White light

• The sun emits white light because sunlight contains all the wavelengths of the visible spectrum in equal proportions.
• Light travelling through a vacuum or a medium is termed white light if it includes all wavelengths of visible light.
• Light travelling through a vacuum or air isn’t visible to our eyes unless it interacts with something.
• The term white light can have two meanings:
  • It can refer to a combination of all wavelengths of visible light travelling through space, regardless of observation.
  • What a person sees when all colours of the visible spectrum hit a white or neutral-coloured surface.
• The human eye sees white when the wavelengths of light associated with the three primary colours – red, green, and blue (RGB) – are projected onto an achromatic surface.
• White light appears as different colours to an observer when some wavelengths of light are reflected by an object’s surface while others are absorbed.
• Artificial light sources usually emit light with a varied distribution of wavelengths or intensities, so they don’t typically emit white light.
• While there isn’t a single, distinct definition for white light, it’s fair to say that in any given situation:
  • White is the lightest possible colour.
  • White is an achromatic colour, which means a colour without hue, but with maximum saturation and brightness.
• Colour constancy, the ability to perceive colours as relatively constant, even under changing lighting conditions, enables human observers to see very different whites as appearing the same.

Why do Light Bulbs Glow if Light is Invisible?

• Incandescent light bulbs pass an electrical current through a fine tungsten filament with high electrical resistance.
• The resistance causes the filament’s electrons to heat up, giving off a bright yellowish-white colour. This colour is produced by:
  • The appearance of the heated tungsten surface.
  • Refraction, reflection, and dispersion as light encounters microscopic water droplets and dust particles on its journey from the light bulb towards the observer’s eyes.
  • Refraction, reflection, and dispersion within the liquids and solids of an observer’s eyes before the light reaches the retina.
• Besides these observed properties, a filament, along with other types of light sources, may emit wavelengths of light:
  • That are outside the visible region of the electromagnetic spectrum.
  • Travel from source to the retina without encountering surfaces or objects (however large or small), so remain completely invisible to an observer.
• The light emitted by a tungsten bulb spans wavelengths between 200 and 3000 nanometres.