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
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.
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
Electromagnetic radiation extends from
gamma rays (γ) through to
long radio waves. This is
often referred to as ‘the
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
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)
carries more energy and
infrared radiation less
According to the wavelengths, the ultraviolet portion of the
spectrum is further divided into:
UVA (315 – 400
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.