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Still waters
The back pages Almost the last word Why do ginger biscuits go soft when exposed to air but cakes go hard?
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?
LLEEROGERS/GETTY
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.
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 54 | New Scientist | 3 August 2019
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
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 socalled 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
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. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms
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?
LLEEROGERS/GETTY
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.
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 54 | New Scientist | 3 August 2019
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
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 socalled 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
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. ❚
Want to send us a question or answer?
Email us at [email protected] Questions should be about everyday science phenomena Full terms and conditions at newscientist.com/lw-terms