Intensity and Angle
This demonstration will show that intensity from a light source
decreases as incidence angle of the light beam increases.
Time requirement: 30 minutes as a demonstration.
- Large ball or globe
- Turn off the lights in the classroom.
- Stand a few feet away from a blackboard (or wall) and shine the
flashlight on the board. The flashlight should be perpendicular to the
board, so that the "angle of incidence" of the light beam is 0 degrees.
Observe the intensity of the flashlight beam on the board.
- Tilt the flashlight so that the incidence angle of the flashlight
beam on the board is about 15 degrees. Try to make sure that the actual
distance from the flashlight to the illuminated spot on the board does
not increase. You might actually have to move a little closer to the
board to do this. Once again, observe the intensity of the beam on the
- Tilt the flashlight so that the angle of incidence of the light beam
is approximately 30 degrees and observe the intensity of the beam on
the board. Repeat the observation for an incidence angle of 45 degrees.
Again, try to make sure that the distance from the flashlight to the
illuminated spot on the board does not increase when you increase the
incidence angle of the light beam.
- Now, hold the flashlight a few feet away from a large ball or globe
and illuminate the ball or globe. Observe how the light intensity varies
over the part of the ball/globe which faces the flashlight.
- While continuing to illuminate the ball/globe, rotate it as the
Earth spins on its axis. Observe that all points on the globe rotate
from light to dark and back, but that the amount of light which any
point receives depends upon its "latitude."
- What happens to the brightness of the beam on the board as the
angle of incidence increases from 0 degrees to 45 degrees?
- What happens to the size of the beam on the board as the angle of
incidence increases? Why does this happen? Why does the intensity
decrease when this happens?
- When the Earth is illuminated by the Sun, do we receive the most
energy when the angle of the incident radiation is large or small?
- Describe the variation of light intensity of the ball or globe
when it was illuminated. Where is the maximum intensity and where
is the minimum intensity? Where are the highest and lowest angles of
incidence on the sphere?
- When the ball or globe was rotated, each point on the surface
went from light to dark and back again. This of course corresponds
to day and night on Earth and any other rotating planet. On which
part(s) of the globe was the highest average light intensity over the
course of a rotation? On which part(s) of the globe was the lowest
average light intensity over the course of a rotation? How do these
correspond to the equator and the poles? How does the received light
energy vary with latitude?
Last updated: September 03, 1997
Joe Twicken /