REFRACTION
Angle of refraction
Dispersion
Partial reflection & refraction
Index of refraction
Snell’s Law
REFRACTION
Refraction is the
change in the direction
of light when it crosses
a boundary between
two substances/media.
A medium is the
substance or material
through which light is
travelling. The plural of
medium is media.
Refraction
• Occurs when the speed of
light changes in different
media
• The particles in a medium
slow down the passage of
light waves
• Different media slow down
light by different amounts
Refraction
• Light only refracts at the boundary
when it is entering or leaving a medium
• The more that light slows down, the
more the light is refracted
Refraction in Water
• Light rays change direction at the surface of the water
• The image of the chest appears to be more shallow than
the actual chest
REFRACTION
• refracted ray: the
ray after crossing a
boundary between
media
• angle of refraction:
the angle between
the refracted ray and
the normal
REFRACTION
I nc
id e
nt r
ay
Angle of
incidence
r
ef
R
d
te
ac
y
ra
Normal
Angle of
refraction
How Light Refracts
• The part of the light beam that hits the
medium first will slow down first
Cause of Refraction
Car Analogy
• Car travelling at an
angle towards a muddy
surface
• One front wheel hits
muddy surface and
slows down
• Other wheels continue
to move at a higher
speed
• Causes the path to
bend
REFRACTION
Normal
Angle of Refraction
The angle of refraction is measured
relative to the normal
When moving from low
to high refractive index
light bends towards the
normal
(e.g. air  glass)
Angle of Refraction
The angle of refraction is measured
relative to the normal
When moving from high
to low refractive index
light bends away from
the normal
(e.g. glass  air)
• The amount of refraction is
different for each colour
• A single beam of white light
can be split into a rainbow of
colours
• Dispersion is the refraction
of white light into separate
wavelengths (colours)
Dispersion
Rainbows
Partial Reflection & Refraction
Refraction is often accompanied by reflection.
Some of the light hitting the surface of a body of water
reflects, and some refracts.
Example: Pond water
• Reflection: sky
• Refraction: fish through the water
Partial Reflection & Refraction
• Ray diagrams can show both
reflected and refracted light
rays
• Angle of reflection (r) follows
the Law of Reflection (i = r)
• Angle of refraction (R) can be
calculated by Snell’s Law (see
later slides)
• Emergent ray: any ray of light
that is leaving a medium (e.g.
both reflected and refracted
rays)
Partial Reflection & Refraction
• A large incident angle of light entering water
results in nearly all the light reflecting.
• As the angle of incidence gets smaller, more of
the light refracts and less reflects.
most reflecting
(large angle of incidence)
most refracting
(small angle of incidence)
most reflecting
(large angle of incidence)
Partial Reflection & Refraction
• Silvered two way mirrors: plastic
or glass with a special coating
that reflects most light, but still
allows some to be refracted
• Results in a mirrored surface
that you can see through, but
others cannot.
Partial Reflection & Refraction
•
•
•
“Don’t Miss A Sec” an architectural artwork by Monica Bonvicini, Dec 2003
Installed at a construction site across from London’s Tate Britain museum
A public toilet enclosed within reflective glass walls that allow the user to
see out but prevent those outside from seeing in
http://www.snopes.com/photos/arts/toilet.asp
Recall: Speed of Light
3.00 x 108 m/s
300 million m/s
300,000 km/s
1.08 x 1011 km/h
108 billion km/h
However, it is impossible
for light to move at this
speed when particles get
in the way
Speed of Light
Speed of light changes
depending on the medium
• Speed of light in a vacuum
= 3.00x108 m/s
• Speed of light in water
= 2.26x108 m/s
• Speed of light in acrylic
= 1.76x108 m/s
Index of Refraction
• the amount by which a transparent medium
decreases the speed of light
• A ratio of the speed of light in a vacuum
compared to the speed of light in a medium
c = speed of light in a vacuum
= a constant of 3.00x108 m/s
v = speed of light in the medium
= varies (e.g. water is 2.26x108m/s)
(since units cancel, a refractive index does not have any units)
Index of
Refraction for
various media
Media
Vacuum
Index of Refraction
1.00
Air
1.0003
Carbon dioxide gas
1.0005
Ice
1.31
Pure water
1.33
Ethyl alcohol
1.36
Quartz
1.46
Vegetable oil
1.47
Olive oil
1.48
Acrylic
1.49
Table salt
1.51
Glass
1.52
Sapphire
1.77
Zircon
1.92
Cubic zirconia
2.16
Diamond
2.42
Gallium phosphide
3.50
Solve using GRASP
Recall GRASP
• Given: List the information given to you using
symbols and numbers. Include units.
