1. Key Areas covered Irradiance and the inverse square law.
Irradiance is power per unit area.
The relationship between irradiance and distance from a point light source.
Line and continuous emission spectra, absorption spectra and energy level transitions.

2. What we will do today:
State how we can calculate the irradiance of radiation.
State how intensity and distance of a light source are related.
Carry out calculations on the above.

3. Irradiance (intensity)

4. Irradiance
When radiation falls on a surface, the irradiance, I, is defined as the power per unit area:
I = P A
I – intensity (Wm-2) [or lux]
P – power (W)
A – area (m2)
(Irradiance is referred to as intensity in some past papers)

5. Experimentally proving the inverse square law
A light meter can measure the intensity of light from a bulb in a darkened room at various distances over a metre.

6. Experimentally proving the inverse square law
Measuring irradiance levels at different distances and plotting a graph of results can be used to confirm the inverse square law.

7. Irradiance and distance
As you move further away from a point source, the intensity of radiation decreases.
The relationship between intensity (I) and distance (d) can be shown to follow an inverse square law:
I α 1 in data book as I = k
d d2
Therefore: I1d12 = I2d22
*This is only the case for a point source of light i.e. a lamp not a laser (or a torch)

8. Example
A lamp shines on a screen of area 2.5m2, which is 3m away. The intensity at the screen is 0.01 Wm-2.
Calculate the power of the incident beam.
If the screen is moved to a distance of 1.5m from the lamp what would the new intensity be?
P = I A = 0.01 x 2.5 = W
I1d12 = I2d22
so I2 = I1d12
d22
I2 = (0.01 x 9)
2.25
I2 = 0.04 Wm-2

9. 2001 Qu: 18 (repeated in 2011!)

10. 2005 Qu: 17

11. 2005 Qu: 13

12. 2003 Qu: 17

13. 2008 Qu: 28

14. Past Paper Questions
Revised Higher

15. Revised Specimen Paper

16. 2012 Qu: 17