An introduction to… **The inverse-square
relationship**

Set up the apparatus, using the side output of a ray box as the light source.

Measure the short-circuit current (I) from the solar cell and the distance (d) between the solar cell and the

**filament**of the bulb. Record your results in a table. Include a column for values of 1/d^{2}.Change the distance between the solar cell and the bulb and repeat the measurements.

Continue until you have 4 pairs of results in the range 7cm < d < 10cm and a further 10 results up to d = 50cm. Suggest why the measurements are not equally spaced.

Plot a graph of current (y-axis) against distance (x-axis) and draw the best-fit curve.

State the simple pattern in results and hence deduce the effect of light intensity on photocell current.

It is suggested that the current is inversely proportional to the square of the distance from the light source. Plot a graph of current (y-axis) against 1/distance

^{2}(x-axis) and comment on the suggested relationship.Use the apparatus to demonstrate the simple relationship between current and area of the cell exposed to the light. Record your results in a table and plot an appropriate graph.

*Apparatus: Ray box*

Power supply

Solar cell

Metre rule

DMM 318 as ammeter

Leads (×2)