If 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.
鈥� If the cube was made out of perfect mirrors then yes, the light would bounce around forever. Unfortunately, mirrors are not perfect 鈥� 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.
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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 Romer, Great Bookham, Surrey, UK
鈥� 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 , 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 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.
鈥淎 fibre-optic cable uses total internal reflection to allow light to travel over long distances鈥�
Physicists have created a 鈥渕irror box鈥�, using a sphere. In a 鈥渉igh-Q microsphere resonator鈥�, 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 Smithwick, Cambridge, Massachusetts, US