Street fashion
Q: While walking at night through streets lit by low-pressure sodium
vapour lights (that is the orange ones), I have noticed a shimmering, horizontally
oriented pattern of lines in the glow surrounding the lamp. As I get further
from the light, the pattern gets coarser. What causes this?
(continued)
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* * *
A: The explanation published on 30 July is only correct if the observer
is at a great distance from the lamp. Although light from each atom in the
sodium vapour will be diffracted as it passes out of the finite aperture
of the lamp housing or your eye, the pattern of bright and dark fringes
is completely smudged out by the overlapping fringes of light from the other
atoms. Even if all the sodium atoms were in the same place, a simple calculation
shows that you would have to be at least 1 kilometre away from the lamp
to notice this effect.
Therefore the halo of bright and dark fringes observed around the lamp
is due to diffraction of the light around small drops of water suspended
in the air around the lamp. As the light passes by each droplet it is scattered
preferentially at a series of angles which are fixed by the size of the
droplet and the wavelength of the light. So some of the light which would
normally miss your eye changes direction and is seen. Since it appears to
come from outside the lamp, your brain interprets it as a halo.
Michael Brimicombe Aylesbury, Buckinghamshire
Twisted logic
Q: I noticed a strange lightning strike scar on a tree after a storm
last year (see photograph in The Last Word, 18 June). Why should lightning
follow a helical path like this one? (Continued)
* * *
A: Jonathan Reynolds, in his answer to why lightning follows a helical
path down a tree, raises the question of the origin of spiral grain in trees
shown by the alignment of the main conducting and structural elements of
wood at an angle to the axis of the trunk (see photograph in The Last Word,
30 July). This occurs not only in pine trees but in other softwoods and
hardwoods, most notably the horse and sweet chestnuts.
External causes of this spiral grain have been sought, wind being a
likely contender. However, the view is also commonly held that spiral grain
and other forms of complex grain development are a purely natural growth
pattern, formed by structural mechanisms in the tree. This view accounts
for variations in the direction and angle of the grain within a single tree
and between trees of the same species.
However, some species seem to be consistent in their growth pattern
鈥� the Douglas fir has a consistent left-handed spiral in those trees that
display the phenomenon.
Jon Hather Institute of Archaeology, University College London
* * *
A: Your published responses to the original query (see Last Word, 30
July) are correct except for one small point 鈥� there are no vessels in
pine trees. Indeed, the vast majority of gymnosperms do not have vessels,
but tracheids that perform both strengthening and conducting functions.
The reasons for inclined grain have been debated for at least 50 years.
Two common theories related it to the phyllotactic spiral of primordia in
shoot apices while the other attributed it to the angle of pseudo-transverse
division in cambial cells.
Others have considered the distribution of photosynthates and auxins
to be involved while an alternative is the reaction to growth stresses
as the tree enlarges. The angle of the grain changes throughout the life
of a tree, often from 5 degrees left of the main axis to 10 degrees right
in conifers. In some hardwoods interlocked grain implies a total change
of direction from left to right every year or two.
Whatever the cause, the result is a decrease in the strength of solid
wood when it is sawn. The strength declines proportionally to the angle
of the grain at the edge of the cut surface.
J. Burley Oxford Forestry Institute
Eggstraordinary
Q: Why does a very fresh egg take appreciably longer to cook than one
that is a day or more old?
* * *
A: Clear egg albumen coagulates irreversibly at about 58 掳C and
becomes white. When cooked in hot water, the heating is achieved by direct
heat conduction from the outside, through shell and egg white, and by convection
currents in the water of the egg white which contains the dissolved albumen
and surrounds the yolk.
A fresh egg has a much larger volume of this clear albumen solution
trapped between the mucin fibrils that make up a gelatinous lattice. This
lattice forms three concentric dense layers of egg white that surround the
yolk. A fresh egg has a deep and strong lattice in its middle dense layer
that causes the egg to sit up more in the pan when it is broken for frying.
After some days the lattice degenerates and the more watery egg can
be seen to flatten out in the frying pan. Convection is much easier in such
an egg and the heating of the albumen through to the yolk is accelerated.
Stephen Tomkins Cambridge
It鈥檚 a cracker
Q: Why does the end of a whip crack?
* * *
A: The crack is actually a sonic boom, caused by the end of the whip
breaking the sound barrier. This is possible because a whip tapers from
handle to tip. When the whip is used, the energy imparted to the handle
sends a wave down the length of the whip. As this wave travels down the
tapering whip it acts on a progressively smaller cross section and a progressively
smaller mass.
The energy of this wave is a function of mass and velocity and since
the energy of the wave must be conserved, it follows that if mass is decreasing
as the wave travels down the whip, then velocity must increase. Therefore,
the wave travels faster and faster, until by the time it reaches the tip
it has attained the speed of sound.
Mike Capp Oxford
* * *
A: When the wave reaches the tip of the whip it must be dissipated.
Some goes to the air and some into a reflected wave that travels back up
the whip.
At the point that the initial wave reaches the tip and is about to embark
on its return it undergoes a brief but enormous acceleration. The result
is that it moves supersonically.
Andrew Plant Lymington, Hampshire
This week鈥檚 questions
Seeing red: My garden has a tree whose leaves are a rich wine red colour
all year round. I thought that chlorophyll was green. If so, how does this
tree photosynthesise? Is the red variety less efficient, and therefore
less common?
Sion Hughes Beckenham, Kent
* * *
White water drinking: Why do anisette-based drinks, such as Pernod or
Sambucca, turn white when water is added to them?
Alexander Hellemans Amsterdam