A few weeks ago I was contacted out of the blue by Nick Day, who said he had made some very interesting observations with a green laser pointer and some red sweet wrappers. It is the kind of communication that I could easily have ignored as being ‘from a nutter’ (sorry Nick), but in fact when I read the letter, the observations were very thorough and detailed. So I felt obliged to at least have a go myself, and since we had some Quality Street to hand, I did some experiments and found there was indeed something very odd going on.
Nicks’ basic observations were very simple. The photographs below summarise the results.
- He shone the laser pointer at a piece of paper: he observed a green dot. In the image below, the white centre of the dot is caused by over loading of the camera sensor.
- Next he looked at the green dot through a red ‘cellophane’ sweet wrapper. He could only just see the dot. You will need to expand the image to see the faint dot. Notice that the dot is red!
- Then he put the sweet wrapper between the laser and the paper – but now he couldn’t see the dot at all! In the image below I held the wrapper over the laser with an elastic band.
Nick reasoned that observations 2 and 3 should give equivalent results because in each case the light had travelled through the wrapper once and reflected off the paper once. Why should the order make a difference to the results? He demanded to know why!
Well I was puzzled too, but I thought it must just be to do with how we perceive light against brighter or darker backgrounds. Actually, as Nick had speculated, the real answer is much more complicated. It is to do with fluorescence, and the picture below makes this clear.
I took this photograph looking down the laser beam with the cellophane covering. Please don’t do this with your eyes: the camera was doing ‘the looking’, not me! And don’t even use a camera if the beam is not attenuated by a filter. At first I didn’t appreciate what I could see, and then I noticed the obvious, the spot was red! How could green light turn red? By carefully adjusting the camera angle I was able to find the original beam, which was still green, but massively attenuated.
What’s going on? The laser emits visible light – a wave in the electric field – with only a single frequency (564 THz) and wavelength (532 nm). When this wave reaches our retina, it creates the sensation we call ‘green’. Importantly, in the simple concept of reflection from an object, the wave cannot change its frequency. The wave shakes electrons which are set in motion, and they then re-radiate at exactly the same frequency. In a fluorescent substance, electrons within the substance are set in motion at one frequency, but then due to the interaction of the oscillating field of the light with the local electric fields within the substance, the electrons change their frequency of oscillation. For reasons I won’t go into, the frequency emitted is always lower than the initial frequency. So the explanations of Nick’s observations is this.
- The red filter massively absorbs the green laser light – probably more than 99.9% at a guess – so shining light through the filter results in no visible green dot (Observation 3)
- However shining light on the paper results in two types of light emission. Firstly, a bright green reflection (Observation 1) but also a light with a lower frequency and longer wavelength. The green light is absorbed by the red filter however the fluorescence – in the red part of the spectrum – which can be transmitted by the filter.
Nick went further and has tried all kinds of experiments including experiments with a ‘day-glo’ fluorescent orange eraser. In composite photograph below (1) shows the eraser in normal daylight, (2) shows the green laser spot – notice that the centre of the spot overloads the camera sensor and appears white. And (3) show the laser falling on the eraser. Notice that the light around the laser looks yellow. This is caused by the combination of green reflections and red fluorescent light.
What have I learned? Well, fluorescence is much more common than I had thought – paper and sweet wrappers both showed it. And this just reminds me, that if we can look at the world , and think about what we see, then we can notice unusual phenomena all around us: ‘the exotic’ masquerading as ‘the everyday’. Talking of which, tomorrow is another day, and I must get to bed! Goodnight.