Light – colour and fluorescence

Visible light is the small part within the electromagnetic spectrum that human eyes are sensitive to and can detect.

Visible light waves consist of different wavelengths. The colour of visible light depends on its wavelength. These wavelengths range from 700 nm at the red end of the spectrum to 400 nm at the violet end.

The colour we see is a result of the wavelengths that are reflected back to our eyes. We see an object as red because there is a pigment in the object that reflects the red light wavelength. When white light (which contains all of the colours – visible wavelengths) shines on a red object, the red light wavelength bounces off, the other colours are absorbed and the energy from the non-red wavelengths is changed, primarily to heat. A white object reflects all colours (wavelengths), and a black object absorbs all colours. If we change from white light to pure-blue light, the shirt no longer looks red. There is no red light to reflect from the object. It absorbs the blue light and appears to be black. Find out more about the colours of light.

Fluorescence

When we look at a fluorescent object, the fish in the photo or a high-vis vest, it appears brighter than other similarly coloured objects. This is because something quite different happens when light hits fluorescent objects.

Fluorescent objects reflect light as well as absorb the energy of the light, turning some of it into heat, and the majority of the light is emitted as the fluorescent colour. The electrons in the fluorescent pigments absorb light energy and are temporarily promoted into higher-energy orbitals. When the electrons settle back to their regular positions, they emit light that is a certain colour – fluorescent.

A fluorescence analogy

Imagine throwing a ball onto a roof and listening to the ball strike the ground when it falls off. Balls that are thrown hard enough to make it to the roof would all make the same sound when they roll off and hit the ground. If the ball isn’t thrown hard enough to reach the roof, it would make less sound.

The light striking the fluorescent object is like the balls being thrown, and the light coming off the fluorescent object is like the sound they make when they hit the ground. In this picture, four balls are thrown, and the three that had enough energy to make it onto the roof will all make the same sound when they hit the ground. In this analogy, the balls are like different colours (wavelengths) of light.

Different colours of light have different amounts of energy. Ultraviolet light has the most energy, and that is why we use sunscreen to protect our skin from the Sun’s rays. Ultraviolet light is like ball 4 in the analogy – it has the most energy and it went higher than the others. Blue and green light are the next most energetic, and they are like balls 3 and 2. Red light has the lowest energy of visible light and is like ball 1 – it didn’t even make it to the roof. The distance the balls fall from the roof will dictate the colour that fluoresces. In this case, it is a colour between red and green on the visible light spectrum – lower-energy wavelengths than the balls initially had, as some light energy is converted to heat in the process.

Examples of fluorescence

In the image below, purple and green lasers are shining on two different surfaces. On the left, the lasers are focused on a piece of black fabric. On the right, the lasers are focused on a fluorescent high-vis safety vest. Notice the distinct colours of the lasers on the black fabric – purple and green – but when shown on the fluorescent fabric, they both look the same colour.

The light from the two lasers has sufficient energy to excite the electrons in the fluorescent high-vis safety vest. This shows as a bright yellow colour. When the excited electrons settle back to their regular positions, they emit light that is a certain colour – in this case, a fluorescent yellow.

Some liquids also have fluorescent properties. Quinine, which is found in tonic water, fluoresces in violet and ultraviolet light. The image on the left shows a green laser shining through a bottle of tonic water without causing fluorescence, while a violet laser shows a clear path of fluorescence.

In this case, the green light does not have sufficient energy to boost the electrons to a higher energy state. The green light is like ball number 1 in the analogy above.