Hands-On Photometry of Light Bulbs

In this section of the lab we will make measurements of the intensity of light at various distances from a pair of light bulbs, giving you a feel for the inverse square law of light. You will then better understand the relationship between the intrinsic brightness (luminosity) of a light source and its apparent brightness (the brightness you actually measure).  You have already learned about one way to get the distances to stars, using parallax. You will now learn to measure the brightness. Together, the brightness and distance give you the luminosity of the object. In the case of the light bulbs you will be using, the luminosity is just the wattage stamped on the bulb. Stars do not have their luminosity stamped on them so you need to use measurements of I and d in order to determine L. As we will see later, the luminosity is a very important characteristic of stars. When one knows both the spectral type and the luminosity one can learn a lot about stars without actually going out to visit them.

Your instructor will show you the experimental setup involving light bulbs and the devices you will use to measure the intensity of light. Please follow these steps. In this part of the lab you will make and record several measurements.

1. Stand 1 meter away from the light bulb filament as you face the front of the room. Use the light meter to measure the amount of light coming from the 100W bulb at the various distances listed below. Make sure that the photometer sensor and not the end of the tube is 1 meter away from the light bulb. Make sure that the light meter is level with the bulb and that it is square to the direction of the light bulb. Keep the sensor at the same distance from the light bulb while moving it slightly from side to side and up and down to get the highest reading on the light meter.
   

2. Repeat step (1) standing 2 meters away. Record your measurements.
   

3. Repeat step (1) standing 3 meters away. Record your measurements.
   

4. Repeat step (1) standing 4 meters away. Record your measurements.
   

5. Repeat step (1) standing 5 meters away. Record your measurements.
   

6. Turn off the 100W bulb, turn on the 40W bulb and record your reading at 1 meter. Do not measure the light for any other distances. You will be asked to predict what the intensities should be at 2, 3, 4 and 5 meters for the 40W bulb.
   

7. Compare your numbers to the inverse square law equation. You will be asked whether your numbers agree with the predictions of the inverse square law.