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Q: My nine-year-old son has come up with a question that has beaten my years of science education. Why can you see through water? Can anybody help us understand what it is about water – or any transparent medium – that lets light through it?

A: The question should be turned around the other way. What matters is not ‘what lets light through’ transparent media, but what stops it in opaque ones.

Hugh Dukes Luton, Bedfordshire

A: Light is electromagnetic radiation which exists as packets of energy called photons. These act like waves and the energy of a photon is precisely related to its wavelength.

Solid objects, or matter, consist of particles (atoms and molecules) which include electrons. Electrons only exist at certain energy states and a pho-ton which makes contact with a particle may excite an electron or molecule to a higher energy state. This can only occur if the energy of the photon is exactly the same as the energy required to raise the electron from one energy state to another. If this occurs, photons of this particular wavelength are absorbed and the matter is opaque to the light radiation.

Some substances are translucent. Photons can pass through matter if they have wavelengths which are not absorbed and many materials allow photons to pass, but some deflect the photons in many directions (scattering). Such substances behave as they do because waves can interact with objects which have a similar size to their wavelength. This is known as interference. Crystals are good examples of translucent substances because they are regular arrays of atoms, and the distances between their atoms can be similar to the wavelengths of the photons. So photons which pass through crystals can be deflected (refraction) and are scattered.

A material is transparent if it transmits photons which continue in their original direction, or are all deflected by the same change in direction. Such transmitted radiation retains any image which it had when it entered the material. Transparent materials contain very little crystalline material and their atoms and molecules are arranged in an irregular pattern making them glassy or amorphous. This induces negligible scattering because the net effect of any scattering that is induced is zero.

Materials may be transparent to some wavelengths of light but not others. Visible light has a limited range of wavelengths. Even so, different wavelengths are seen as different colours. Some materials may be transparent to some colours but opa-que to others which explains, for example, tinted spectacles.

Richard Courtney Cheltenham, Gloucestershire

It’s a gas

Q: New ÐÓ°ÉÔ­´´ has recently printed a number of letters discussing the scum that forms on the tea of some drinkers and not on that of others. This is not the only drinker-dependent phenomenon I have noticed. Australian beer is always gassed, and something similar happens with the foam that forms the head. Many drinkers end up with the full head of froth in their glasses when the liquid is consumed while others seem to ‘kill’ it after only a few sips. This appears to be independent of the beer brand. Can anyone explain it?

A: I have been told by several bartenders that glasses should never be rinsed with dishwashing materials as these kill the foam. This happens with all brands, as far as I know.

Martijn Hover Helmond, The Netherlands

A: Beer froth can be collapsed by an antifoam agent. These have two essential properties. First, they must be totally insoluble in the beer, and second, their surface tension must be lower than that of the beer. Animal or vegetable fats have these properties. So an Aussie pure in mouth and thought keeps the foam, but his mate with greasy chops causes collapse.

R. C. Watkins Oxford

Mixed reaction

Q: Why, when I am running water using mixer taps, does the hot water always get very much hotter once I have turned on the cold water tap, before settling down to a cooler temperature?

A: This is a demonstration of the difference between temperature and heat. There is a moving column of hot water between the mixer tap and you. With just the hot tap open, it delivers heat to your body at a certain rate. When you turn on the cold water as well, a cool column of water begins to enter the pipe which will hit your skin in a few seconds. But first it has to push the existing hot column through and, by turning on the cold tap, the flow rate is increased. The existing hot water will therefore deliver its heat to you at a faster rate, raising the local temperature of your body until the cool water reaches you.

Gordon Judge Horsham, Sussex

A: The effect can be shown in another simple experiment. When getting into a parked car on a cold morning the whole car will be at about the same temperature, but try touching the tyre with one hand and a large metal part with the other. The metal feels much colder. Because of the differences in thermal conductivity and capacity of metal and rubber, the heat flow from the hand into the metal is much greater than into the tyre.

David Miller Halesowen, West Midlands

A: If a gas-fired water heater is being used, there can be an increase in water temperature when the cold tap is turned on. The water heater automatically reduces the flame height if the rate of flow is reduced. How-ever, there is a lag between the flame height being reduced and the heating effect being reduced. This causes an increase in temperature because a reduced rate of water is passing through the system but is still being heated briefly by the larger flame. Turning on the cold tap brings about this effect because it can reduce overall mains pressure, reducing the rate of flow through the water heater and causing a transient increase in temperature.

Mark Wareing Braintree, Essex

This week’s questions

Trunk drunk: How do elephants drink? Elephants suck water into their trunks but how do they transfer the water to their mouths? If the elephant squirts the water from its trunk by exhaling how can it swallow at the same time? Try it; it’s impossible to breathe and drink at the same time.

T. A. Scott York

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Run rabbit: I was walking in the country with my dog when a rabbit crossed our path. My dog did not see the rabbit, but on reaching the point where it crossed our path he put his nose to the ground and followed the trail. After 10 metres he sensed he was going the wrong way and turned around. How could he smell which way the rabbit was running?

Giles Camplin Dorking, Surrey

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Anticipation: The ants under my patio become very agitated when exposed to the smell of the blue variety of methylated spirits. Pure ethanol or methanol do not have this effect. Does meths contain an ant pheromone or is there another reason?

Sjoerd Wadman Hertford