Download - Higher Physics Unit 3 Irradiance
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Higher Physics
Unit 3Irradiance
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Irradiance 1Irradiance is a measure of the ‘brightness’ of a light source (or any other wave).
Consider a 100 W tungsten filament light bulb (25% efficiency).
The lamp emits 25 J of light energy in all directions each second.The irradiance of the light falling on a surface is a measure of the amount of energy per second per square metre.
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Irradiance 2
We have the equation -
I = P A
where –I – irradiance (W m-2)P - power (W)A - area (m2)
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Irradiance 3
As the light radiates out from the source the energy is spread out over a larger area. This causes a reduction in the irradiance as the distance from the source increases.How can we find a relationship for the irradiance of a light source, I , at a distance, d ?
lampLight
sensordistance, d
Interface
PC
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Irradiance 4
By measuring the irradiance, I, at a range of distances, d, we get the following graph -I
(Wm-
2)
d (m)
This seems to show an inverse proportion relationship, but the situation is more complex.
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Irradiance 5
Redrawing the graph for I against 1/d 2, we get the following -I
(Wm-
2)
1/d 2 (m -
2)
According to this graph, I is directly proportional to 1/d2.i.e. I = k
d2
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Irradiance 6