Talk:Inverse-square law

the inverse square law for xray is as follows...

I=

D(squared)=

I2=X

D2(squared)=

I=intensity

D=density

e.g. to find x

If the intensity of a source is 100 mR/h at a distance of 50 feet what will the intensity be (x) at 75 feet?

I=100mr/h

D(squared)=50ft

I2=

D2(squared)=75

50squared=2500 which is multiplied by 100 which equals 250000 which you will devide by (75squared=5625) which gives you the answer for X X=44.4 so the intensity ffrom the source at 75ft= 44.4 mR/h!!!

not bad for a highschool dropout turned army grunt then druggie then father now a clean straight walking radiographer trainee!! btw i have been in training for only three days!

I just want the inverse square law for x-rays. Can any1 help me? Bold text

Inverse-square laws are about point sources, and yet it sounds like point sources can't actually exist. (Doesn't the Planck length, among other things, imply that things must have a certain minimum size?) In light of this apparent paradox, can someone explain why inverse-square laws are to any extent successful? Perhaps there are empirically-derived bounds along the lines of "if two masses both have volumes less than V_small, then Newton's gravitation law will give results that shall not deviate by more than 1x10^(-8) newtons from the true value"? --Ryguasu 17:31 Feb 6, 2003 (UTC)

Partly it's something along those lines. Notice that one of Newton's laws says that if no force acts on an object it continues at the same velocity, but one may object that there has never been an object on which no force acted, and the objection is somewhat parallel to what you're saying. Newton demonstrated mathematically that if the earth has uniform density then its gravitational field outside of the sphere in which its mass exists, is the same as if all the mass were concentrated at the center. So that's another reason. Michael Hardy 00:23 Feb 7, 2003 (UTC)

For example, Sol "provides" 9140W at the distance of Mercury (0.387AU); but only 1370W at the distance of Earth (1AU)

It would be useful if the formula to get the above values (9140 and 1370) were given too.

It does not make sense to use watts as the unit here. Perhaps watts-per-square-meter or something like that was intended instead? If so, it needs to say so. Michael Hardy 22:58, 12 Jan 2004 (UTC)

Looking at the Principia Mathematica first edition -- it may BE Robert Hooke who advance the Inverse-Square in relation to gravity first... - Sparky

The diagram

I just looked at the diagram I drew again and I realised it doesn't look terribly good. I am going to redraw it. Does anybody have some suggestions as to how I can make it better? -- Borb 00:39, 31 December 2005 (UTC)

Looks good to me. If you were specific about what you think needs improvement in it, maybe I could say more. Michael Hardy 02:44, 31 December 2005 (UTC)

It doesn't look very high quality, I was going to draw it again anyway, I just wondered if there was a different way I should draw it. A diagram could be either showing a quarter of the lines going through the same sized square, as in my diagram, or it could be showing the same number of lines going through a square 4 times the size of the original. Does anybody have a preference on which type I should draw? Also, technically the "squares" should be part of a sphere so I might try making them look like that. -- Borb 20:14, 2 January 2006 (UTC)

If you can access a copy of Edgar Allan Poe's Eureka, you will find a perfectly clear visual illustration of this law.Lestrade 15:21, 12 February 2006 (UTC)Lestrade

The diagram is wrong. Look at the red lines, one of them is actually going the wrong way, and end up in a completely new place in the 3r area. Irios 11:28, 3 March 2010

Diagram

The first diagram I ever saw of the inverse square law was better, in my opinion. It showed ...

Nevermind, I found some similar ones: [1] [2] — Omegatron 18:16, 19 February 2006 (UTC)

What about lasers?

- I agree on the diagram comment. Looking at the two linked images here the first did not work. The second is a great diagram. Displaying the four and then nine squares at each of the given distance values displays how the light is being spread out over greater distance.

p. 47 of Bill Bryson's A Short History of Nearly Everything explains that Halley (of Halley's Comet fame) was the one who went to Newton to ask him about a math problem concerning the inverse square law. Halley and Hooke were acquaintances and were involved in a bet to see who could solve the problem first. After Newton started publishing books with his findings, Hooke said he had already discovered the answer and accused Newton of plagiarism.

