Sound and light

[phl666]

Two questions in one really:

1. If I place a mirror behind a light source, do I get double the amount of light?

2. If I hit two piano keys with the same force as I hit one, is the sound produced double?

[Peter Pedant]

try it and see - I like candlelight and save on candles by having a few mirrors in the room. (not quite double)

Try parallel mirrors - you get about eight reflections -

The hall of mirrors at Versailles was said to be sufficieintly lit by one candle

[gen2]

1 It depends what was behind the candle before the mirror was placed ther. If it was a matt black surface with 100% absorbtion, then the answer is yes, you do get double. In real life whatever was behind the original candle would have reflected some light so in practice the answer is 'a bit less than double, how much less depends upon how reflective the original surroundings were'.

[fo3nix]

1. i think the idea of "double the light" is a bit mis-leading. you only have one candle, so the light (the number of photons i mean here) cannot be doubled by adding a mirror. what the mirror will do is reflect the light to more places, thus illuminating a greater area (giving the effect that there is more light being produced, maybe).

2. hit one key, and you get a loud noise. hit two, and you get two loud noises, at different frequencies. the amount of sound (i.e. the sound waves) you produce will be double, but the volume will be just the same. the volume depends only on how hard you press the keys.

[mibn2cweus]

Because of the way we perceive sound, doubling the sound energy is not easily perceived as louder. The range of sound volume from the barely perceptible to the loudest sound is about one million to one. In terms of the actual power required to produce the sound this ranges from one trillion (a million, million) to one.

Within this range of loudness you would have to play ten piano keys on ten different pianos at once to produce an apparent doubling of the loudness of one key played with the same force. It is not hard to appreciate the skill of a pianist and the quality of a piano required to play music involving a wide range of softness and loudness.

[qapmoc]

Hi Fo3nix, I think you got it the wrong way around there, the mirror will reflect the light to less places because it takes the light which would go to the back and puts it in the same place as light which goes to the front illuminating a smaller area but with more light.

[phl666]

I was under the impression that energy can neither be created nor destroyed - only converted to another form. How then can light energy be increased at all by placing a mirror near it?

[gen2]

phl666, of course no exra light is created by the mirror, but light that would not previously have reached the viewer's eyes will be reflected towards the viewer. Think of it not so much as doubling the light, but a redistribution of the available light so more of it is seen.

[fo3nix]

gen2: precisely.
qapmoc: not quite sure what you mean. i don't think i got it wrong, no.

[bobjim1705]

I think wat qapmoc was getting at, is that a reflective surface just redistributes the light that would otherwise shine on the area that the mirror is now blocking.

Or have I got it wrong too?

[qapmoc]

OK lets put it this way - ( Assuming the front is 180 degrees and the back is 180 degrees before someone asks about the sides ! )

With no mirror 50% of the light goes to the front 50% area and 50% light goes to the back 50% area,

Now put a mirror and 100% light goes to the front 50% area and 0% light goes to the back 50% area,

So the front area gets twice as much light but the total output is still 100% but the total area receiving light is now 50%

BTW Happy new year to all.

[fo3nix]

qapmoc: yup, that is indeed true :)

[Hamish]

Errm on the sound front, there is a complex interplay between the volume and the fequency of a note, hit two keys of diferent notes with the same force and the overall vol will not quite be double.

[makinsonc]

1) no there is less light

2) I don't know

[bymis]

the light will be more that doubled if you use a conically shaped mirror :)