Laboratory work: dependence of illuminance on luminous intensity
The aim is to check that the illuminance E is directly proportional to the luminous intensity I (E = I/r²). Equipment: an incandescent lamp of known luminous intensity (lamp 2), a test lamp (lamp 1) connected to a supply with adjustable voltage, a luxmeter (photocell) and a measuring tape. Method: lamp 1 is switched on and at distance r₁ the luxmeter reads E₁. Then lamp 1 is switched off, lamp 2 is lit and moved nearer or further to find the distance r₂ at which the reading is again E₁. The illuminances are equal: I₁/r₁² = I₂/r₂², so I₁ = I₂·(r₁/r₂)². Raising the voltage of lamp 1 step by step (40, 80, … V) and finding I₁ each time, we plot E = f(I): the points should lie near a straight line through the origin. In an additional task we vary r and check the graph E = f(1/r²). Safety: connecting lamps to the supply and changing the voltage is done only by the teacher; do not touch hot lamps.
Draw tables and graphs for E = f(I) and E = f(1/r²) in your notebook using the numbers of the example. Do the experiment in the school laboratory with the teacher; at home you can check how E depends on distance with a lamp and a phone app.