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?
:rolleyes:

Interesting question. Give me some time to reflect on it.

What's the wall colour?

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.

Wot Fox3 said.

Even the Pope agrees. Better accept that or you won't be expecting what happens next

Spanish Inquisition - YouTube

Having spent some considerable drinking time pondering this question Mr D, I suspect it's not such a matter of twice the light now, but more, half the light before.

But now I'm thinking, if you put two mirrors up, was it a quarter of the light before ?

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.
:uhoh:

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.

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.

What about a lighthouse? One light source and lots of mirrors produces a lot more light. So it seems.

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.

Sorted, thanks.

:ok:

would your light meter show the room as twice as bright now?

Here's a quick stab at it:

Assuming:

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.

Practical result:

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 laser 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.

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 lasers.

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 laser 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)

Just as well we invented the electric light bulb up here methinks.:rolleyes:

Question - will see Miss June's naked torso clearly now?
sure and you'd be totally confused, as there'll be several of them. :E (the exact number will depend on how many reflections you'll be able to absorb, I guess).

is the candle on a conveyor belt ?


ok.. how about one takes the mirror and makes it slightly concave, in order to focus the light emited from the candle...

... hang on, i'm on to something here.. if i could find a way to make an electrical candle, i could mount perhaps two of them on the front of a horseless carriage.....

Will looking at girly magazines in candle light can make you go blind? :rolleyes:

The latest research indicates that it will make you short-sighted, not blind. But then so will reading the Bible in poor light.

The light (from a girly mag or anything else) spreads out, as arcniz says, over the surface area of a sphere, which is 4*PI*r ^2, so squared, not cubed as mentioned earlier.

You can't be serious!
http://t0.gstatic.com/images?q=tbn:ANd9GcRX3T58lPd64vGVawS8Ros7poPf_9-5tZ6C6KahCUg3qbqFnKmk2w

I think its all to do with light absorption and reflection.

Mirrors don't reflect 100% but they do reflect alot more than a gloss wall and more than a mat wall.

You could work out an equation that took the size of a mirror room the amount of reflected light the power of the candle and work out the increase in light at points in the room.

I think they have computer lighting design programs now that do it for interior designers.

In fact here is a free one

DIALux evo 1 - BETA Download (http://www.dial.de/DIAL/en/evodownload.html)

But what if it is a candle in the wind?

. . . or even, with the wind.:}



Remember, the photon that comes out of the mirror is not the one that went in.



I thought it important you knew that.:8

Remember, the photon that comes out of the mirror is not the one that went in.

I've always found checking the serial numbers to be too much bother. ;)

will see Miss June's naked torso clearly now?


You might be able to, but none of us can.

Remember, the photon that comes out of the mirror is not the one that went in.

.. but then again, it might be the same one ....

.... cos there might only BE the one. See discussion here!! (http://www.physforum.com/index.php?showtopic=8190)

One of Pprune's better light hearted threads!
:ok:

(the exact number will depend on how many reflections you'll be able to absorb, I guess).

On such occasions, it helps immensely for one to be in a reflective mood...

...reflective mood...

Foregoing was meant to be a shining example of illuminating banter, but a look backward in the rearview shewed 'er 'ready done.

Look it's quite easy. You light a candle in space, well, no, not in space as there's no air, well, you illuminate a one-candle-power electric bulb in space. Somewhere well away from the earth. Maybe at one of the Lagrange points. Now remove yourself from the vicinity of the bulb and watch what happens. Nothing, right. Now silver one side of the bulb and try it again. Aha ! It moves off to one side, doesn't it. Now get the original bulb and scrape the paint off, place a mirror 10cms to one side and try the experiment again. See which way the bulb moves. Then place the bulb between two mirrors, one each side, and try it again.

That should answer your question, Mr D. If it doesn't go and contemplate the June 2012 playmate, or a picture of her if you can't get the real thing. Place her at the Lagrange point, and see which way she moves.....

What's a playmate? Is it a bit like a Black and Decker, or more like a Karcher?

As I sit here in my cell, I now realise that my mate said "go to B'n Q , get a Black & Decker.......being a bit hard of hearing, I thought he said " black, and Deck her "

Hat, coat.......

Hmmm,let me see, a individual photon leaves the candle flame trundles across the room at the speed of light hits the mirror(we will assume the surface is silvered not the back) screeches to a halt instantly,does a 180 then accelerates back up to the speed of light and trundles back across the room to the candle flame.
On the other hand a individual photon leaves the candle flame as simultaneously a individual photon leaves the candle flame in the mirror they meet at the surface of the mirror and change places?
:confused::rolleyes:

In fact, the reflection model only fully works if we consider the photon to have taken every possible path (not one of them, but all of them), at the same time, or even going backwards in time, and work out the probability of each possible path happening.

Quantum ElectroDynamics (QED) is such fun. It's impossibly weird, but works.

Note: this is the model. What the photon actually does is anyone's guess!

Fox3 has it right, of course, but there is a further consideration. If the walls of the room are perfectly reflective, the light will continue to accumulate without limit until you measure it. Then the measuring apparatus will absorb some as might the source itself.

After an excellent landing etc...

I suppose amount of light in a room lit by a candle depends on the albedo of the mirror & walls.

Small mirror & matt black walls would be darker than huge mirror & gloss white walls.

On the other hand a individual photon leaves the candle flame as simultaneously a individual photon leaves the candle flame in the mirror they meet at the surface of the mirror and change places?

Y'aint thinkin' fourth-dimensionally (i.e. the Time component)
Drapes.

A photon takes time to go from a candle and reach the mirror
but the tiny photon itself (if it wore a Seiko) will measure the
time taken for the journey as nil (see Bert Einstein, genius).

In your above example the wave theory of light (if you was to
go and apply Jimmy Maxwell's equations to light) and not the
photon model will solve your specular light question, and that
there ain't no transpositioning of photons in the way you are
chewing the cud over.