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Butterfly Light - Download this Text

By Professor Jonathan K night

Some suggestions for further thinking and discussion

1. Things glow when they are heated – take a look at the furnace in the video!! What colour do they glow when heated? Compare an electric kitchen hob, a coal fire, and a high-temperature flame as used by a welder. Which of these has the higher temperature? Which is closer to the temperature of the furnace in the video?

• All things above absolute zero give out electromagnetic radiation constantly. This is called blackbody radiation. When the object is quite cold – like room temperature or below – the wavelength of this radiation is very long, but when it gets hotter shorter wavelengths are emitted. Eventually, when the body is pretty hot (say, 800 degrees C) the emission is partly at visible frequencies, in the red. When the body gets hotter still, more visible radiation is given out. The emitted light becomes much brighter. When the body is very hot – say, 1500 degrees C or above, the emitted light is a mixture of all visible colours as well as infra-red. All the colours added together make the light look white.

2. Light from the sun looks white as well – and can burn hands with a magnifying glass. Why is the light from the sun white? How hot is the sun? Why does sunlight burn if it is focused using a lens?

• The sun emits white light because it is very hot indeed – much hotter than anything else in the video. The sun emits a lot of invisible (infra-red) radiation as well as the visible light which we use for seeing. Some of this light is reflected by our hands (which is why our hands have color) and some of it is absorbed. Some of it even passes through our hands – X-rays, for example – but there are very few of these getting to the surface of the earth from the sun. The light which is absorbed by our hands gets converted to heat. Normally, this heat is spread out over quite a big area, and so our body can get rid of it without the skin temperature getting too high. However, it you take the light from a large area (say, the size of a lens) and focus it to a small spot, then you deliver heat to that much smaller area faster than your skin can get rid of it, and the skin temperature rises. After a few seconds, if the sun is bright, the temperature is high enough to burn.

3. The fibre can be used to convert laser light into “rainbow” light – white light which contains all the colours of the rainbow! How can the fibre change the colour of light? Does the colour of light change when it passes through a window? What is the difference between the window glass and the short pulses in the fibre?

• Light is made up of photons, each of which has a fixed and distinct energy or wavelength (colour). To generate the rainbow, the total energy must be re-distributed amongst the photons. Practically, this cannot happen in vacuum – it must be “mediated” by the presence of a material. Even then, it does not happen easily or readily, but only when the light fields are very strong. The intensity of light in the fibre is about 100,000 million (!) times as bright as the intensity of sunlight. When the intensity gets that high, things can react strangely. One useful analogy is with a swing (the electrons in the material) being pushed by someone (the electric field of the light waves). If they push gently, the swing rocks back and forth in a simple way, oscillating at just one frequency. Now, imagine pushing 100,000 million times as hard! The swing might do something pretty strange as well! It would no longer oscillate at just one frequency! When this happens in a material, the electrons get shaken about in new ways. They use some of the incident energy to generate new frequencies (or colours) and so cause the rainbow of light you see in the video. When the driving force is light, this type of response is called “nonlinear” and we talk about a “nonlinear optical process”.

4. What applications can you think of for any of the fibres demonstrated in the video? Why would the fibres in the video be better than conventional fibres?

• Some properties of the new fibres which might be useful for applications are…hollow cores will not be damaged by high intensities. Vacuum core would not have any nonlinear response. Hollow cores might have very low attenuation (attenuation in fibres is usually caused by scattering). Solid-core fibres can be used to generate laser-like white light supercontinua. Some general areas to consider would be in medicine and biomedicine, surgery, materials processing, marking, projection television, telecommunications…

Jonathan Knight
March 2007

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