Higher Physics Unit 3 Irradiance.

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Presentation transcript:

Higher Physics Unit 3 Irradiance

Irradiance 1 Irradiance 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.

Irradiance 2 We have the equation - I = P A where – I – irradiance (W m-2) P - power (W) A - area (m2)

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 ? lamp Light sensor distance, d Interface PC

Irradiance 4 By measuring the irradiance, I, at a range of distances, d, we get the following graph - I (Wm-2) This seems to show an inverse proportion relationship, but the situation is more complex. d (m)

Irradiance 5 Redrawing the graph for I against 1/d 2, we get the following - I (Wm-2) According to this graph, I is directly proportional to 1/d2. i.e. I = k d2 1/d 2 (m -2)

Irradiance 6