# You have a light bulb in the middle of a room and .

1. Jan 31, 2006

### cluelesskiddy

You have a light bulb in the middle of a room and .......

Hi people, heres an easy question for you. I was asked this by a guy down the pub a few days ago, I think I know the answer, but they don't believe me.

If you have a light bulb switched on in the middle of a room and then you put mirrors on the walls is there any more light in the room or does there just appear to be more light in the room, but actually there is no more light in the room ?

Please state your reasoning and qualifications to answer this question. I have a degree in Physics and they don't believe me so why should they believe you ?

2. Jan 31, 2006

### chroot

Staff Emeritus
What does "more light in a room" really mean?

Does it mean:

a) An appearance to the eye of being brighter? This doesn't make sense, because you can't see photons which don't hit your retina. A room can be "very bright" even if you see none of it.

b) A larger number of visible-wavelength photons in the volume at any given time? This makes more sense, and is physically calculable, but does not correlate with the visual perception of "brightness" for the same reason given in a.

The room full of mirrors will certainly have more visible-wavelength photons at any given time than will the room without mirrors. This is obvious: mirrors reflect nearly 100% of the photons which hit them, while ordinary walls do not.

Will the room be perceived as brighter? Perhaps not, if the observer sits in a carefully chosen position so that none of the direct reflections strike his eye. (Eyes are directional detectors with a field of view of less than 180 degrees, after all.) After many hundreds or thousands of reflections, the mirrors will still have absorbed almost all the photons.

On the other hand, if the observer sits in an advantageous location, he'll see two or more images of the light bulb created by a single reflection. Instead of hitting the far wall and being absorbed, light going away from the observer is reflected back to him. In this case, the room will certainly seem brighter, in the sense that more photons will be hitting his retina.

My conclusion is that the mirror room will appear equally bright, or brighter, than the non-mirrored room, depending upon where the observer is located. I don't believe it can appear less bright however.

- Warren

3. Jan 31, 2006

### pallidin

Look at it this way: You have 100-watt bulb shining in a room with flat black paint on all walls, ceiling and floor. Then, you have the same 100-watt bulb in a room that is reflective(mirrors, etc...)
Now, a person enters the first(black) room. You can see them within a certain degree of illumination.
A person enters the second(mirrored) room. You can see them a great deal more from much higher reflective illumination.

4. Jan 31, 2006

### cluelesskiddy

What I am really interested in the answer to is:-

Is there actually more light in the room with mirrors in than the room without. YES or NO

5. Jan 31, 2006

### Gokul43201

Staff Emeritus
For someone with a degree in physics you should be a heck of a lot more precise in your question. From my best interpretation it looks like chroot already answered this in the affirmative in his part (b).

6. Jan 31, 2006

### chroot

Staff Emeritus
As Gokul indicated, the question is too imprecise to have a definite YES or NO answer. If you care to make the question more precise, the answer will be more meaningful. I've already attempted to answer your question in several ways, depending upon interpretation.

- Warren

7. Jan 31, 2006

### Mk

I think I understand your question, and I would say the answer is "yes." The mirrors reflect photons around that were released by the lightbulb's filament. However, it is completely unnoticeable to a measuring device such as the human eye.

8. Feb 1, 2006

### krab

With no mirrors, the light leaves the light bulb and hits the wall, where some of it is absorbed. With mirrors, the light is (almost all) reflected, so goes farther before being absorbed. So, yes, there is more light in the room. In fact, more hits your retina too, since there will be direct light from the bulb, and also light coming form the bulb's images.

9. Feb 1, 2006

### vanesch

Staff Emeritus
Can I reformulate your question classically: is the EM field energy HIGHER in the room with mirrors than in the room with no mirrors ? In other words, is the radiation field *more intense* in the room with mirrors ?

Then the answer is a definite YES.
There are different ways to arrive at this result, but probably the simplest one is this: when the room is "in steady state", 100W of heat energy needs to be converted into heat. The radiation field needed to impinge on a MIRROR to loose 100 W must be much higher than the radiation field impinging on a wall. In fact, if the "loss" of the mirror is 2% and the wall ~100%, then the radiation field needs to be 50 times more intense for the mirror case than for the wall case.

10. Feb 1, 2006

### DaveC426913

Guys, this is a PUB question - it needs to have a layperson's answer. (It can still have a *succinct* answer, it just needs to be plain language.)

In a room with black walls, virtually all light emitted from the bulb will immediately be absorbed. In a room with mirrors, that same light will bounce around more, making it continue to be useful as illumination.

(Think of shadows - in a black-walled room, person will have a dark side away from the bulb, and a strong shadow. In a mirrored room, the person will be lit from all sides and cast little shadow.)

The room will *most definitely* be brighter with mirrored walls. Anyone who has ever painted a room with dark colours knows perfectly well that the room is actually darker.