Light and other electromagnetic radiation

It would be appropriate to add the word "Interestingly" to the start of this section, I am perplexed attempting to explain connection Omeganumber (talk) 02:26, 1 March 2008 (UTC)

citation

how do you cite it? —Preceding unsigned comment added by 24.236.97.242 (talk) 18:34, 7 March 2008 (UTC)

New Diagram!

Well I finally got around to drawing a new diagram of the inverse square law. Comments/Suggestions etc? It's not the easiest law to illustrate... -- Borb (talk) 14:05, 2 April 2008 (UTC)

Criticism of Acoustics entry

This is intended as constructive criticism regarding the "acoustics" entries on the inverse square law page. I don't like to see popular misconceptions perpetuated. I guess if I knew how to present what I feel is the correct information on the page I would have edited it rather than posing a comment here, but I am an absolute novice when it comes to Wikipedia editing. But to get to the point…

The propagation of sound *does* obey the inverse square law in exactly the same way as electromagnetic radiation does. Sound *pressure* is not a valid equivalent to electromagnetic radiated power as a source – you are not comparing like with like. As the distance from the source increases, the acoustic *power* fall as the inverse square of the distance – double the distance means a quarter of the power, or 6·02dB; this equates to (and is the cause of) a halving of sound *pressure*, since pressure is proportional to the square root of power times radiation resistance, the latter being constant.

If my knowledge of acoustics is faulty, then please disregard the foregoing comments, if not, then maybe you might consider editing the relevant sections of the page accordingly.

S Sycamore (talk) 19:16, 8 August 2008 (UTC)

Above copied from User_talk:Borb -- Borb (talk) 20:58, 8 August 2008 (UTC)

Light Emitting Diodes (LEDs)

Is it possible that the LED device .. designed to remove infrared radiation (IR) from the white light spectrum does not follow the Inverse-square law as is? Sorta meaning IR helps carry light through distance?Greg0658 (talk) 16:42, 9 February 2009 (UTC) More clearly put .. does LED light fall-off at a different rate than full spectrum electromagnetic waves and white light?Greg0658 (talk) 13:58, 13 May 2009 (UTC)

No the inverse square law is a spreading function and is independent of the spectrum of radiation. The typical LED has a built in reflector and lenses that direct the light in one direction. Inverse square still applies but it's as if the point source was originated somewhere far to the rear of the actual LED so the fall of is less per unit distance after the LED surface. 12.106.237.2 (talk) 17:15, 15 April 2010 (UTC)

accuracy - Point Source Approximation

From ISBN 0-9658356-9-3 Ryer, A, "The Light Measurement Handbook", 1997 : p.26. (The chapter is also available online here[3])

Under the heading "Point Source Approximation" Ryer describes 1) some practical limitations of the inverse square rule, 2) a more accurate and encompassing alternative formula which includes non-point sources.

If I have interpreted correctly: for illuminance from a non-point-source - ie where the diameter of the light source is more than 1/5th of the distance - the following formula may be more useful: E = Lr²/(r² + d²); where r= source radius and d= distance.

This would certainly seem like a valid addition to the Inverse-square_law#Light_and_other_electromagnetic_radiation section. Perhaps it would also be applicable in other contexts ? --Redbobblehat (talk) 14:07, 10 February 2009 (UTC)

Watts Versus Candles

I was trying to look up a solution to a problem I had run into regarding Inverse Square law calculations given a distance variable of 0 (which would involve division by 0, which is unallowed), although I see someone has made a comment referring to limitations of inverse square law, and that may be one of them.

However, much of the work I am doing involves simulated sunlight, and I noticed the potentially useful mention of the relative illumination of the sun at the surfaces of Mercury and the Earth; but I was a bit puzzled as to why this is presented in watts? Watts are a measurement of energy consumption, not illumination: for that reason, it's impossible to convert between watts and candles since energy consumption given an illuminative power is variable: in other words, different lights produce varying amounts of illumination given the same energy consumption. So I can't convert the value to a useful illuminative power. Perhaps this should be given in candles instead? LSmok3 ^Talk 12:10, 7 December 2009 (UTC)