Watching a prog here,a bloke describing the effect of candle light on a room a mirror on the wall reflecting same ,a thunk struck, you have a room,you light a candle,you measure the light level in the room with a meter and get a level,you then bring in a mirror hang it on the wall and you see a perfect reflection of the candle emitting said light in said mirror,so you now have two candles one real and one a mere reflection but seemingly just as bright, question?you then take out your light meter and take another light level,would your light meter show the room as twice as bright now?
If it's white, and the detector isn't very directional (they usually aren't) then the mirror makes very little difference to the reflectance of wall it's covered (100% vs 80% typ.), so a very small change. (There are other factors like multiple reflections too).
If the walls were matt black and the candle was very close to the mirror, then yes, double.
In the case of the room that made me thunk it were Georgian wall paper,none to bright I imagine, I just thought the original reading would measure the light heading for the mirror the second reading would also measure the light returning from the mirror as well, ergo twould be twice as bright.
PS,I was imagining a perfect unidirectional light meter not a real one. Hmmm, lerrus assume a room with matt black walls ceiling and floor for the purpose of this thought experiment. Just seems to me that getting double the amount of light for nowt(not supplying any additional energy) seems to be contravening the laws of physics somehow.
Last edited by tony draper; 13th Jun 2012 at 23:15.
The meter receives a small amount of light direct from the candle - straight line between the two. Depending on the room shape and light meter position, most of the rest of the light will hit the meter after a few reflections. With 80% reflectance, that means 80% of the light after one reflection. 80x80=64% for light that does two reflections, etc. It's a complex calculation, but your mirror won't make that much of a difference in a white room (unless it's a huge mirror). The image of the candle will be less bright (intense) than the original because it's travelled further - to the mirror and back instead of direct.
further to your edit. If the candle is 1m from the light meter, and it's 1m to the mirror the other way. Direct light - path = 1m. Say light meter reading of 1.0 Mirror reflection = 1m there + 2m back = 3m reflection travels 3 times further. reflection appears linearly 3 times smaller than real candle and has 1/9 (3x3) the area, so 1/9 the brightness. 0.111 relative to the original, so the light meter with the mirror would read 1.111
Without the mirror, that light energy is absorbed into and warms up the black wall. Energy conservation still works.
Last edited by Fox3WheresMyBanana; 13th Jun 2012 at 23:21.
trying to think of this logically Mr D, and there's something about light reflected vs light absorbed.
I suspect the answer lies with energy rather than light.
First thing I learnt in "A" level physics was that said energy can neither be created, nor destroyed, so I reckon, in a way, we start with some total energy, the rest is reduced by absorption? (The Georgian wall paper).
So, holding a mirror up, just reduces the absorption, rather than increasing the original source.
Lighthouse lights use fresnel lenses rather than mirrors, and generally have done since 1822. The light is refracted into a few direction to increase brightness in those directions, but obviously at the expense of the light in other directions. Refraction is preferred to reflection as less light is absorbed, so more reaches the desired direction.
Lighthouses use prisms of glass or crystal to magnify and give direction to the single light source. No mirrors in the one closest to me and at one time in its history, it was the brightest lighthouse in the southern hemisphere.
would your light meter show the room as twice as bright now?
Here's a quick stab at it:
a. Candle output is constrained (simplified) so as to be constant and uniform in all directions.
b. Light meter is looking at some point on a surface in ( front of the mirror for case 2) that is directly illuminated by the reflection of the candle from the mirror and is also illuminated by the candle.
MN = Case with no mirror:
Light meter sees some fraction of the total light from the candle, the sum of light direct from the candle and light reflected from other surfaces, not including the mirror.
MY = Case with mirror:
Light meter sees the same amount of light as in case MN, with following differences:
1) does not see the light reflected from surfaces behind the mirror, because the mirror masks them out, mostly.
2) does see the light reflected from the mirror, intensity of which will depend on distance of mirror to candle (angle subtended), reflective efficiency of mirror, and atmospheric light losses due to the longer light path from candle to mirror to light meter, and secondary and n-ary reflections from other surfaces.
A few percent increase might be measured in a typical (if there is such a thing) diy practical case, with distances, mirror type and quality, and aperture of view of the light meter being the most significant variables.
Solar collecting systems using mirrors can add ten-percent or more for each mirror that is tracked to maintain focus on the collector target surface. Sun is a large-ish point source, but not collimated & wide rangee of wavelengths, of course.
Solution could be simplified considerably for analysis if a l@ser were substituted for candle, (collimated beam, point source).
A fully predictive analysis for serious purposes would need to be accomplished by ray-tracing, with all surfaces, elements, and variables fully specified. This is the method used for creating visual quasi-reality in synthesized images and animations from folks like PIXAR, NASA, etc. Ray tracing essentially involves taking a point on the illumination rediating surface, drawing a path from that to every other surface in the environment, treating each one of those points as a new radiating surface and tracing the radiation from there to every other surface in view from that point, and so on, for however many levels of reflection one desires, all of the above repeated for every point on the original radiating surface while summing up the energy levels for each individual surface point.... until the final result is a physically and visually realistic model of the entire space, which then can be portrayed in snapshot views calculated from any angle, at whatever image resolution is desired. The process is repeated from scratch, in toto, for each image frame, which really exercises the computing things that do the calculating to make a whole movie.
The intensity of the light decreases with the cube of the distance from the source - so a reflection, being 2 x further away, produces 1/8 the light.
Seems 'bout right for light radiating uniformly in all directions from a point source or from a spherical source.... the initial energy is dispersed over a sphere of ever-expanding radius.
Not so for collimated/focussed non coherent light sources (lighthouse) or coherent (narrow frequency band) sources like l@sers.
Candle is a funny-shaped, multi-multi frequency spherical source up close, but more like a spectrally-dispersed point source when far distant. Put a clever lens around a candle and much of the total light output can be captured and focussed on a point far distant, keeping the perceptable footprint of the source - and thus the light output available from the source) equivalent to that on the surface of the lens-mirror array -- out to 2x the focal distance. Atmospheric dispersion, seagulls, topless babes on sailboards are plausible attenuating factors, however.
Using l@ser light that is made to be collimated and therefore inherently nearly non-dispersive, along with first-surface mirrors (that do not do a refractive transform 3 or more times as the light waves transit thru, the way yer ordinary household mirror does), source-mirror dispersion can be very close to nil, meaning that the power reduction of the lotal light package thus reflected is also very small.
Ok taking this Gedankenexperiment further, say I'm in a 20ft by 20ft dark room with completely black walls - and I light a very small candle in the centre of the room. I'm near the door and note there's not enough light to clearly see the centrefold of a girly mag. Id only need a small amount of more light and I'd be able to comfortably see Miss June 2012.
I blow out the candle, grope for the door and go into another exact same room - except for the floor, the walls of the room are constructed by one huge mirror. I light the same candle in the centre of this room, move over to the door and hold my girly mag exactly as before.
Question - will see Miss June's naked torso clearly now?
As a supplementary question, I would like to ask that if one places a globe of water beside the candle as in a lace makers lamp, it focuses a beam of bright light, does this mean the rest of the room has less light in it? (assuming a matt black room)
Last edited by green granite; 14th Jun 2012 at 07:03.