Law of Reflection:
Angle of Incidence = Angle of Reflection
Why can’t you see yourself in a
sheet of white paper?
How does the image compare to
the object?
The image made in a plane mirror
is called a Virtual Image.
Geometry still applies to the
Law of Reflection!
How big does a mirror need to be for
you to see your entire self’s image?
Refraction of Light
• Light travels at different speeds in different media.
• 3.0 X 108 m/s is the fastest light can travel, and EMwaves move at this rate while in a vacuum (and air).
• When light travels from one medium to another, it
changes speed, and we “see” the effects of that
change in speed by the light changing direction, or
bending.
• Light can travel at slower speeds when in different
media… how to find that speed depends on the
substance’s index of refraction, n.
• n = Speed of light in a vacuum = c
Speed of light in the material
v
For Example…
• What is the speed of light in diamond if diamond has
an index of refraction of 2.419? What is the speed of
light in ice which has an index of refraction of 1.309?
• n = c/v
• Diamond: 2.419 = (3.0 X 108 m/s) / v
• vdiamond = (3.0 X 108 m/s) / 2.419
• vdiamond = 1.240 X 108 m/s
• ICE: vice = (3.0 X 108 m/s) / 1.309
• vice = 2.292 X 108 m/s
• In general, the higher the index of refraction (larger
than one), the slower the speed of light in that
material compared to a vacuum.
• Table 18.1, page 582, gives the Index of Refraction
for a variety of substances.
Snell’s Law of Refraction:
n1sin1 = n2sin2
Light is said to “bend toward
the normal” as light travels
from a less dense medium to
a more dense medium.
If the angle of incidence is 46o,
what is the angle of refraction?
Light going from a more dense medium to a
less dense medium is said to “bend away
from the normal.”
2 = 74o
Where does the chest appear to a
viewer on the boat? Deeper or
shallower than its actual depth?
Total Internal Reflection
Note: This only works when n1>n2 as shown
Fiber Optics use Total Internal
Reflection to send signals moving at
the speed of light!
Dispersion of Light
• The spreading of light into its color components is called dispersion.
• Different frequencies of light travel at different speeds in transparent
materials, they will refract differently and bend at different angles.
• Red light, wavelength 660 nm, has an index of refraction in crown glass
of 1.520
• Violet light, wavelength 410 nm, has an index of refraction in crown glass
of 1.538
• The greater the index of refraction, the greater the bending, hence, violet
bends more than red light when entering and exiting a prism.