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Light fantastic
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THE LAST WORD Light fantastic I have a luminous watch that I wear at night. Even though I put it on a south-facing window ledge to refresh it during the day, the dial has always been very faint. One night, finding it impossible to read the time, I flashed an LED bicycle light at it for a second. The watch immediately became very bright. How can one second of torchlight have more effect than a whole day of sunlight?
n Most glowing watch dials are coated with a paint containing a phosphorescent substrate such as strontium aluminate. When electromagnetic radiation strikes this, some incoming photons will have just the right amount of oomph to kick electrons in the atoms of the substrate up to higher energy levels. These excited electrons then fall back to a calmer state, emitting a new photon in the process. Whereas the energy exchange in fluorescent materials is almost instant, phosphorescent materials can tie up the photon’s energy in a “triplet state” that can take minutes or even hours to decay. The result is that phosphorescent materials continue to emit low-intensity light for a considerable time after the initial radiation charge. The average intensity of solar radiation on Earth is about 1.37 kilowatts per square metre: the solar constant. So every second, about 1 joule of energy would hit a luminous watch face of 7 square centimetres kept in
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
constant sunlight. However, if it remains facing south throughout the day in the northern hemisphere, it will not be in constant direct sunlight and will receive much less energy. Assuming the writer observes the dial soon after sundown, it is faint because the westerly setting sun has only indirectly, and weakly, illuminated the south-
“White LED light not only has more energy than sunlight, it also tends to be more blue-shifted” facing watch. In any case, the dial’s energy is continuously leaking away as electrons settle into lower energy levels. By contrast, an LED bicycle light has a typical power rating of 3 watts. If flashed on a watch face at close range for a second, practically all of the 3 joules of energy emitted will be incident on the face and the dial will glow brightly. White LED light not only has more energy than direct sunlight, it also tends to be more blueshifted, so it gives off higherenergy photons that are more likely to cause electron excitation in the dial. By contrast, sunlight consists principally of lowerenergy yellow and infrared light. However, the dial’s luminosity would almost certainly persist for longer after 3 hours of afternoon sunlight, say, than after a blast of LED light. This is because increasing the duration of irradiation increases the probability that each electron in
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to lastword@newscientist. com or visit www.newscientist.com/topic/ lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
the phosphor will be hit by a photon, so more triplet states can potentially be set up. Sam Buckton Chipperfield, Hertfordshire, UK
Some glow brightly after exposure to light but fade to invisibility in a matter of hours or less. Other materials are not so bright but last longer. I have a watch with a dial that is still discernibly luminous after 14 days. Exposing the watch on a southfacing window ledge all day could be counterproductive: after the phosphorescent material has become saturated, further exposure adds nothing to its performance, but sustained exposure to solar rays will age the material. Rather than leaving the watch to age in the sun, giving it a blast from an LED torch at bedtime might produce enough phosphorescence to last through the night. Looking slightly to one side of the watch might also help. There
n In the past, the paint on luminous watches contained a radioactive material and a phosphor. The radiation continuously excited the phosphor, so it would glow with the same intensity all night. As a schoolboy, I put a Geiger counter to my watch and was surprised at how strongly it reacted. Now, luminous watches contain a long-persistence phosphor that is excited when exposed to light. This lasts a while but decays exponentially, and is only really bright soon after excitation. So a whole day’s exposure to daylight counts for little – it’s only the last few minutes that matter. “As a schoolboy, I put a I was outdoors most of the Geiger counter to my afternoon, and half an hour after watch and was surprised sunset (in the Australian winter) how strongly it reacted” the luminescence was invisible. A quick blast with an LED torch is a higher concentration of rod made it bright again. High-power white LEDs emit a lot of ultraviolet cells away from the eye’s centre of vision, and these function at low light, and this is efficient at light levels. exciting the phosphor. Terence Collins Incandescent lamps work too, Harrogate, North Yorkshire, UK but are less effective. Guy Cox Australian Centre for Microscopy and Microanalysis This week’s question University of Sydney ON the SIDE Australia Why do sore throats often seem to switch sides? n Watch manufacturers use Tim Turner different phosphorescent materials on their luminous dials. Sutton, Surrey, UK
Will we ever speak dolphin? The latest book: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphins
THE LAST WORD Light fantastic I have a luminous watch that I wear at night. Even though I put it on a south-facing window ledge to refresh it during the day, the dial has always been very faint. One night, finding it impossible to read the time, I flashed an LED bicycle light at it for a second. The watch immediately became very bright. How can one second of torchlight have more effect than a whole day of sunlight?
