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

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• otterandseal1
In summary: The square of the distance is d^2. The 1/d^2 in the inverse square law is the reciprocal of the square of the distance.
otterandseal1
What's the difference between distance and the square of the distance?
Many Thanks

otterandseal1 said:
What's the difference between distance and the square of the distance?
uhh...one is squared?

otterandseal1 said:
What's the difference between distance and the square of the distance?
Many Thanks
The square of the distance is bigger.

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.

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?

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.
Unless the distance is < 1.

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?!

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?

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.

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.

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?

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.

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?

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.

jtbell said:
Unless the distance is < 1.
Hey, watch it! Nitpicking here is MY job

Irradiance is the amount of radiant flux (or energy) per unit area received from a source. It is typically measured in watts per square meter (W/m2).

2. What is the difference between distance and the square of the distance in irradiance?

The difference between distance and the square of the distance in irradiance is the inverse square law. This law states that the irradiance of a source decreases as the square of the distance from the source increases. In other words, the farther away an object is from a source, the less irradiance it will receive.

3. Why is the inverse square law important in understanding irradiance?

The inverse square law is important in understanding irradiance because it helps us calculate the amount of irradiance received at different distances from a source. This is crucial in many scientific fields, such as astronomy, where the distance between objects can greatly affect the amount of radiation they receive.

4. How does the inverse square law apply to other physical phenomena?

The inverse square law applies to many physical phenomena, including light, sound, and gravity. In general, it states that the intensity of a physical quantity decreases as the square of the distance from the source increases. This can be seen in the brightness of a light source, the loudness of a sound, and the strength of a gravitational force.

5. Are there any exceptions to the inverse square law?

While the inverse square law applies to many physical phenomena, there are some exceptions. For example, if the source is not a point source (i.e. it has a finite size), the inverse square law may not apply. Additionally, some phenomena, such as radioactivity, follow a different law called the inverse cube law, where the intensity decreases as the cube of the distance from the source increases.

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