Can light bounce?
 Yes! We call this a reflection.
 Humans have been looking at reflections for
thousands of years, however they’re not always clear.
Reflections
 Why is the
reflection
blurred?
 How could you
get a clear
reflection?
The Egyptians
 Almost 4,000 years ago the Egyptians realized that
only smooth ponds produce clear reflections.
 However it wasn’t until 1835 when Justus von Liebig, a
German scientist, developed a method of coating glass
with silver to produce a sharp, well-defined, reflected
image.
The Law of Reflection
 Consider what happens when I bounce pass a
basketball.
 How can I change the angle of the bounce pass?
The Law of Reflection
 Light reflects the same way the basketball does.
 Laser Demo
 The Law of Reflection: The angle that a reflected ray
makes as measured from the normal to a reflective
surface equals the angle that the incident ray makes as
measured from the same normal.
The Law of Reflection
 𝜃𝑖 = 𝜃𝑟
 𝜃𝑖 is the angle of the
incident ray
 𝜃𝑟 is the angle of the
reflected ray
 The normal line is
perpendicular the surface
Bellringer
Updates
 Sound Quiz
 SLC today
 First Regents Review HW due tomorrow
 1-50 MC
Objectives
The Law of Reflection
 Smooth vs rough surfaces
 Which will be a better reflector?
 Laser demo
The Law of Reflection
 Specular Reflection is when parallel light rays are
reflected in parallel
 Diffuse Reflection is when parallel light rays are not
reflected in parallel but instead they are scattered.
 This is what allows us to read text or see images from
various angles.
Specular Reflection
Diffuse Reflection
Checkpoint
What is the law of reflection?
1.
2. What is the difference between specular and diffuse
reflection?
1.
Which one is used by mirrors?
The Law of Reflection
 The law of reflection still applies to both types of
reflection.
 No matter how much light reflects off a wall or a sheet
of paper, you will never be able to use them as mirrors.
 The light that reflects off the wall or paper is how we
see the wall or paper.
Changing Reflection Types
 How could you make a diffuse surface a specular
surface?
 Polish it, or coat it with something to smooth it over.
Flipped Images
 Why is the reflected
image of this pond upside
down?
Law of Reflection
 What is the angle of
incidence of the highest
leaf if the tree is 10 meters
tall and its reflection is 20
meters long?
 If the observer is 1.83
meters tall, how far away
from the base of the tree are
they?
Law of Reflection Tree Solution
 𝑡𝑎𝑛𝜃 =
10𝑚
20𝑚
 𝜃 = 26.6°
 𝜃𝑖 = 90° − 26.6° = 63.4°
 tan 26.6° =
1.83𝑚
𝑥
 𝑥 = 3.7𝑚
 𝑑 = 20𝑚 + 3.7𝑚 = 23.7𝑚
Practice Problems - 467
 Page 467 numbers 1-6
 8 minutes
Practice Problem Answers
1.
2.
3.
4.
5.
6.
Water fills in the rough areas and makes the surface
smoother. The surface normals are then parallel.
35
42, 48, 84
51
30
90
Bellringer
Updates
 Sound Quiz
 SLC today
 First Regents Review HW due tomorrow
 1-50 MC
Light and Boundaries
 What happens when light encounters a boundary like
a window?
 Some light is transmitted through the window and
some light is reflected back.
 Laser out the window demo
 Is this a property of a wave or a particle?
Thirsty??
 What is wrong with this
pencil??
 Have you ever looked into
a pool and it seemed
shallower than it was?
Refraction
 These phenomena happen because the light rays bend
as they enter a new medium.
 Refraction: The change in direction of waves at the
boundary between two different mediums.
Refraction
 The incident angle in all three cases is the same. What
is different?
 The medium, and the angle of refraction.
Refraction
 The change in direction of a light beam depends on
the properties of the mediums that the light rays are
traveling from and into.
 Notes
 𝜃𝑟 on the reference table is the angle of reflection not
the angle of refraction!
 The angles are measured from the normal line.
Willebrord Snell
 Snell came up with a mathematical law to relate the angle
of incidence with the angle of refraction.
 He did this using a light source and a piece of glass.
 He shot the light at various angles and measured the results.
 Then he plotted his data and found a mathematical
relationship that fit and could correctly predict the outcome.
 Let’s try it!
 Watch your eyes…laser beam
Snell’s Findings
 He found that the sines of the angles were related!
 Woo trig!
 He also found that each material had its own “index
of refraction” (n) which determines the angle of
refraction of light as it crosses the boundary between
two mediums.
Snell’s Law of Refraction
 𝑛1 𝑠𝑖𝑛𝜃1 = 𝑛2 𝑠𝑖𝑛𝜃2
 𝑛1 = 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑟𝑠𝑡 𝑚𝑒𝑑𝑖𝑢𝑚
 𝜃1 = 𝑎𝑛𝑔𝑙𝑒 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑛𝑜𝑟𝑚𝑎𝑙 𝑖𝑛 𝑡ℎ𝑒 𝑓𝑖𝑟𝑠𝑡 𝑚𝑒𝑑𝑖𝑢𝑚
 𝑛2 = 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑟𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑒𝑐𝑜𝑛𝑑 𝑚𝑒𝑑𝑖𝑢𝑚
 𝜃2 = 𝑎𝑛𝑔𝑙𝑒 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑛𝑜𝑟𝑚𝑎𝑙 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑒𝑐𝑜𝑛𝑑 𝑚𝑒𝑑𝑖𝑢𝑚
 This is found in the wave section of your reference table.
Index of Refraction
 By doing experiments with
other combinations of
mediums Snell found the index
of refraction of many different
materials.
 A table of the different indices
of refraction can be found in
the reference table.
Beaker Demo
 Will light bend as it goes from air to corn oil?
 Why?
 Will light bend as it goes from corn oil to corn oil?
 Why?
 Will light bend as it goes from corn oil to another medium
that has the same n as corn oil?
 Why?
 Can we make something invisible?
Bellringer
Updates
 Missing sound quiz
 Physics club on Tuesday
 New SLC roster
 Review HW due Friday
Snell’s Law
Example
 Find the angle of refraction if a light ray is shot