11. Feb 1, 2006

### Moonbear

Staff Emeritus
I usually hesitate to question the physicists on topics of physics, but that particular definition really makes no sense to me. The number of photons hitting your retina really doesn't necessarily correlate to the brightness of the room, but would rather depend on things like how close you are to the light source and whether you are facing toward or away from it. If I go stare into a bright light bulb, the fact that what my eyes are seeing is very bright doesn't mean the rest of the room has suddenly gotten brighter (assuming that same light bulb has been illuminating the room all along).

He asked if there's more light in the room, not how that light is perceived in terms of brightness. I think krab and DaveC have answered the question fairly well, at least in terms of the layperson's pub question.

Usually the question is asked by referring to the room being completely coated in a fully reflective surface, which is a tad different than saying the walls are covered in mirrors (but, yeah, it's a pub question, so the nuance isn't going to be appreciated unless you're hanging out in a pub near MIT). Most people (yeah, yeah, not the physicists) would think of a mirror as just your typical bathroom mirror...a piece of glass with a silver-colored coating on the back to make it reflective. Those are not 100% reflective. Likewise, if there are only mirrors on the walls and not the floor and ceiling, then there are more places for light and heat to dissipate out of the room. So, that would be quite a bit of a difference in terms of how much heat and light stay in the room compared to a room entirely coated in a completely reflective surface. I think you're supposed to ponder whether the light and heat keep accumulating so that the room gets infinitely brighter and hotter (of course this requires ignoring things like the combustion temperature of the materials the room is made from and when the light bulb is going to break). The question usually seems far more worthy of pondering when you've already downed a few pints of beer.

12. Feb 2, 2006

### Gokul43201

Staff Emeritus
For what it's worth, I think vanesch's post is the most illustrative proof of the solution - even though it applies only in the steady state. It's elegant; it tells you what the ratio of intensities is; and it's easily laymanizable !

13. Feb 2, 2006

### DaveC426913

Then perhaps someone could finish the job and laymanize it.

14. Feb 6, 2006

### kungfool

The room would be brighter to you I think, on account that more of the bulbs light is being directed into your eyes by all the mirrors. If the room had parabolically shaped walls and you were sitting at its focal point I think an ordinary light bulb would have the same effect on your eyes as staring into the face of god would. In a room without all the mirrors most of the light blows right past you and gets swallowed by the wood paneling and cigarette smoke.

The bottem line is that mirrors do not create light and must work with the light being thrown at them, so its really a question of how much light is entering your eyes, the rest of the light doesnt matter, as you do not have any physical access to it--the key is, if you see it, it is only because its light is entering your eye.

15. Feb 9, 2006

### Robin

Why do you think mirrors should make the room lighter ?
They don't scatter the light, including not scattering it in the direction of your eyes.
Just like the sky on the moon is all but black, becuase there is no atmoshere to scatter the sunlight.
The light bounces back & forth between the mirrors & gradually gets completely absorbed, since mirrors aren't perfect reflectors.
The only time you might happen to see more light, is if your eye is in the parallel beams in line with the light bulb but then you would probably block & or scatter the primary beam so you'd just see an illuminated image of yourself in the mirror & the light bulb in front of you.

Last edited: Feb 9, 2006
16. Feb 9, 2006

### leandros_p

You would have the same light in the room either with mirrors, or without them, if you are counting the energy of light, but you would have less shadows in the room with the mirrors !

Without the mirrors the objects in the room are lighted with a directional light, thus they cast shadows.

But in a room full of mirrors, the light illuminates the objects in the room from everywhere with the help of the reflectors, thus the lighted objects are without shadows and they are 3D illuminated (illuminated from every side/direction).

So if you make a light measurement with a simple photographic camera, in the room with the mirrors, you will get much bigger measurement of the light entering into the camera, in comparison to measurement in the same room without mirrors.

This is a technique which is used in photographic studios with big light reflectors in order to "erase" the shadows from the objects of a picture.

Leandros

Last edited: Feb 9, 2006
17. Feb 9, 2006

### Tide

Pardon me, but if the OP is asking for your "qualifications" to answer the question in order to settle a dispute that he is having with his barmates then isn't he obligated at least to reveal what HIS thinking is on the issue before doing his "homework" problem for him?

18. Feb 10, 2006

### pervect

Staff Emeritus
I thought you had a degree in physics? So why don't you do it?

It's a little hard to gauge an answer to the appropriate level of the questioner. It's especially hard when people "play games". Information is lost in the process of "laymanizing" an answer. Thus if you do have a degree in physics, you should be able to understand the answer that was given by Vanesch, and over-simplify it as needed for your particular audience.

Last edited: Feb 10, 2006
19. Feb 10, 2006

### DaveC426913

I am flattered. I would have this belief continue.

20. Feb 10, 2006

### pervect

Staff Emeritus
Hmm, somehow I confused you with the Original Poster (OP), "clueless kitty"