Materials: A 200 watt clear glass light bulb, lamp holder, and ruler.

1. On a windless, sunny day, stand facing directly at the sun with your eyelids barely closed. Please, beware: it is harmful to look at the sun with your eyes open! Now feel the brightness of sunlight as it passes through your eyelids. It is very important to remember this sensation.

2. If you do not have a lamp, go indoors to trailer 6B (MWF 1:30 -3 pm), switch on the light bulb, close your eyes exactly as before, and feel the brightness of this light. Find the distance r between your eyes and the lamp's filament, that replicates the brightness of the sun. Repeat steps 1 and 2 four times (two trials for each partner) and record distances r in meters.

3. The intensity I of a lamp at a distance r from the source is proportional to

                                           Power of lamp in watts/(r^2)                                                                                  (1)

because light is uniformly spread over the surface of a sphere of radius r .

4. The intensity of the Sun on Earth is also uniformly spread over a spherical surface of radius R  = 1.5x10^11 m (distance Earth-Sun), thus being proportional to

                                            Power of the Sun in watts/(R^2)                                                                                (2)

5. Equate intensities (1) and (2) (same brightness) to find the power of sun's radiation, Power of Sun = (Power of lamp)*(R^2)/(r^2) . Find the average Power and its error.

6. The total power used by humans in all forms of energy is approximately 10^13 watts (or Joules/s). What fraction of the Sun's power is this value ?

7. Discuss uncertainties in your estimates and any insights you may have derived from the experiment.