New_Scientist_3_08_2019

54 | New Scientist | 3 August 2019

Special effects

I took this picture of the sky over

Wimbledon, London, on 13 May at

around 6 pm, two and a half hours

before sunset. How are these

spectacular effects created?

Bernard Burton

Wokingham, Berkshire, UK

The effect is known as irisation

or iridescence and is caused by

diffraction of sunlight or

moonlight by very small particles.

These are usually water droplets

but, in theory, any particles

that are sufficiently small and

numerous, such as very fine

volcanic ash, can create the effect.

Diffraction, or the bending of

light rays by obstacles in their path

such as water drops, is the same as

that produced by a small aperture

in a screen, according to Babinet’s

principle. The amount of bending

varies with the wavelength, so

resolution into different colours

occurs in the case of visible light.

David Muir

Edinburgh, UK

The clouds that look like ripples

on a sandy beach are called

altocumulus undulatus.

The rainbow fringe effects

have formed in a layer of thin

cloud, which is probably a patch

of altostratus translucidus. If a

cloud’s tiny water droplets or

ice crystals are uniform in size,

sunlight is diffracted to produce

beautiful pastel colours. Each

wavelength of light is dispersed

by a different amount, creating

the band effect.

The ribbon of cloud running

roughly parallel to the slope

of the first roof is a persistent

condensation trail from the

exhaust of an aircraft. Cloud

experts sometimes call this

cirrus homogenitus, a name

that indicates its origin.

Mike Follows

Sutton Coldfield,

West Midlands, UK

The ripples in the clouds are

created by wind shear, when

different layers of air move at

different speeds. This causes air at

the boundary to oscillate, creating

ripples like those made by a

breeze on the surface of a pond.

The striped pattern in the cloud

is due to an oscillation called the

Kelvin-Helmholtz instability. As

the air from the lower layer moves

upward, it cools and condenses,

creating cloud. As it moves

downwards again, the moisture

will re-evaporate, resulting in a

gap in the cloud. Pilots normally

take a detour around these so-

called billow clouds because they

betray the presence of potentially

dangerous turbulence.

Chris Daniel

Llansanffraid Glan Conwy, UK

The iridescence effect is named

after Iris, the Greek goddess of

rainbows. But unlike rainbows,

which are due to reflection and

refraction and which give ordered

colour displays, iridescence is

caused by diffraction.

The effect happens within

10 degrees of the sun or moon

when light scatters as it passes

through layers of very small and

similar sized water droplets or

ice crystals. The proximity of

the sun to the cloud as seen by

the observer can wash out the

effect unless the sun is shielded

from sight, as it is in this photo

by a building.

If the sun is between 10 and

40 degrees away from the cloud,

the effect may be due to

interference similar to that seen

in soap bubbles. Light is divided

as it passes through the water

or ice particles and recombines

with a phase difference that

is detectable by a change in

colour or brightness.

Iridescence is just one of

dozens of beautiful and intriguing

effects, collectively known as

photometeors or atmospheric

optical phenomena.

A key question

We keep the key to our holiday

cottage in a four-digit key safe

by the door. Because of my poor

eyesight, I move only one of the

digit rotors when I leave, so it is

easier to open next time. Most

visitors rotate all rotors on leaving.

Is my behaviour riskier? (continued)

Peter Bauer

Egg Harbor City, New Jersey, US

The risk is quite high, regardless

of how you lock it.

If a rotary barrel lock of this

type has only one rotor scrambled,

simply put tension on the lock by

pulling on it. All the rotors except

one will then be easy to move.

Turn that one until the resistance

disappears and the lock will open.

This approach usually works

even if all the rotors are scrambled.

Due to manufacturing variations,

one rotor will carry more of the

tensile load than the others and

resist most when you try to turn it.

Turn that one until it releases

and another will take its place.

Repeat with the next most

resistant rotor and so on, then

the lock will be open in seconds.

You may have to pull hard to sense

slight differences in resistance

and it may not work with very

precisely machined locks, but this

method has always worked for me.

My wife uses one of these locks

to secure our bicycles and I never

had to learn the combination.  ❚

This week’s new questions

Hard baked Why do crisp ginger biscuits go soft if left

exposed to the air for a couple of days when other baked

products, such as cakes and bread, go hard? Edward Sexton,

Leeds, UK

Still waters When a container of liquid is rotated on its

base in an upright position, the liquid inside doesn’t turn

with it and remains static. Why is this? Stefan Badham,

Portsmouth, Hampshire, UK

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Why do ginger biscuits go

soft when exposed to air

but cakes go hard?