• Required: List the item that needs to be solved.
Use symbols.
• Analysis: Write mathematical equation(s) that
will be used.
• Solution: Replace the equation with values listed
in the Given. Solve the equation.
• Phrase: Write the answer to the question in the
form of a sentence.
Given:
v = 1.91 x 108 m/s
c = 3.00 x 108 m/s
Recall GRASP
• Given: List the information given to you using
symbols and numbers. Include units.
• Required: List the item that needs to be solved.
Use symbols.
• Analysis: Write mathematical equation(s) that
will be used.
• Solution: Replace the equation with values listed
in the Given. Solve the equation.
• Phrase: Write the answer to the question in the
form of a sentence.
Given:
v = 1.91 x 108 m/s
c = 3.00 x 108 m/s
Required: n
Recall GRASP
• Given: List the information given to you using
symbols and numbers. Include units.
• Required: List the item that needs to be solved.
Use symbols.
• Analysis: Write mathematical equation(s) that
will be used.
• Solution: Replace the equation with values
listed in the Given. Solve the equation.
• Phrase: Write the answer to the question in the
form of a sentence.
Given:
v = 1.91 x 108 m/s
c = 3.00 x 108 m/s
Required: n
Analysis:
Solution:
n= c
v
= 3.00 x 108 m/s = 1.57
1.91 x 108 m/s
Recall GRASP
• Given: List the information given to you using
symbols and numbers. Include units.
• Required: List the item that needs to be solved.
Use symbols.
• Analysis: Write mathematical equation(s) that
will be used.
• Solution: Replace the equation with values
listed in the Given. Solve the equation.
• Phrase: Write the answer to the question in the
form of a sentence.
Given:
v = 1.91 x 108 m/s (glass)
c = 3.00 x 108 m/s
Required: n (glass)
Analysis:
Solution:
n= c
v
= 3.00 x 108 m/s = 1.57
1.91 x 108 m/s
Paraphrase: The refractive index of glass is 1.57
Given:
n = 1.33
c = 3.00 x 108 m/s
Required: v (water)
Analysis: n = c , therefore v = c .
v
n
Solution: v = 3.00 x108 m/s = 2.26 x 108 m/s
1.33
Paraphrase: The speed of light in water is 2.26 x 108 m/s
Snell’s Law
The index of refraction can
also be calculated using sines
of angles (trigonometry)
n1 sine i = n2 sine R
i is the angle of incidence
R is the angle of refraction
Actually written as:
n1 sinθ 1 = n2 sinθ 2
A light ray moves from water to glass. The angle of incidence in
water is 260. Calculate the refracted angle given the index of
refraction for water is 1.33 and glass is 2.04.
Given:
θ1 = 26o, n 1 = 1.33, n2 = 2.04
Required:
θ2
Analysis:
n1 sinθ1 = n2 sinθ2
therefore
n1 sinθ1 = sinθ2
n2
Solution:
sinθ2 = 1.33 x sin 26o = 0.497
2.04
θ2 = sin-1 0.497 = 16.6o
Phrase:
The refracted angle of a light ray moving from
water to glass is 16.6o
Given the diagram below, calculate the angle of
refraction and draw the refracted ray.
Glass
n = 1.52
Water
n = 1.33
• Step 1: draw the normal
• Step 2: measure the angle
of incidence
• Step 3: calculate the angle
of refraction using Snell’s
Law
• Step 4: Draw the refracted
ray using the angle
calculated in step 3