Members: 77,663   Posts: 1,715,249   Online: 1018

1. Originally Posted by fretlessdavis
Same thing with everything I did in acoustics. SPL is standardized at being measured at 1m for speakers and such... makes up for the errors in not having sound coming from a point source...
SPL can be measured at any distance. It's usually one metre and one watt to work out speaker efficiency.

Steve.

2. Originally Posted by Tom1956
Stone, you're wrong on 2 counts: firstly politicians do it every day when they make a speech. Secondly, when you walk up to the Sun and put your light meter on the surface and take a reading. THAT is point zero. You're not interested in digging through 433 thousand miles of plasma with a shovel to get the "true" reading.
That meter (if it does t melt) can hover just above the surface but will never be INSIDE the light point, it's impossible, can't occupy the same space, just making quarters and quarters closer for infinity...

3. Originally Posted by StoneNYC
My dad is a real honest to god physicist... I'll ask him tomorrow...



Approaching
Sometimes you can't work something out directly ... but you can see what it should be as you get closer and closer!
Let's use this function as an example:

(x2-1)/(x-1)

And let's work it out for x=1:

(12-1)/(1-1) = (1-1)/(1-1) = 0/0

Now 0/0 is a difficulty! We don't really know the value of 0/0, so we need another way of answering this.

So instead of trying to work it out for x=1 let's try approaching it closer and closer:

x (x2-1)/(x-1)
0.5 1.50000
0.9 1.90000
0.99 1.99000
0.999 1.99900
0.9999 1.99990
0.99999 1.99999
... ...
Now we can see that as x gets close to 1, then (x2-1)/(x-1) gets close to 2

We are now faced with an interesting situation:

When x=1 we don't know the answer (it is indeterminate)
But we can see that it is going to be 2
We want to give the answer "2" but can't, so instead mathematicians say exactly what is going on by using the special word "limit"

The limit of (x2-1)/(x-1) as x approaches 1 is 2

So it is a special way of saying, "ignoring what happens when you get there, but as you get closer and closer the answer gets closer and closer to 2"

4. Originally Posted by Curt
So it is a special way of saying, "ignoring what happens when you get there, but as you get closer and closer the answer gets closer and closer to 2"
Actually, the answer to everything is 42.

5. Originally Posted by lxdude
Actually, the answer to everything is 42.
+1

6. Originally Posted by MattKing
Stone:

Take some university physics courses and you will find that what you say here isn't actually true.

Or for that matter, watch a few Star Trek episodes .
Of the Original Series... the subsequent series played kind of loose with science (worse than the original, I should say).

7. Originally Posted by Steve Smith
SPL can be measured at any distance. It's usually one metre and one watt to work out speaker efficiency.

Steve.
Sound waves propogate much like light does. True it can be measured at any distance, like light can, but for larger sources, 1m is pretty standardized at any power level to check SPL. A speaker is a larger source, so getting closer and closer you're not getting the full output of the speaker-- just like light.

8. He didn't really give a detailed answer... This was his reply....

There is always a physical limitation to a light source, which itself has an associated radius. So in practice this mathematical situation never arises. But theoretically, yes.

"Stone wrote:
So, on the photo forum, this question arose....

This is confusing me

according to the inverse square lawB=I/d^2,theIllumination from a light sourcequadruples every time the distance from subject to light source is cut in half.Inconsequence doesn't that mean that the light source approaches infinite intensitywhen the distance to the light source approaches '0'?Hoew can this be?is there a flaw in the inverse square lawor is it limited to certain conditions? "

9. Originally Posted by StoneNYC

There is always a physical limitation to a light source, which itself has an associated radius. So in practice this mathematical situation never arises. But theoretically, yes.

"Stone wrote:
So, on the photo forum, this question arose....

This is confusing me

according to the inverse square lawB=I/d^2,theIllumination from a light sourcequadruples every time the distance from subject to light source is cut in half.Inconsequence doesn't that mean that the light source approaches infinite intensitywhen the distance to the light source approaches '0'?Hoew can this be?is there a flaw in the inverse square lawor is it limited to certain conditions? "
It's limited by the real world application of the math.

In Physics, theories and laws seem to be based off of perfect conditions. There is no true point light source possible-- as it would occupy no space. As mentioned before, other effects start happening with different light sources as you get really close. There is no problem with the law, but perfect conditions for it are never attained in real life.

When learning physics, early on you learn to 'deal with it', later on you learn why, and even later on, you basically throw everything out of the window and start fresh. At least that's how my education was... from basic mechanics, electricity, light, etc, then up through Quantum Mechanics. I never went further than with light and optics than a good engineer would, though.

The law works best for comparing relative distances as it falls off, but there's too much difference in light sources as you get really close to them, not to mention your measuring area would have to be so tiny, it would have little real world application.

10. Couple of things I didn't see mentioned although I may have missed them, are that it is true for all electromagnetic waves be they IR, UV, Sound, Visible Light, Gamma and X-Rays and that it is only true in an open space, if you are in a room or a place where there can be reflected waves then the results will be muddied and will likely not follow the inverse square law.

Page 3 of 8 First 12345678 Last

 APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY: