The last word–

LIGHTBOXIf you have a hollow cube whose

internal sides are made of perfectly

reflecting mirrors, and you switch on

a torch and wave it around, then

turn it off, would the light keep

bouncing around the cube or would

it go dark? If it goes dark, where does

the light go? If the light continues to

bounce around, how long would it

do so? This question has perplexed

me since I was a boy.

● If the cube was made out of perfect mirrors then yes, the light would bounce around forever. Unfortunately, mirrors are not perfect – some of the light that falls on them is absorbed. A domestic mirror reflects only about 80 per cent of the light falling on it. If you stand between two large mirrors, set up so you can see the series of reflections, you find they soon get noticeably darker. Even a high-quality telescope mirror only reflects between 95 and 99 per cent of the light.

The other factor to consider is the speed of light. In a 1-metre cube made of mirrors with 95 per cent reflectivity, light would be reflected 300 times in a millionth of a second and lose 5 per cent of its brightness each time, so reducing it to under a millionth of its original brightness.

Where does it go? As the light is absorbed it warms up the surface that absorbs it, so the cube would be ever so slightly warmer.John RomerGreat Bookham, Surrey, UK

● The answer is yes, if the mirrors are perfect and there is absolutely

nothing inside the box, including air. Unfortunately, common mirrors are imperfect. After a large number of bounces, which occur quickly given the speed of light , the light would be almost completely absorbed by the mirror. Light would also be absorbed by the air within the cube, but to a much lesser extent.

Perfect mirrors do exist, relying on the principle of total internal reflection. For example, light travelling from water to air can only escape at steep angles. At shallower angles, the light is perfectly reflected back into the water. This can be easily seen in an aquarium or even a glass of water. Look at the surface of the water from

a shallow angle underneath and you will observe an image due to total internal reflection. Interestingly, in this case the mirror is neither the water nor the air, but rather the air/water interface.

In order for total internal reflection to occur, the material in which the light is travelling must have a higher refractive index than the adjoining material. However, although the reflection is perfect at the interface between the two, the first material will absorb some of the light as it travels through. So unfortunately there are no perfect mirror boxes, but you can come close.

A fibre-optic cable uses total internal reflection to allow light to travel along it with very little loss over long distances. Such a cable is analogous to a mirror box with two ends extremely far apart. And

diamonds are cut to take advantage of total internal reflection, so light will bounce around many times before escaping undiminished . This gives diamonds their brilliance.

Physicists have created a “mirror box”, using a sphere. In a “high-Q microsphere resonator”, light is trapped in a tiny glass sphere, continuously bouncing off the inside surface at shallow angles by total internal reflection.

Light is continuously pumped into the sphere, and because no light can escape and only very little is absorbed by the glass, the light inside the sphere builds to very large intensities.

These microspheres are used as very sensitive sensors, detecting impurities landing on their surface because of the way the impurities disrupt total internal reflection.Quinn SmithwickCambridge, Massachusetts, US

THIS WEEK’S QUESTIONSIce belt

I took this half-full tub of pasta sauce out of the freezer to thaw (see Photo). The sauce had been put into the freezer when warm. Why did the ring of ice not stretch from the level of the sauce to the base of the tub?Ewan DrysdaleHead of ChemistryKimbolton SchoolHuntingdon, Cambridgeshire, UK

Finger clickin’ good

What makes the noise when you click your fingers?Andy HeadBelper, Derbyshire, UK

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