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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?

* * *

A: The shimmering of sodium vapour lights may be caused by single aperture
diffraction. This occurs when light waves from different parts of an aperture
(in this case the sodium light) meet. The waves interact alternately in
phase or antiphase along a horizontal line perpendicular to the light source.
When the waves are in phase, the light will appear brighter. As distance
from the streetlight increases the areas where the waves meet in phase will
become less intense and, consequently, will not appear to be as clearly
defined.

Samantha Copestake Stockport, Cheshire

Double vision

Q: I have a mirror made in about 1800. It produces a clear secondary
image above the main image, regardless of whether the object being reflected
is above, below or to the side of the direct line of sight. The displacement
is about 10 millimetres at 1 metre (slightly less towards the bottom of
the glass). The glass is about 6 millimetres thick and so could hardly produce
the effect by internal reflection even in one direction. Can anyone suggest
a reason for this effect?

* * *

A: The mirror is tapered and the effect is caused by internal reflection.
The second image is displaced from the first by about twice the angle that
the two surfaces of the glass vary from the parallel. Consequently, as you
move back from the mirror, the second image will be displaced by a greater
distance higher up the mirror with the effect reduced towards the bottom
of the mirror.

Incidentally, there should be more than two images, but they will be
much fainter and displaced progressively upwards by twice the amount of
the first displacement, then four times, eight times and so on.

Adrian James Bridge of Weir, Strathclyde

* * *

A: Glass is a liquid and not a solid, as most people believe. If a sheet
of glass was placed vertically and left undisturbed for a period of a few
hundred thousand years, it would eventually become a horizontal sheet spread
over the floor on which it was resting.

If the 200-year-old mirror was examined along its edge, it would be
found to be considerably thicker towards its base. This is true for all
glass. In older buildings, windows are thicker at their bases. The double
image is therefore likely to be caused by refraction between the two surfaces
of the mirror which are angled towards each other rather running parallel,
as should be the case. One can assume that if the owner of the mirror were
to rehang it upside down and wait another 200 years the image would be perfect
again.

D. Baines Hayle, Cornwall

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?

* * *

A: The helical path followed by the lightning strike in the photograph
is due to the spiral grain of the pine tree. Most trees have a twist in
their trunks, although this is not always obvious because it can be hidden
by the bark. The vessels in the sapwood just under the bark, which carry
water and nutrients from the tree roots, may circle around the stem once
or twice before arriving at the branches they are supplying. In this particular
case the lightning probably only struck a single branch. The current then
took the path of least resistance to the ground by following the water-saturated
vessels which supplied that branch. This also explains why fungal infections
in a single tree root may affect individual branches at different points
around a tree.

Barry Gardiner Forestry Authority Roslin, Midlothian

* * *

A: The lightning puzzle is explained clearly by the above picture,
which demonstrates that, under their bark, pine trees grow in a spiral fashion.
This, however, raises another question: how and why do pine trees grow like
this? Have they something in common with narwhal tusks?

Jonathan Reynolds Salisbury, Wiltshire

In a spin

Q: What makes the Earth rotate?

* * *

A: The Earth rotates simply because it has not yet stopped moving.
The Solar System, and indeed the Galaxy, were formed by the condensation
of a rotating mass of gas. Conservation of angular momentum meant that any
bodies formed from the gas would themselves be rotating. As frictional and
other forces in space are very small, rotating bodies, including the Earth,
slow only very gradually. The Moon, a much smaller and lighter body, has
effectively already stopped rotating because of the gravitational drag
exerted by the Earth, and now always keeps the same face turned towards
us.

Glyn Williams Derby

This week’s questions – Misterious

Misterious: On foggy days I can hear the sound of rain in my garden.
It is falling under a small Jacaranda mimosifolia and nowhere else in the
garden, nor anywhere else in my whole neighbourhood except under other jacaranda
trees. The precipitation is quite considerable, and my son tells me he has
seen something similar under she-oaks, which are, superficially at least,
dissimilar to jacarandas. Why does this happen?

Rod Hibberd Leichhardt, New South Wales

This week’s questions – Red eye

Red eye: When watching an electric bar fire immediately after it is
switched on and before the bars glow red, I observe a brief phase when
the exposed metal element coils appear to go matt black. A friend suggested
that I am one of the few people whose eyes are infrared sensitive. Could
this be the explanation? If not, what am I seeing?

Mike Rogers Berkhamsted, Hertfordshire

This week’s question – Ice screens

Ice screens: When ice forms on the windows of a stationary car during
the winter, the ice on the front and rear screens is hard and difficult
to remove while that on the door windows is softer and more powdery. Can
anyone explain this?

R Moorshead London