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Announcements
• Happy Groundhog Day!
• Pick up old labs at the end of class
• Why not start preparing now for the
final exam?
ÆRead through the old lab again, making sure you know the
answers to the quiz questions
ÆTake another look at the lecture notes (on the web) and see if
they now make more sense
• We were lenient in the grading
• In the future, be sure to
ÆAnswer question explicitly being as quantitative as possible
ÆShow your calculations
ÆRarely (if ever) will more that 3 sig figs be appropriate
Light – Some History
• Early 1800’s: Light is a wave traveling through the aether
(not quite right)
• 1865 James Clerk Maxwell: Light is a wave of
electromagnetic radiation (yes)
• 1905 Albert Einstein: Light is both a particle and a wave
(amazing but true)
wavelength, λ
PARTICLE
WAVE
Electromagnetic Spectrum
• All electromagnetic
waves travel through
vacuum with a speed c
(3 X 108 m/s)
• The visible portion of the
spectrum forms a tiny
portion of the total EM
spectrum
• For all EM waves, c=λf
(true for any type of
wave); λ = c/f
Views of Crab Nebula
X-ray
optical
infra-red
increasing wavelength
radio
Reflection and Refraction
When ever light
travels from one
medium to another,
two things can
happen:
Æ Reflection
Æ Refraction
Reflection
• Draw a line normal
(perpendicular) to the
surface
• Consider the angle
between the incident ray
and the reflected ray
• The angle of reflection is
equal to the angle of
incidence: θ1 = θ1’
Æ just what you’d expect
if light were a stream
of particles bouncing
off the surface
Specular and Diffuse Reflection
Refraction
• Some of it is transmitted
(refracted) into the 2nd
medium
• Not all of the light is
reflected at the surface
• Note that the refracted
angle is not equal to the
incident angle
• In fact, the angles are
related to the velocities
of light in the two media
Æ sin θ1/sin θ2 = v1/v2
Refraction
Can be understood from wave properties of light.
The angle changes
because light travels
SLOWER inside some
materials than others. The
speed of light inside any
materials is always less
than in vacuum,
v(inside) < c.
Refraction
Snell’s law
• Light travels at a speed c in
vacuum, but slower in other
media.
• Define index of refraction n
= c/v, where v is the speed
that light travels in a given
medium (glass, water, etc).
• sin θ1/sin θ2 = v1/v2
can be rewritten as
Æsin θ1/sin θ2 = n2/n1
Æn1 sin θ1 = n2 sin θ2
Refraction
The angle can change in either direction,
depending on the relative index of refraction
(or speed) of each material.
Reflection and refraction
Fig. 22.6b, p.691
Total Internal Reflection
When light travels from a medium with a larger
index of refraction to a lower index of refraction, it
bends out
At some incident angle, the refracted angle is 90o
ÆFor any angles greater than that, no light makes it
into the refracted medium
ÆNO REFRACTED LIGHT; 100% of it is reflected
Fiber Optics
Used for example for fiber
optic transmission of phone
calls
Cladding has a lower index of refraction
than core so for any angles greater than the
critical angle, the light is totally internally
reflected