GOAL: To measure the luminosity of the sun with a wax photometer and a

standard luminosity source (for comparison).

The basic idea is to place a 200-watt light bulb and the sun on opposite sides of a wax photometer (an intensity-measuring device) and vary the distance from the photometer to the 200-W bulb until the intensity of the bulb at the photometer matches the intensity of the sun at the photometer. See picture below.

EQUIPMENT: 200-watt unfrosted light bulb with socket, cord, and clamp

wax photometer ( = 2 slabs of wax separated by opaque

aluminum foil)

meter stick

(pinhole camera)

PROCEDURE: (remember lab journal expectations)

(To determine the luminosity of the sun, we need to know the distance of the sun from the earth. We will assume that this distance is known.)

1) Clamp the bulb onto something stable, such as the fence around the observing platform.

2) How should the bulb's filament be oriented to the photometer face? Why? How/where should the photometer be oriented relative to the bulb and sun? Why?

3) Note any observations that you can make about the colors of the light recorded in the two halves of the photometer.

4) One partner should be the intensity judge and decide where the photometer should be held such that the sun and bulb have equal

intensities on the two photometer faces. This person should then hold the photometer steady at this distance while the other partner(s) measure(s) and record(s) the appropriate data.

5) Repeat step 4 so that each partner estimates the position of balanced

intensity (sun vs. light bulb) at least once.

RESULTS

1) Explain your color observations using appropriate laws.

2) Using the average value of your group's measured distances (of balanced

solar and bulb intensity), calculate the luminosity of the sun.

3) Give one good reason that would explain why your calculated value of

the solar luminosity could be higher than the known value. Explain

clearly why your calculated value could be higher. Be as quantitative

as possible.

4) Repeat question 3 with "higher" replaced by "lower."

5) Mars is about 1.5 times farther away from the sun than earth. If you

repeated this experiment on Mars, how far would you have to hold the

photometer away from the 200-W bulb so that the two sides of the

photometer were illuminated with equal intensity?

(Use the known value of the solar luminosity in answering this

question.)

CONCLUSION