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Light
Chapter 27 and 28
What is light?
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The only thing you can see.
Most objects you see reflect light from
some other source.
If you remove the light source, the object
cannot be seen.
However some objects can also produce
their own light (fire, light bulbs).
Light is an electromagnetic wave
(EM wave or radiation)
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Electromagnetic waves need no medium to
travel through, but if they have a medium it
will effect the speed.
All electromagnetic waves have an
oscillating electric field and a magnetic
field at right angles to each other.
Frequency and wavelength of
light
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Frequency of light corresponds to color.
Visible light- what we can see, spectrum goes
from red (lowest frequency) to violet (highest
frequency)
The colors follow the order:
ROY G BIV
Red orange yellow green blue (indigo) violet
Other types of EM radiation
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There are more types of electromagnetic
waves, however, if the frequency is outside
of the visible range the waves are invisible
to the human eye.
The wave with a frequency just higher than
violet is called ultraviolet, the frequency
just below red is called infrared.
What are infrared and ultraviolet
used for?
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Ultraviolet lights are used as black lights, to
sterilize things, and are used in tanning beds.
Infrared are used as warmers (heat lamps) and
for thermometers (infrared cameras).
They are both invisible to the naked eye.
Both lights also have visible light mixed in so
you know it is on.
The EM spectrum
Visible Light
Wavelength (m)
104 102 1 10-2 10-4 10-6 10-8 10-10 10-12 10-14
Ultraviolet Gamma Rays
Microwaves
Radio Waves
Infrared
X-rays
104 106 108 1010 1012 1014 1016 1018 1020 1022 1024
Frequency Hz
The speed of light
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People used to think the speed of light was
infinite when all conventional speed tests
failed since the light was so fast.
Michelson devised an experiment to test the
speed of light in 1880.
All electromagnetic waves travel at the
fastest speed known to man.
The symbol for the speed of light is c.
c =3.00 x108 m/s
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How fast is that?
Someone from Copley decided they had to be the
first to hear a concert. They went to LA to see it,
however, it was also being broadcast by HBO
(TV waves are electromagnetic). Los Angeles is
3,800,000 m from Copley, but we will double that
distance to 7,600,000 m since the signal has to go
by way of satellites. If the student has good seats,
10. m from the speakers (the speed of sound is
345 m/s), and physics students in Copley watch
the concert on HBO who hears it first?
Answer
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Student in LA
v = d/t
345 m/s = 10 m /t
t = .029 s
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Students in Copley
3x108 m/s =
7,600,000 m / t
t = .025 s
These students hear it
faster
Yes, light is really that fast. If it were a straight path, light
could go completely around the planet in 0.13 s
For most practical purposes the speed of light is
instantaneous on the planet Earth
Homework
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Read section 27.1, 27.2 and 27.3
Pg 548 1-8; 24-25