Physics - LDRs...

Why is the graph of light intensity against resistance for an LDR a curve?
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thanks

The behaviour of an ldr follows the equation (Ra/Rb)=(Ia/Ib)^-k where Ia and Ra are intesity and resistance figures in one state and Ib and Rb are the figures for a second state. K is a constant depending on the construction of the ldr and is usually somewhere betwen 0.6 and 0.9. The graph from this will give a curve - usually in ldr datasheets a log scale is used giving an almost perfect straight line.

The resistance of the ldr decreases as intensity increases (negative sign in front of k) because from I=nAve the area, drift velocity and electron charge are the same, but the incoming photons excite electrons in the material which increases the number of charge carriers (n) and hence the current increases for a given voltage.

As the light intensity increases, the rate at which electrons are freed also increases, causing the resistance to drop rapidly at higher light levels. I think this is something to do with the "avalanche effect" where the free electrons somehow knock other electrons free and hence increase the current even further - not sure on that one though.