Lab 4: --Changing Light Intensity

Problem: What is the relationship between light intensity and distance from the light source?

Background and Inquiry: Have you ever noticed how bright it is sitting in the front row of a movie theater, and how much dimmer it is sitting in the last row? We all know that the further away you go from a light source the dimmer it gets. But, what type of relationship exists as you move further away from a light source? Would you expect an inverse proportion? That means if the light in a movie is half as bright in the 10 row it will be one fourth as bright in the 20th row. Before beginning this lab try to predict how light intensity behaves as you move further away from alight source.

The equipment you will use today consists of a light source, photo detector (light detector) and a meter. When light hits the photo detector it is changed to electricity. The electricity is recorded by a meter. The greater the light intensity the more the meter will read. Make sure to turn off the light bulb when not taking any measurements. You can do this by disconnecting the battery.

On a separate sheet of paper write your hypothesis to the experiment. What type of relationship do you expect to see?

State your hypothesis. Justify your statement!

Materials: Light source, meter stick, battery, microamp meter, photocell

Diagram:

Procedure;

1) Copy Table I into your lab notebook.

2) Set up the equipment as shown in in class.

3) Set the light source about 30 cm. from the photocell. Turn on the light source and direct it at the photocell. Notice the reading on the meter. Start by bringing the light source closer to the photocell, notice how the meter begins to read more current. Continue to move the light source closer until the meter reads exactly 25 micro amps. Do not go past the 25 micro amp reading since it may damage the meter. Measure the distance and record this distance into table I. Measure the DISTANCE from the light source for the following Intensities (AS YOU MOVE AWAY FROM THE LIGHT SOURCE)  25, 20, 15,10 ,5, and 2.5 micro amps.

Part II: Effect of different color lighting.

4) Cover the photocell with the red filter and repeat the experiment. Use the tape provided, but do not cover the filter.

5) Cover the photocell the the blue filter and repeat the experiment.

6) Cover the photocell with the green filter and repeat the experiment.

Results;

D = distance I = Intensity.

TABLE I: NO FILTER

D = distance I = Intensity.

TABLEI I: RED FILTER

D = distance I = Intensity.

TABLE III: BLUE FILTER

TABLE IV: GREEN FILTER

Graphing: 1) Plot Intensity of Light (I), vs. Distance, (Intensity on the y axis, Distance on the x axis).

Advanced Graphing Activities:

Plot Average Intensity vs. 1/D2 . (Intensity on the y axis and 1/D2 on the x axis).

Discussion:

1) What are the independent and dependent variables?

2) How are the variables changing with relationship to each other?

3) What happens to the dependent variable when the independent variable increases? decreases?

4) How does the relationship shown in this experiment compare with other relationships you have so far seen?

5) How does the equation for this relationship compare with those of other equations you have studied?

Applications: What is difference between a laser beam and a flashlight? What are applications for laser light sources where regular light sources cannot be used?