Emission Spectroscopy Electrons jump from higher levels to lower ones.

Emission Spectroscopy Electrons jump from higher levels to lower ones. Energy is released or emitted in the form of light of specific energy (or colour). Electrons must first be excited up to higher levels. This is done either with heat (eg. flame tests) or light (atomic emission spectra).

Absorption Spectroscopy Electrons jump from lower levels to higher ones.

Absorption Spectroscopy Electrons jump from lower levels to higher ones. Energy is absorbed in the form of light of a specific energy (or colour). This is the method used in UV-Vis Spectroscopy and Atomic Absorption Spectroscopy (AAS) and many others.

We know that white light can be split into a spectrum of colour. This effect is called DISPERSION.

The colours come from the different energies, frequencies and wavelengths of light. Energy, frequency and wavelength are properties of light and they are all interconnected.

The wavelength is the distance from the crest of one wave to the crest of the next wave. The longer the wavelength, the less frequent these waves will be. ie. Long wavelength means low frequency. The less frequent the waves, the less energy they will carry. ie. Long wavelength means low energy.

Red light has the longest wavelength of visible light. This also means it has the lowest frequency and the lowest energy. Violet light has the shortest wavelength of visible light, and has the highest energy. Other colours are in between.

How are colour and light absorption are related?

Just as light can be split into its various colours, so these colours can be recombined to form white light. Mixing all these coloured lights would give us white, but we can also use combinations of fewer colours to get white.

We can see that red, blue and green light add together to make white light. But where green overlaps magenta, white light is also formed. Same with red and cyan. And with yellow and blue.

We call these colours ‘complementary”. Complementary colours add to give white light. What is the complementary colour of green?

Complementary colours are on the opposite sides of the colour wheel. They help us understand the colours we see when light is absorbed.

In the same way, if we take blue light away from white light (ie. absorb blue), what colour remains? Yellow light – which is blue’s complementary colour.

The absorption spectra of individual atoms have very distinct lines showing which energies of light have been absorbed as the electrons jump to higher levels.

The absorption spectra of compounds tends to have broad overlapping regions for all the possible electron jumps.

This is the type of absorption spectra which is obtained using a UV-VIS spectrophotometer. The position of the peak shows the wavelength of the light most strongly absorbed and what % is absorbed.

This substance is absorbing the red/orange light (~600nm) and transmitting the rest which our eye mixes and interprets as blue/green.

White light shining on a white object. All light is reflected.

White light shining on a red object. Red light is reflected – other colours absorbed (particularly green and similar colours).

This is a more realistic view of this. The reflected colours combine and are mixed in the brain to give the overall red colour.

White light shining on a blue object.

White light shining on a black object. All light is absorbed. Black is the absence of light. White light shining on a grey object. A small amount of each colour is reflected.