through air at an angle of 25 degrees into glycerol.
𝑛1 𝑠𝑖𝑛𝜃1 = 𝑛2 𝑠𝑖𝑛𝜃2
(1)𝑠𝑖𝑛25° = 1.47 𝑠𝑖𝑛𝜃2
𝑠𝑖𝑛𝜃2 =0.287
𝜃2 = 16.68 𝑑𝑒𝑔𝑟𝑒𝑒𝑠
Practice
 Complete practice problems 1-5 on page 494 of the
textbook.
 Answers
1.
2.
3.
4.
5.
26.3 degrees
34.2 degrees
17.0 degrees
1.5
The medium is float glass
Where does “n” come from?
 The index of refraction (n) describes how much light
bends as it enters a medium, but why does it bend?
 The light bends because it is changing its speed!
The speed of light
 The speed of light can never be faster than when it is
traveling through a vacuum, but it can be slower when
traveling through a different medium.
 The index of refraction is the ratio of the speed of light
in a vacuum to the speed of light in the medium.
What is the speed of light?
 The speed of light is
𝑚
8
3.00𝑥10
𝑠
in a vacuum.
 Its symbol is “c”.
 It is found on the front page of the reference table.
The Index of Refraction
𝑛=
𝑐
𝑣
 n is the index of refraction
 c is the speed of light in a vacuum
 v is the speed of light in a medium
Checkpoint
1.
What is the speed of light in water?
2. What is the index of refraction of a medium that
makes light travel at
8𝑚
2.75𝑥10 ?
𝑠
Critical Angle
 As the light travels from a medium of higher n to lower
n, the angle of refraction is larger than the angle of
incidence.
 Critical Angle: A certain angle of incidence where the
refracted light ray lies along the boundary of the two
mediums.
Critical Angle
Total Internal Reflection
 Total Internal Reflection occurs when light traveling
from a region of higher n to a region of lower n strikes the
boundary at an angle greater than the critical angle such
that all light reflects back into the region of higher n.
 𝑛1 sin 𝜃𝑐 = 𝑛2 sin(90°)
 sin 𝜃𝑐 =
𝑛2
𝑛1
 𝑛1 is the medium of the incident ray (higher n value)
Total Internal Reflection
 If you’re under water in a pool and looking up at the
surface you may not be able to see what’s above the
surface.
 Fiber optic cables use this phenomena to transmit data
very efficiently and at the speed of light.
Fiber Optics
Fiber Optics
Mirages
 As the ground heats up from the sun it creates a thin
layer of much warmer air near the ground.
 This layer of hot air has a smaller n value than the air at
eye level.
 This difference in n makes the light rays of the sky and
usually an object reflect of the cool and hot air
boundary.
Mirages
Checkpoint
1.
What conditions are necessary for total internal
reflection to occur?
2. What are two examples of total internal reflection in
real life?
3. What causes mirages?
Index of Refraction Lab
 Take a moment to read the procedure.
Bellringer
Updates
 Physics Club today
 New SLC Roster
 HW due Friday
 Next Quiz?
Sound Quiz
 Mod 2: 83
 Mod 8: 81
Index of Refraction Lab
 How many calculations must you show?
 Once you’ve calculated your three different average
experimental n values come ask me for the theoretical
n values.
Sample Long Answer Problem
Index of Refraction Lab
 How many calculations must you show?
 Once you’ve calculated your three different average
experimental n values come ask me for the theoretical
n values.
Bellringer – 2 mins to hand in
 Find the critical angle of a
light beam traveling from
diamond to zircon.
 2.42 sin 𝜃𝑐 = 1.92sin(90°)
 sin 𝜃𝑐 = 0.7933884298
 𝜃𝑐 = 52.5°
DO
STOP
WORK
Bellringer – 2 mins to hand in
 What is the difference
between a reflected wave
and a refracted wave?
DO
STOP
WORK
Objectives
 Be able to answer all questions about reflection and
refraction of light
 Begin your journey through space and astronomy.
Lyrid Meteor Shower
 Tonight is the peak of the shower.
 Look northeast?
Index of Refraction Labs
 Please hand in your completed index of refraction labs.
Lab Checklist - Missing
 All labs (except “Series-Parallel Circuits”) are out of 50




