Electromagnetic waves can interact with a
material medium.
When EM waves encounter a material medium, they can interact with
it in much the same way that mechanical waves do. They can transfer
energy to the medium itself. Also, EM waves can respond to a change
of medium by reflecting, refracting, or diffracting, just as mechanical
waves do. When an EM wave responds in one of these ways, its direction
changes. When the direction of the wave changes, the direction in
which the energy is transferred also changes.
Potential energy comes
from position or form;
kinetic energy comes
from motion.
Transferring Energy
A mechanical wave transfers energy in two ways. As it travels, the
wave moves potential energy from one place to another. It also
converts potential energy into kinetic energy by moving the medium
back and forth.
In a vacuum, EM waves transfer energy only by moving potential
energy from one place to another. But when EM waves encounter
matter, their energy can be converted into many different forms.
In what form do EM waves transfer energy in a vacuum?
Wave Behavior
SKILL FOCUS
How do EM waves interact with matter?
Designing
experiments
PROCEDURE
1
Observe the radiometer on a table or desk.
MATERIALS
2 Write a hypothesis in the form of an “If . . . ,
radiometer
then . . . , because . . .” statement to answer
the question: What makes the radiometer
vanes move?
3 Develop an experiment to test your hypothesis.
TIME
30 minutes
vanes
WHAT DO YOU THINK?
• How does light affect the vanes?
dark side
• Based on your observation of the vanes, does light affect
the light and dark surfaces differently? If so, how?
• How would you modify your design now that you have seen
the results?
CHALLENGE Based on your observations, what does a
radiometer measure? Explain your answer.
508 Unit 4: Waves, Sound, and Light
radiometer
light
side