Light and colour

Section 1 of 4

The orange of a carrot, the green of a leaf and the purple of a blueberry all come from molecules. To see how, start with light.

White light, such as sunlight, is a mix of all the colours of the rainbow. A glass prism, or a raindrop, spreads it out into its colours: red, orange, yellow, green, blue and violet. Light travels as a wave, and each colour has its own wavelength, the distance from one wave crest to the next. Violet has the shortest waves we can see, about 400 nm, and red the longest, about 700 nm. The carbon-carbon bond lengths you have seen were in nanometres too: a wave of red light is about 5,000 times as long as a C=C double bond.

The colours of visible light, from violet to red A bar of colour running from violet at 400 nanometres on the left, through blue, green, yellow and orange, to red at 700 nanometres on the right. Beyond violet is ultraviolet, UV, and beyond red is infrared, IR, both invisible. Light toward the violet end carries more energy, and light toward the red end less. UV IR 400 nm 500 nm 600 nm 700 nm violet blue green yellow orange red more energy less
Visible light runs from violet, about 400 nm, to red, about 700 nm. Ultraviolet (UV) light, beyond violet, and infrared (IR) light, beyond red, are invisible to our eyes.

Light carries energy, and shorter waves carry more of it. Violet and blue light carry the most energy of the colours we see, and red light the least. Ultraviolet light, beyond violet, carries more still: it is the part of sunlight that causes sunburn.

Taking in some colours

A molecule can take in, or absorb, light when the light's energy matches a jump its electrons can make. Small molecules such as water and ethanol absorb ultraviolet light and none of the visible colours. All the visible colours pass straight through them, so they look colourless.

A coloured molecule absorbs some of the visible colours. The rest of the light travels on to your eyes, and you see the colours that are left. Put the colours round a circle, and each one sits opposite a partner. A molecule that absorbs one colour looks roughly like its partner: absorb blue and it looks orange, absorb green and it looks red, absorb violet and it looks yellow.

A colour wheel: each colour sits opposite its partner A ring of six colours in order: red at the top, then orange, yellow, green at the bottom, blue and violet. Dashed lines join each colour to the one opposite: red with green, orange with blue, and yellow with violet. A molecule that takes in one colour of a pair looks the other colour. red orange yellow green blue violet
A colour wheel. Red and green, orange and blue, and yellow and violet are partners: a molecule that absorbs one colour of a pair looks roughly like the other.

Carrots look orange because a molecule in them absorbs blue light. The next section shows which part of that molecule does the absorbing.

Quick check

A dye absorbs blue light and sends back all the other colours. What colour does it look?