wavelength (about 12 cm) that is absorbed by water molecules to cause heating in food. Also used for point-to-point radio communication, the GPS system, and radar systems.
  • Infra-red – 700 nm to 1 mm. Popularly known as ‘heat radiation’. Used for heating, thermal imaging, and remote controls for consumer equipment.
  • Visible light – 400 nm to 700 nm. The very narrow range of electromagnetic radiation detected by the human eye, seen as red at the long end, through orange, yellow, green, and blue, to violet at the short end.
  • Ultra-violet – 10 nm to 700 nm. Used as a germicide to sterilize workspaces and tools, and in the food and water treatment industries. Also used for security marking, photo-etching processes, and in medical therapies.
  • X-rays – 30 pm to 10 nm. Used for medical, security and industrial imaging.
  • Gamma rays – shorter than 30 pm. Used in security and medical imaging, and medical therapies. Also used as a germicide in food processing.

Note: 1 nm (nanometre) = 1/1,000,000,000 m and 1 pm (picometre) = 1/1,000,000,000,000 m.

All matter emits electromagnetic radiation according to its temperature: the hotter an object is, the more radiation it emits and the shorter the wavelengths of that radiation. This is why objects begin to glow as they get very hot, starting with a red colour, then becoming white as they get even hotter and emit more and more light across the whole of the visible range. Objects at normal room temperatures emit radiation most strongly in the infra-red region, around 0.01 mm.

The Sun is extremely hot – about 5,500 °C – and emits radiation with a peak wavelength of about 500 nm, towards the blue end of the visible spectrum. This peak, however, is pretty broad, with the solar spectrum extending well into the ultra-violet and infra-red regions. The Earth’s atmosphere also lets most of this radiation through to the surface. Wavelengths shorter than the middle of the UV (ultra-violet) region are very effectively blocked by the upper atmosphere, including the ozone layer, while much of longer end of the infra-red region is absorbed by water in the atmosphere.

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