A Law of Brightness
A basic physics lab activity
using a light sensor to measure brightness of light source(s) at various
Computer light sensor or
• Hook the
photocell to the ohmmeter, and check that the reading on the ohmmeter
changes as light to the cell changes (you can do this by turning off the
lights or by shielding the cell with your hand).
• Place a lit
light bulb in an otherwise darkened room. It is important not to have
any background light behind the light bulb or objects obstructing light
from the light bulb.
• Hold the light sensor or photocell one
meter away from the light bulb and take a reading of brightness from the
4.7. Record your light brightness reading, and then before
taking additional readings at new distances, make predictions for what
you would expect at two and three meters. Take readings at two and three
meters, record your data, and compare these with your predictions.
Gather and record data for five additional distances.
4.8. Plot your
data (there should be at least eight points) on a graph with distance
on the horizontal axis and light reading on the vertical axis. Draw a
smooth curve that approximately connects the points.
4.9. On the
basis of your graph, which of the following relationships between
brightness, B, and distance, D, can you rule out?
Which function most
matches this curve ?
4.10. Square each of your distance
measurements. Plot light reading vs. distance squared. Draw a straight
line through the points on the graph.
A. What is the slope of this
B. What is the math function for the light reading in
terms of the distance?