Why does my hair look different colours in the light?

How do we view colour? Your hair can look different colours in the light. But why?

Beaut's new science expert Dr Regina Kelly, Beaut's new Science Editor, is a Science Educator and a Dean’s Postdoctoral Fellow in STEM Education at EPI*STEM, the National Centre of STEM Education, School of Education, Faculty of Education and Health Sciences, University of Limerick.

Have you ever sat in a hairdressers admiring your new hair colour under the bright lights of the salon, only to sit into the car and see a reddish colour in the mirror? Or the dress you thought was blue-grey in the changing room appears more yellowish grey when you take it home. Why do colours appear different?

Isaac Newton observed that the surface of an object reflects some colours and absorbs all of the other colours. What does this mean? The colour of a dress is not 'in' the dress. We perceive colour as the reflected light off an object into our eyes. The dress contains atoms that selectively absorb one or more frequencies of light. Light is made up of a range of frequencies, each of which corresponds to a specific wavelength. A light wavelength is representative of a specific colour. The range of these wavelengths is 700 nm (red) to 400 nm (violet). In the case of a yellow dress, the surface of the dress is reflecting the wavelength of light we see as yellow (590 -560 nm) and absorbing all the other colours.

Not all black and white

What about white and black? White light contains all of the colours of the spectrum. Remember in school when you used a prism to disperse light into a spectrum? An object appears white when it reflects all wavelengths and absorbs none. An object appears black when it absorbs all the wavelengths that shine on it.

The appearance of an object is also dependent on the colours of light that shines on it. If an object is illuminated with cyan light (cyan consists of green and blue light) and absorbs green, then blue light will be reflected from the object so it will appear blue. This is known as the process of colour subtraction. If blue light is illuminated on a material, that absorbs blue. The shirt appears black due to the absence of reflected light. What colour will red trousers appear when the room lights are turned off and the room is entirely dark? If there is no light shining on the trouser, there can be no reflected light. The trousers appear black.

To answer our question, the colour a material appears to be depends on the light shining on the material and the nature of the atoms of the object. How can you use this information? Use white light, or nearly white light from the sun to view the colour of an object. The spectrum of light from an artificial light will change depending on the bulb you use.