n Most glowing watch dials are coated with a paint containing a phosphorescent substrate such as strontium aluminate. When electromagnetic radiation strikes this, some incoming photons will have just the right amount of oomph to kick electrons in the atoms of the substrate up to higher energy levels. These excited electrons then fall back to a calmer state, emitting a new photon in the process. Whereas the energy exchange in fluorescent materials is almost instant, phosphorescent materials can tie up the photon’s energy in a “triplet state” that can take minutes or even hours to decay. The result is that phosphorescent materials continue to emit low-intensity light for a considerable time after the initial radiation charge. The average intensity of solar radiation on Earth is about 1.37 kilowatts per square metre: the solar constant. So every second, about 1 joule of energy would hit a luminous watch face of 7 square centimetres kept in
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
constant sunlight. However, if it remains facing south throughout the day in the northern hemisphere, it will not be in constant direct sunlight and will receive much less energy. Assuming the writer observes the dial soon after sundown, it is faint because the westerly setting sun has only indirectly, and weakly, illuminated the south-
“White LED light not only has more energy than sunlight, it also tends to be more blue-shifted” facing watch. In any case, the dial’s energy is continuously leaking away as electrons settle into lower energy levels. By contrast, an LED bicycle light has a typical power rating of 3 watts. If flashed on a watch face at close range for a second, practically all of the 3 joules of energy emitted will be incident on the face and the dial will glow brightly. White LED light not only has more energy than direct sunlight, it also tends to be more blueshifted, so it gives off higherenergy photons that are more likely to cause electron excitation in the dial. By contrast, sunlight consists principally of lowerenergy yellow and infrared light. However, the dial’s luminosity would almost certainly persist for longer after 3 hours of afternoon sunlight, say, than after a blast of LED light. This is because increasing the duration of irradiation increases the probability that each electron in
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to lastword@newscientist. com or visit www.newscientist.com/topic/ lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
the phosphor will be hit by a photon, so more triplet states can potentially be set up. Sam Buckton Chipperfield, Hertfordshire, UK
Some glow brightly after exposure to light but fade to invisibility in a matter of hours or less. Other materials are not so bright but last longer. I have a watch with a dial that is still discernibly luminous after 14 days. Exposing the watch on a southfacing window ledge all day could be counterproductive: after the phosphorescent material has become saturated, further exposure adds nothing to its performance, but sustained exposure to solar rays will age the material. Rather than leaving the watch to age in the sun, giving it a blast from an LED torch at bedtime might produce enough phosphorescence to last through the night. Looking slightly to one side of the watch might also help. There
n In the past, the paint on luminous watches contained a radioactive material and a phosphor. The radiation continuously excited the phosphor, so it would glow with the same intensity all night. As a schoolboy, I put a Geiger counter to my watch and was surprised at how strongly it reacted. Now, luminous watches contain a long-persistence phosphor that is excited when exposed to light. This lasts a while but decays exponentially, and is only really bright soon after excitation. So a whole day’s exposure to daylight counts for little – it’s only the last few minutes that matter. “As a schoolboy, I put a I was outdoors most of the Geiger counter to my afternoon, and half an hour after watch and was surprised sunset (in the Australian winter) how strongly it reacted” the luminescence was invisible. A quick blast with an LED torch is a higher concentration of rod made it bright again. High-power white LEDs emit a lot of ultraviolet cells away from the eye’s centre of vision, and these function at low light, and this is efficient at light levels. exciting the phosphor. Terence Collins Incandescent lamps work too, Harrogate, North Yorkshire, UK but are less effective. Guy Cox Australian Centre for Microscopy and Microanalysis This week’s question University of Sydney ON the SIDE Australia Why do sore throats often seem to switch sides? n Watch manufacturers use Tim Turner different phosphorescent materials on their luminous dials. Sutton, Surrey, UK
Will we ever speak dolphin? The latest book: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphins