Irradiance: difference between distance and the square of the distance?

Click For Summary

Discussion Overview

The discussion revolves around the concepts of distance and the square of distance, particularly in the context of irradiance and the inverse square law. Participants explore the mathematical and physical implications of these terms, seeking clarity on their relationship and relevance in practical scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the square of the distance is larger than the distance itself, but emphasize the need for specificity in the question being asked.
  • There is a discussion about how textbooks state that increasing distance results in decreasing irradiance, and that irradiance is inversely proportional to the square of the distance.
  • Participants question whether "distance" and "the square of the distance" refer to the same concept, with some suggesting that they are not the same.
  • One participant explains that squaring a distance results in a number that does not have physical meaning as a distance, but rather serves as part of an equation.
  • There is a clarification that the symbol 1/d^2 represents 1 divided by the square of the distance, not the square of the distance itself.
  • Some participants mention the need for adjustments when considering extended sources of light, as opposed to point sources.
  • One participant raises a question about the purpose of placing a black sheet underneath a light meter in irradiance experiments, suggesting it may reduce reflected light.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and interpretation regarding the relationship between distance and the square of distance, particularly in relation to irradiance. No consensus is reached on the clarity of the original question or the implications of the inverse square law.

Contextual Notes

Some participants indicate that the discussion may lack clarity due to vague phrasing of the initial question, and there are unresolved aspects regarding the physical meaning of squared distances in the context of irradiance.

otterandseal1
Messages
16
Reaction score
0
What's the difference between distance and the square of the distance?
Many Thanks
 
Physics news on Phys.org
otterandseal1 said:
What's the difference between distance and the square of the distance?
uhh...one is squared?

You'll need to elaborate considerably about your question if we are going to be able to help you!
 
otterandseal1 said:
What's the difference between distance and the square of the distance?
Many Thanks
The square of the distance is bigger. :smile:

I think you need to be more specific with your question. What is it you are trying to figure out?

EDIT: Ah, I see russ beat me to it
 
phinds said:
The square of the distance is bigger. :smile:

I think you need to be more specific with your question. What is it you are trying to figure out?

EDIT: Ah, I see russ beat me to it
Its to do with irradiance, I see textbooks saying "distance increases will result in irradiance decreasing" and "irradiance is inversely proportional to the square of the distance" What is the distance between the two? Sorry if I was too vague.
 
russ_watters said:
uhh...one is squared?

You'll need to elaborate considerably about your question if we are going to be able to help you!
Its to do with irradiance, I see textbooks saying "distance increases will result in irradiance decreasing" and "irradiance is inversely proportional to the square of the distance" What is the distance between the two? Sorry if I was too vague.
 
phinds said:
The square of the distance is bigger. :smile:
Unless the distance is < 1. :oldwink:
 
otterandseal1 said:
Its to do with irradiance, I see textbooks saying "distance increases will result in irradiance decreasing" and "irradiance is inversely proportional to the square of the distance" What is the distance between the two? Sorry if I was too vague.
The distance in question is the distance between the source of light and what is receiving it.

...I'm still not sure that is what you are asking though...
 
russ_watters said:
The distance in question is the distance between the source of light and what is receiving it.

...I'm still not sure that is what you are asking though...
Yes, but sometimes it refers to the distance between a source of light and then generally the square of the distance. Are they the same things? If it's more helpful, the inverse square law is the square of the distance is inversely proportional to irradiance/illuminance. What does the square of the distance mean?
 
otterandseal1 said:
Yes, but sometimes it refers to the distance between a source of light and then generally the square of the distance. Are they the same things?
"distance" is how far apart 2 points in space are. The Earth is about 150 million km from the sun, for example.
If it's more helpful, the inverse square law is the square of the distance is inversely proportional to irradiance/illuminance. What does the square of the distance mean?
Squaring something is just squaring it. 150 million km squared is 2.25 x 1016 km2.

Maybe you are asking what physical meaning this new number has? It has none, it's just a partial piece of an equation. It is not itself a distance. Notice that it no longer has units of distance...but note what units it has?!
 
  • #10
russ_watters said:
"distance" is how far apart 2 points in space are. The Earth is about 150 million km from the sun, for example.

Squaring something is just squaring it. 150 million km squared is 2.25 x 1016 km2.

Maybe you are asking what physical meaning this new number has? It has none, it's just a partial piece of an equation. It is not itself a distance. Notice that it no longer has units of distance...but note what units it has?!
Is the symbol 1/d^2 the square of a distance then?
So if I said as the distance from a point source increases, the irradiance decreases, would this be an effective conclusion of the inverse square law?
 
  • #11
otterandseal1 said:
Is the symbol 1/d^2 the square of a distance then?
So if I said as the distance from a point source increases, the irradiance decreases, would this be an effective conclusion of the inverse square law?
Near enough when the source can be regarded as a point source. An extended source like big nebulae or galaxies *Sun and Moon, too) need some adjustment.
Go into the garden at night and see the effect of the light from a picture window as you walk away, trying to read a book. Definitely not ISL.
 
  • #12
otterandseal1 said:
Is the symbol 1/d^2 the square of a distance then?
So if I said as the distance from a point source increases, the irradiance decreases, would this be an effective conclusion of the inverse square law?
Yes. The irradiance decreases faster with a square in the equation than without. That's it.
 
  • #13
russ_watters said:
Yes. The irradiance decreases faster with a square in the equation than without. That's it.
Sorry just to double check 1/d^2 is not the same as the square of the distance is it?
 
  • #14
otterandseal1 said:
Sorry just to double check 1/d^2 is not the same as the square of the distance is it?
No, it's 1 divided by the square of the distance.
 
  • #15
russ_watters said:
No, it's 1 divided by the square of the distance.
In irradiance experiments, why is a black sheet placed underneath the light meter and the light bulb?
 
  • #16
otterandseal1 said:
In irradiance experiments, why is a black sheet placed underneath the light meter and the light bulb?
Probably to cut down on reflected light to make the experimental results better fit the inverse square law.
 
  • #17
jtbell said:
Unless the distance is < 1. :oldwink:
Hey, watch it! Nitpicking here is MY job :smile:
 

Similar threads

High School Distance between two points on Earth doesn't increase with altitude?
  • · Replies 18 ·
Replies
18
Views
4K
Undergrad Effect of fast-neutron irradiation on the tensile properties of steels
  • · Replies 2 ·
Replies
2
Views
3K
High School Why exactly do we need the gravitational constant?
  • · Replies 11 ·
Replies
11
Views
3K
Rayleigh Scattering: Blue/Red Irradiance Ratio & Distance
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad Discussion about least squares method
  • · Replies 2 ·
Replies
2
Views
2K
Question about irradiance from a UV lamp
  • · Replies 6 ·
Replies
6
Views
3K
High School Meaning of physical quantities and division
  • · Replies 7 ·
Replies
7
Views
2K
High School Significant figures for special functions (square roots)
  • · Replies 2 ·
Replies
2
Views
6K
High School Calculating solar irradiance at each planet?
  • · Replies 10 ·
Replies
10
Views
5K
Spectrometer sensitivity (photons/count) at different wavelengths
  • · Replies 5 ·
Replies
5
Views
2K