White light

• The sun emits white light because sunlight contains equal amounts of all of the wavelengths of the visible spectrum.
• As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
• As light travels through a vacuum or the air it is invisible to our eyes.
• The term white light has two meanings. It can refer to:
  •  A mixture of all the wavelengths of visible light travelling through space without reference to its observation.
  • What an observer sees when all the colours that make up the visible spectrum strike a white or neutral-coloured surface.
• The human eye also sees white when the wavelengths of light corresponding with the three primary colours red, green and blue (RGB) are projected onto a neutrally coloured surface.
• White light appears coloured when some wavelengths of light are reflected by the surface of an object but others are absorbed.
• Artificial light sources typically emit light with an uneven distribution of wavelengths or intensities and so do not emit white light.
• Whilst there is no single, unique specification for white light, it is reasonable to argue that in any specific situation:
  • White is the lightest possible colour.
  • White is an achromatic colour, meaning a colour without a hue but with maximum saturation and brightness.

Why do light bulbs glow if light is invisible?

• Incandescent light bulbs work by passing an electrical current through a fine tungsten filament that has a high electrical resistance.
• The resistance causes electrons within the filament to heat up and to appear a very bright yellowish-white colour. This colour is produced by:
  • The appearance of the surface of the heated tungsten.
  • Refraction, reflection and dispersion as light encounters microscopic water droplets and the surfaces of particles of dust on its journey from the light bulb towards an observer’s eyes.
  • Refraction, reflection and dispersion in liquids and solids within an observer’s eyes before light reaches the retina.
• Independent of these observed properties, the filament also emits a full range of wavelengths of light that propagate without encountering obstacles and so remain entirely invisible to an observer.
• The light emitted by a tungsten bulb spans wavelengths between 200 and 3000 nanometers.