Probably a Simple Physics Question - Settle Our Argument

DampCat

DampCat

DampCat

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Right, we've been arguing in the studio today about something that all of us should know.

If you have a light shining on an area (the size of the area does not matter, assume that the light covers the area completely), and then you add another light of identical power and spread... does the area become twice as bright?

I'm saying no. The light isn't accumulative. The kind of situation where it would appear that you are "doubling the amount of light" would be, for example, in your sitting room where adding a second light increases the spread of the illumination... but the strength remains the same, just over a wider area. My example for why it ISNT the case was "If you had a stereo with one speaker plugged in, adding a second speaker wouldn't make it twice as loud"... I dont know if it's a foolproof example but it conveyed what i meant (obviously this is assuming that the speakers are independantly powered and adding one doesnt reduce the draw of the first).

My colleague is suggesting that adding identical lights will increase the power of the available light, using vague and confusing examples of previous photography problems.

Thanks OCUK.
 
Hmm, think about what you're saying for a second. Take two flashlights and point them at a wall. Overlap the beams. Is it twice as bright at the conversion points? Yes, indeed!

Light a candle. Test luminosity in the room. Light another candle and place it directly next to the first. Is the room brighter? Yes, indeed!

Add twice as much energy to anything and you will get twice the results. I think there's an equation for that..... ;)

As far as speakers (sound pressure), that's a different issue. It requires twice as much energy to increase the pressure by 3dB (something like that).
 
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Surely an identical beam of light will just merge with the first with no observable result? :(

If I was doubling the power output of the first light, it'd be twice as bright, but a separate identical light isn't producing twice the power, its producing the same power... isnt it?
 
The example of speakers is problematic as you're not taking into account wave interference (both constructive and destructive).

I have absolutely no idea if this extends to light because my grounding in physics doesn’t involve this area and the physics of light can get reasonably complex at the quantum level which I think would be the scope for determining the answer in this case. Hopefully someone can come in with an answer based with theory.

My answer based in day to day living would be that of a LED torch. They use multiple LED’s for greater intensity of light, so I’d assume that yes, adding a second light source of identical value would increase light intensity, doubling doesn’t seem at all likely but an increase of intensity definitely does.
 
starting with basic physics where else does the light and heat energy from the second bulb/flame/whatever go ?
 
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"andy" said:
starting with the basic physics where else does the light and heat energy from the second bulb/flame/whatever go ?
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Entertaining the idea of light as a wave (that'll upset physicists but its useful to explain here) and that there is phase coherence wave interference can occur. If the second light were to superpose with the first light source it can disrupt the propagation of the first reducing the amplitude. The first can also do the same to the second, reducing the amplitude of the second. This results in the final result of an overall increase in intensity at the point of observation but not a doubling effect.

Simply put, the movement of light can disrupt the other movement of light providing a higher level of intensity than any single source alone but significantly less than twice the intensity at the point of observation.

The full answer is probably hidden in this very famous experiment but I'm being too lazy to find it (just turned 1am here): http://en.wikipedia.org/wiki/Double-slit_experiment
 
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The OP seems to be confusing "twice as loud" for loud-speakers with "appears to be twice as loud". Actually when you add the second speaker you do double the sound energy, but since the human ear hears on a logarithmic scale it only sounds a little louder. Much the same happens with light as the human eye is so good at adjusting fro brightness: the light level will be twice as high, but you won't think of it as such.


M
 
Raist said:
As far as speakers (sound pressure), that's a different issue. It requires twice as much energy to increase the pressure by 3dB (something like that).
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And as we all know +3dB is a doubling.

Adding a 2nd light does make it twice a bright, but due to what Meridian says, it won't look that impressive to the human eye. Get your SLR out and look at how its light metering changes. You should only need half the exposure time with two lights on rather than one, all else being equal.
 
Interference isn't really relevant in this case. Overall, the photon flux will be doubled by adding a second lamp, and so, therefore, will the scattered photon flux, meaning the surface is radiating twice as much light. Whether we perceive that as a doubling in brightness is another question, though.
 
Don't know!

The idealised 50% contructive/destructive interference thing probably due to coherent, monochromatic rays, so I bet with two torches it doesn't really happen the same way.

Try it out?
 
clv101 said:
And as we all know +3dB is a doubling.

Adding a 2nd light does make it twice a bright, but due to what Meridian says, it won't look that impressive to the human eye. Get your SLR out and look at how its light metering changes. You should only need half the exposure time with two lights on rather than one, all else being equal.
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This. CLV101 and Meridian have the right of it.

You can see this with the use of stage spot lighting. Also the more lights being shone on the same point the more energy imparted to the surface being lit, to see this you simply touch the surface to feel the increase in temperature. If you've ever been under stage lighting this is quite apparent.
 
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