How do mirrors increase light?

SPECULATIVE SCIENCE

If you shine a light into a mirror, does the fact that the light is reflected back mean that you are effectively doubling the amount of light in the room? If so, couldn't the energy problem be at least partially solved by filling rooms with mirrors? If not, why not?

Mark Lloyd, Bath UK

• A room lined with mirrors would be more brightly lit than one lined with the usual mixture of wallpaper, wood, windows, etc. The mirrors would reflect most of the light falling on to them back into the room, whereas wallpaper absorbs most of the light reaching it. This would mean that someone reading the Guardian in a mirrored room with a 60 watt bulb would see the page as more brightly lit than in a normal room with the same bulb. However, the difference would be slight if the room was normally furnished as usual with chairs, carpets, cats, etc. It would have even less effect on the energy crisis as a lot of energy is used for things like cooking, cleaning, computers, etc. and not just for lighting.

  Robin Wilson,

• Yes and no. Mirrors can't create light, only reflect it. Normally, much of the light from an electric light is absorbed by the walls of a room (and a lot is also reflected which is why you can see!). Mirrors are much more reflective and will bounce the light back so of course they can be used to increase the general brightness in a room. However, the light will not bounce around indefinitely; even mirrors absorb some of the light that hits them and eventually the reflected light would dim to the point that it made no difference. If this were not true, the entire world could be lit with one bulb and a very large number of strategically positioned mirrors!

  Max Wurr, Stanmore United Kingdom

• No, you would not increase the amount of light in the room. The light waves are merely being reflected off of the mirror. To add more light to the room, you would have to increase the amount of light waves being emitted. The reflection only changes the path of the waves, not the amount or intensity of the light.

  Kayla Iacovino, Tempe United States

• Yes. "Daylighting" is the science/technology behind avoiding waste of light when it is absorbed in walls and other surfaces. On the simplest level you paint walls light colours to reflect light and brighten a room (unlike a teenager who finds it hard to read in her dark painted cavern despite lightbulbs and daylight)

  Craig Napier, Eagle Heights Australia

• Firstly, a light source only emits a certain amount of energy, so we can dispense with the idea of solving the world's energy problems with mirrors. If you shine a light into a mirror, you simply divert the energy in another direction. If the questioner is talking about placing a mirror behind a light source such as a table lamp or candle, the mirror only reflects the light which would otherwise have been absorbed by the wall. There is more light in the room, to be sure - but only because you're wasting less of it.

  Robert Hanstock, Pangbourne England

• No. That would be a perpetual motion machine with more energy being available than you put in. The reality is that the mirrors absorb a small fraction of the light energy. If you want to do anything useful with the light in a box you have to shine it somewhere to do work (generate electricity, warm something, excite the rods & cones in your eye) and it is therefore converted to some other form of energy.

  Eamonn O'Riordan, Dublin Ireland

• No, if you throw an apple against a wall and it bounces back at you, you don't have more apples than you started with. Having said that, covering your room with mirrors will certainly discourage you from throwing things indoors. So that's a yes. I think.

  Phil Cohen, Sydney Australia

• Although you may not actually double the light, your eyes get two goes at it, or even more, so the room does appear lighter. In the past candles and lamps often had polished metal reflectors behind them or were installed in front of mirrors for precisely this effect, the room seemed lighter without any extra expense or heat (candles and oil lamps generate quite a lot of heat)

  Susan Deal, Sheffield UK

• Hmm, the recent invention of the auto absorb/discharge phalengial dark light energy pixel unit by Professor Specchio of Devon County Silver'd-Mirror college (a secret establishment in the heart of rural England), may well change established opinion. His pixel actually produces light from dark. In much the same way as anti-matter exists so does anti-light or 'dark light' Similarly as anti-matter cannot be in the presence of matter without the destruction of both, so light and dark light cannot be in the same space. Easy to see when you switch on a light. Before Professor Specchio's invention, such units were very large and required more power to operate than they could generate from absorbing darkness. Previously dark was seen as the absence of light now it is agreed that dark exists and actually destroys light. This had lead many to speculate on the possibility of a 'dark weapon' or a device dropped on a country that will absorb all the light and so lead to chaos. It is rumoured that such a device was trialed in Iceland over 60 years ago. In fact the international courts are still arguing about compensation for Iceland who still suffer from an annual 6 months of darkness after a test that all the involved powers claim never occurred. Within 10 years the first domestic application will be launched. In the future, if you wish to nap during the day, you will simply turn on your dark bulb to plunge the room into darkness. A better use will be to limit the amount of light in countries such as Australia and so restrict harmful UV rays.

  Taki LePeace, Odourville, USA

• Yes - this is the technology used in lighthouses, most flashlights, and your car headlights. You're not creating more light, as has already been said, just re-directing it back into the space you want lit.

