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1. Introduction - What is light?

    Light is electromagnetic radiation which is visible to the human eye. Electromagnetic radiation is generated by the oscillation or acceleration of electrons or other electrically charged particles. The energy produced by this vibration travels in the form of electromagnetic waves. These waves are characterised by their wavelength (λ) which is the distance between successive peaks and is measured in units of length, and by their intensity, or amplitude, which is the height of each of those peaks.

    To explain how light travels, it is considered a wave. However, light can also be considered particles when describing how it interacts with matter.

    These particles called photons carry each a specific amount of energy. Light intensity increases with the number of photons. For example, intense red light used on a theatre stage and a traffic red light may consist of photons of the same energy but the first one is more intense due to the larger number of photons emitted.

    Electromagnetic radiation extends from gamma rays (γ) through to long radio waves. This is often referred to as ‘the electromagnetic spectrum’. The energy of a wave depends on its wavelength: the longer the wavelength, the lower the energy. Therefore, in the electromagnetic spectrum, gamma rays have the highest energy, and long radio waves the lowest.

    The sun emits visible light, but also infra-red (IR) and ultra-violet (UV) radiation.

    The visible part of the electromagnetic spectrum only covers a small range of wavelengths, from 380 nm to 750 nm. In the electromagnetic spectrum, shorter wavelengths (from 10 nm to 380 nm) are ultraviolet (UV) and longer wavelengths (from 750 nm to 1 mm) are infrared (IR) radiation. Ultraviolet radiation carries more energy and infrared radiation less energy than visible light.

    According to the wavelengths, the ultraviolet portion of the spectrum is further divided into: UVA (315 – 400 nm), UVB (280 – 315 nm) and UVC (100 – 280 nm). All radiation from the sun with a wavelength below 290 nm, that is most high-energy UV-radiation, is filtered out by the atmosphere before reaching the Earth’s surface. More...

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