The experimental formula for reaction of the body to stimulus of strength  is .

(a)

Sensitivity  is defined as rate of change of reaction with respect to .

Consider reaction .

Differentiate on each side with respect to .

Quotient rule of derivatives: .

. 

(b)

Graph :

Graph of the sensitivity and reaction of the body functions.

From the graph, it is to be observed that at the lo level of brightness , the reaction of the body is very high.

The sensitivity is decreasing for the low values of brightness.

It is to be expected that brightness should be optimum for good sensitivity.

(a) Sensitivity .

(b)

it is to be expected that brightness should be optimum for good sensitivity.