Ghostly flock
Question: I live in the countryside and I have noticed that when walking home
in the twilight the spring blossom on trees鈥攁nd even a flock of white
sheep鈥攁ppear almost to glow in the near darkness. Is there something
special about the twilight that produces this effect or is it just me? It is
certainly very beautiful.
Answer: This glowing effect is caused by the re-radiation or fluorescence of
light at longer (visible) wavelengths from scattered ultraviolet, violet and
blue light in the atmosphere. Remember that the daytime sky is blue because of
the preferential scattering of blue light known as Rayleigh scattering.
Many flowers fluoresce, and also reflect in near-ultraviolet light. This is
beneficial because pollinating insects can see these wavelengths. It is
particularly noticeable on flowers of the Geranium family, both the wild species
and the cultivated pelargoniums.
Advertisement
Because there is no other source of light present, the fluorescence shows up
proportionately more, giving rise to the extra-luminous effect described. It is
not an optical illusion; it is a real phenomenon.
G. N. G. Tingey
London
Answer: Both the sheep and the blossoms are fluorescing. The atmosphere
conducts enough low-energy ultraviolet light to produce the effect. The
fluorescence is more noticeable at dawn and dusk, when total ambient light
levels are lower. As a skier, I鈥檝e seen particularly striking effects on
fluorescent ski wear, which can glow so brightly at dusk that it looks like it鈥檚
plugged into the mains electricity supply.
I can see why fluorescent blossom would provide an advantage in attracting
insects. Indeed, flowers use markers visible in ultraviolet light to identify
pollen sources. Some nice examples are available at
http://vertigo.derby.ac.uk/BiologicalImaging/Shows/fys97/Eddie/biology.html#Guides.
However, I have no idea what advantage a sheep gains from being fluorescent.
Chris Harris
Wotton-under-Edge, Gloucestershire
Thirsty work
Question: Alcohol causes dehydration. How does it do this and how much water
do you have to drink to compensate for the effect?
When drinking a gin and tonic, does the water in the tonic compensate for the
effect of the alcohol, or does the fact that it is mixed with gin make it act
differently in some way?
Answer: The water balance of the body is achieved in the kidneys, more
specifically in the distal tubule and collecting ducts of the nephron and is
controlled by the hormone vasopressin, otherwise known as antidiuretic hormone
or ADH. This hormone allows water to be reabsorbed from the urine into the
blood, reducing the volume of urine produced.
Alcohol inhibits the release of ADH from the posterior pituitary gland and
therefore water is not reabsorbed from the urine, resulting in an increased
urine volume. If this water loss is not subsequently replaced by drinking enough
water then you鈥檒l experience dehydration.
The diuretic effect of alcohol depends on the amount of alcohol ingested and
not the volume in which it is dissolved. Therefore a unit of alcohol (listed by
the British government鈥檚 medical advisers as a pub measure of spirits, a
125-millilitre glass of wine or half a pint of normal strength bitter or lager)
should have a standard effect on an average human. But the duration of this
effect will be influenced by the rate of ingestion and rate of metabolism of the
alcohol, and there will be quantitatively different effects on different people
depending on how well their liver is functioning and the rate of consumption.
The degree of dehydration will depend on the fluid lost in the urine and the
volume of water ingested. So the greater the fluid intake, the less the
dehydration. Drinking tonic with your gin will partly offset the fluid loss
caused by the alcohol in the spirit. Drinking alcohol dissolved in large
volumes, such as bitter or lager, will have less of a dehydrating effect.
Of course, the hangover many of us have experienced the morning after
consuming alcohol is caused partly by dehydration and it can be avoided or at
least reduced by drinking a pint of water before retiring to bed.
Gwynfryn Chivers
Homerton School of Health Studies
Cambridge
Answer: Alcohol inhibits the release of ADH, so for any given water intake we
excrete more of that water in the urine.
Homer Smith (author of The Kidney: Structure and function in
health and disease, Oxford University Press) reckoned that 1 millilitre of
ethanol inhibited ADH to the extent that we lost an extra 10 millilitres of
water in urine. Using this calculation we can roughly calculate that if your gin
and tonic consists of 9 per cent gin, your kidneys will be neutral as far as
water balance is concerned. But because you still lose water via non-kidney
routes you will actually make a net loss. Drink wine, which is generally greater
than 9 per cent alcohol by volume, and you are even worse off.
Most beers are about 5 per cent alcohol by volume, so you benefit if that is
your tipple, although switching to whisky late in the evening鈥攁s many
drinkers may be tempted to do鈥攊s not a good idea.
Light beer or a gin and tonic made up to less than 2 per cent alcohol will
generally provide a net gain of water. Outside on a hot day, however, all bets
are off because sweating increases our water loss by a large amount. The tip to
drink water to offset such losses, especially before bedtime after a heavy
evening, is a good one.
Shane Maloney
Perth, Western Australia
This week鈥檚 questions
Count the rings: My partner鈥檚 father was born in Cameroon before proper
records were kept. He would like to know how old he really is. Is there any way
to determine the exact age of a living human being?
Peter White
Cardiff
Blitz power: Considering the enormous amount of electrical energy contained
in a lightning strike, why has no one tried to tap it for domestic use?
Lightning energy is grounded by sending wired rockets into clouds, so could this
energy be used?
Patrick Fahy
Kilmeena, Ireland
Nutty request: How would you check for the level of stickiness or
adhesiveness in different types of peanut butter?
Chris Hewitt
Ontario, Canada
Neon attraction: What causes the different colours you see in neon signs? I
realise that additives to the neon gas in the light tube might do the trick but
this does not really explain how in some displays the same tube show several
different colours.
Yolande
By e-mail, no address supplied