• Question: Why are certain wavelengths of light reflected and others absorbed?

    Asked by delaram123 to Dilwar, Lou, Rachel, Simon, Susan on 18 Nov 2013.
    • Photo: Simon Langley-Evans

      Simon Langley-Evans answered on 18 Nov 2013:

      Light contains all of the visible colors (and some wavelengths that are invisible), and we see it as white. When that white light strikes an object some light will be reflected and some absorbed. The wavelengths that we see are the wavelengths that are reflected and the colours that we don’t see are being absorbed by the material. For example, the leaves of green plants absorb red and blue light efficiently and reflect green, so we see green. Carrots absorb blue and green light and reflect yellow, orange and red.

      So we perceive leaves to be green and carrots as orange. The exact shades of the colors that we observe depend upon the precise wavelengths that are absorbed. Smooth and rough surfaces reflect light differently, making some objects appear shiny and others dull.

      When light is absorbed it is because the energy carried by the light is interacting with electrons in the molecules that are in the surface that is illuminated. That light energy excites electrons in the molecule. The energy to excite certain molecules is associated with a particular colour. In a plant the molecule that absorbs light is chlorophyll where elections get excited exactly by the energy in blue and red light. An object we see as black absorbs all of the light energy and does not reflect any of it. In contrast, a white object is absorbing nothing and reflecting all wavelengths.

    • Photo: Susan Skelton

      Susan Skelton answered on 19 Nov 2013:

      Hi delaram!
      I hope you enjoyed the chat yesterday?

      This is a great question and something that I have to think about often in my research. In most cases it is to do with the structure of the material.
      Light with different wavelengths carries different amounts of energy. Light with short wavelengths (blue light) has loads of energy; light with longer wavelengths (red light) has much less.

      In a previous question, you asked about the quantum energy levels of electrons. The same theory is required here, so I’ll use the same analogy of our electron climbing our ladder of energy levels. Whether a material absorbs light or not depends on the size of the gaps between the energy levels in the material. If the energy that the light carries and gives to the electron is enough for the electron to reach the next rung of the ladder, then the electron will take the energy, and the light will be absorbed. If the light doesn’t carry enough energy for the electron to make it to the next rung, he’ll fall back down and the light will not be absorbed; it will be reflected instead.

      If the ladder has very big rungs, then lots of energy is required to get to the next rung, so blue light will be absorbed and the other wavelengths will be reflected. On the other hand, if the rungs are close together, the the electron doesn’t need so much energy to jump up the ladder, so red light will be absorbed and the other colours are reflected.

      This is true in almost all circumstances, except in the nano world…where something very strange happens.

      When I look at tiny nanoparticles, I see that their colour depends on their size! For example, gold particles (which I use a lot) start off a normal yellowy-gold colour as expected, when they are reasonably big. But as they get smaller and smaller, they turn from yellow, to red, to blue, …and then when they get very small, they are too small to reflect any colour of light at all and become invisible!

      So things aren’t always what they seem in the nano world and our normal understanding of physics can break down!

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