  Liz, Samoia, US

• There have already been sufficient amount of explanations and facts proving that amount of light is not being doubled when it gets reflected by a mirror. If at all it does then the "Law of Conservation of Energy" would become false !

  Ram Kumar, Tenkasi, India

• If I put a mirror behind a candle I see two identical flames. How can two flames of the same size not give more light than just one of them?

  Mike, Roadtown Virgin Islands

• I'm certainly not a scientist -- but I can tell you that the wattage "effect" certainly is noticed, as i make judicious use of mirrors and use the smallest wattage possible. Lifelong habit. My cooking is all done on electric, only "high" or "off." taking far less time and using residual heat to finish off the dish. I suppose the mirrors make use of "residual light." My electric bills have been forever less than anyone I know of -- despite having all creature comforts and not being miserly. I just live "differently," and what i assume is "smarter." Those electric bills are, in fact, lower than those of folks having both gas and electric, thinking they'll save using gas. BIG huge help is flipping the breaker off and on for the hot-water heater if it's electric. 15 minutes before a bath, flip it on. Immediately afterward, flip it off. You'll have hot water enough for a day and possibly half the electric bill by following these principals.

  Emma Peel, El Paso Texas USA

• Light is a tricky thing, a light placed against a mirror will reflect almost all of the light however the light itself will be in the way causing a bit of a shadow, wasting more of the light. As you get farther from the light source the amount of light drops exponentially, so mirrors 10 feet from a bulb may only receive 10 percent of the usable light energy so the reflection at this point will be a little less than 10 percent of the original power. Then this 10 percent reflected drops exponentially again.

  michael sediway, san diego usa

• Mirrors absorb less light. They absorb some, significantly less. While the light cannot go on forever, it can certainly be spread further with well placed mirrors. The fact that the light is being reflected rather than absorbed means it is there longer. While it is technically the same wave, it is longer, therefore it is having more light in the room. You are not doubling, or adding light, you are just losing less light than what you were if you had say a black wall.

  Mikey A. Amith, Valdez United States

• The combination of (AgNO3 + NH3 + C12H22O11 + NaOH) create a reflective absorption surface capable of creating an exact opposite image by a 180° view. Depending on the perspective of an individual viewer it will appear as if one can view such distance as one would physically. In these areas of the surface are what allow light to absorb and and reflect emitting a wider range of light, herein giving the semblance of "more" light.

  Zyan Kol, Pumbaya Malta

• I just observed a very bright bathroom. No lightbulbs were lit. The morning Sun was shining in the window and the rays were hitting the vanity mirror, then hitting a smaller mirror on the opposite wall (next to the window). I feel positive the level of light in the room was MUCH higher with all the light rays bouncing around. Isn't there a a test meter that can measure the amount of general light in a room?

  Steve Robbins, San Antonio USA

• If I hang a picture of the sun in my living room, can I get a tan while I watch cartoons?

  Herb Eaverstank, East Overshoe US of God Damned A!

Add your answer

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Dear Sev, first of all, a simple thing. Light in the vacuum always moves by the speed 299,792,458 meters per second: this fact is exactly true because of the modern definition of one meter. The speed of light in the air is just 0.03% smaller than the speed of light in the vacuum. Nothing ever travels faster than light in the vacuum.

Second, mirrors typically reflect less than 100% of coming light - something like 70%. But that's not the main problem here.

Third, the energy is conserved, so you can't produce much more "light" by putting mirrors. You can't illuminate 100 households by having 1 light bulb and "copying" it by mirrors. Why not? While you may increase - and almost double - the "amount of light" that is hitting a particular area, this fact is (more than) balanced by the fact that the light that would be absorbed by the area occupied by the mirror itself is not absorbed.

So when it comes to the energy budget, ideal mirrors (that reflect 100% of light, just for the sake of simplicity) only rearrange the distribution of light - which areas finally absorb it and which areas just reflect it. The total amount of light that is absorbed is given by the total amount of light that is emitted on the light bulb - and it only depends on the light bulb (and its power).

If you consider light as a "practical thing allowing us something to to be seen", then you want the light to be reflected, e.g. by a book. But the counting for a book that reflects some light - so that we can read it - is similar as the counting for an object that absorbs the light. Mirrors may increase the amount of light reflected by a particular book, but they may not increase the amount of light reflected by the whole room, assuming it has a uniform albedo.

If you look at a light bulb and a nearby mirror, you may see "two light bulbs" and a doubled amount of light, so to say. But this is only true from certain directions. From other directions, the outcome is different and often opposite. For example, if you place your eyes behind the mirror, so that the light bulb is on the opposite side of the mirror than you, then you see no light bulb directly - and no unreflected light from a light bulb (and no light bulb light reflected only by mirrors).

This lesson is much more general. Mirrors - and any other gadgets - may move energy from one place to another, or transform it from one form to another. But they never change the total amount of energy.

Cheers LM