points.
You need a grade of 33 or higher for the lab to count
towards your lab minutes.
Check off the labs you have in your folder and calculate
your total number of lab minutes.
If you have someone’s lab give it to them or me.
When you’re done bring me you lab folder and checklist.
Homework
 Complete all the multiple choice questions.
 We will spend one mod on Friday reviewing it.
 I’d rather you not do it at all than cheat.
 The Regents exam is less than two months away.
 If anyone fails I will delete you from my memory.
 If you do really well you may be sent to the Physics
Hall of Fame.
Bellringer
Updates
 New SLC Roster
 HW due Friday
 Next Quiz?
Index of Refraction Lab
 Calculations and Conclusion Questions
Physics Hall of Fame
 If you have one of the
three highest scores on
the Regents exam your
name will be placed on
the “Physics Hall of
Fame” plaque!
Refraction Bend
 If light goes from a lower n to a higher n (slows down)
it will bend towards the normal line.
 If light goes from a higher n to a lower n (speeds up) it
will bend away from the normal line.
Refraction Bend
Refraction Bend Practice
 Which n value is greater?
 n2 because the refracted
light is bending towards the
normal.
 Which medium is the light
traveling through faster?
 The top one because it has
a lower n value.
Refraction Practice
 Complete questions 37 to 40 on page 514 in the next 8
minutes!
37. The angle of incidence is larger than the angle of
refraction b/c air has a smaller index of refraction
38. The angle of incidence is smaller than the angle of
refraction b/c glass has a larger index of refraction
Refraction Practice
39. The term critical angle refers to the incident angle
that causes the refracted ray to lie right along the
boundary of the substance when a ray is passing
from a region of higher index of refraction to a region
of lower index of refraction. If the incident angle
exceeds the critical angle, total internal reflection
will occur.
40. C>D>B>A
Pyro Board
 Oh what you can make once you know physics!
 https://www.youtube.com/watch?v=2awbKQ2DLRE
 What is a standing wave?
 What is an antinode?
 What determines what frequencies will stand in a
pipe?
RR Test – MC Review
 Law of Reflection
 Two types of reflective surfaces
 Snell’s Law
 Calculate n values using velocity and c
 Ratios of index of refraction, velocity, and wavelength
of light at boundaries.
Find the angles of refraction
30°
Air
Corn Oil
𝜃2 = 19.89°
Diamond
𝜃2 = 11.93°
Water
𝜃2 = 22.09°
Crown Glass
𝜃2 = 19.21°
Air
𝜃2 = 30.00°
 𝑛1 𝑠𝑖𝑛𝜃1 = 𝑛2 𝑠𝑖𝑛𝜃2
 (1)𝑠𝑖𝑛30° = (1.47)𝑠𝑖𝑛𝜃2
 𝜃2 = 19.89°
 1.47 𝑠𝑖𝑛19.89° = (2.42)𝑠𝑖𝑛𝜃2
 𝜃2 = 11.93°
 2.42 𝑠𝑖𝑛11.93° = (1.33)𝑠𝑖𝑛𝜃2
 𝜃2 = 22.09°
 1.33 𝑠𝑖𝑛22.09° = (1.52)𝑠𝑖𝑛𝜃2
 𝜃2 = 19.21°
 1.52 𝑠𝑖𝑛19.21° = (1)𝑠𝑖𝑛𝜃2
 𝜃2 = 30.00°
Mathematical Proof
 Use 𝑛 =
𝑐
𝑣
𝑛2
to prove
𝑛1
=
𝑣1
𝑣2
 How is this equation shown of the reference table?

𝑛2
𝑛1
=
𝑣1
𝑣2
=
𝜆1
𝜆2
 Can you write a mathematical proof for the last part of
the equation?
The Missing Piece
 Light has both a frequency and a wavelength so its
speed can be expressed using 𝑣 = 𝑓𝜆
 However when light changes its speed only the
wavelength is changed.
 The frequency of light always remains the same.
 The frequency of light is what determines the color of
light.
Back to the Proof

𝑛2
𝑛1
=
𝑣1
𝑣2
=
𝑓1 𝜆1
𝑓2 𝜆2
 But 𝑓1 = 𝑓2
 So
𝑛2
𝑛1
=
𝑣1
𝑣2
=
𝜆1
𝜆2
 Now we can easily calculate the index of refraction,
velocity, or wavelength.
Practice – answer in nm
 If a light beam has a wavelength of 570nm in diamond
what will be its wavelength when it travels into air?

𝑛2
𝑛1

1.00
2.42
=
𝑣1
𝑣2
=
=
𝜆1
𝜆2
570𝑛𝑚
𝜆2
 𝜆2 = 1,379.4𝑛𝑚
 If a light beam has a wavelength of 650nm and a velocity of
199,861,639m/s in Lucite what is its velocity in Zircon?

𝑛2
𝑛1
=
1.92

1.50
𝑣1
𝑣2
=
𝜆1
𝜆2
𝑚
𝑠
199,861,639
=
𝑣2
 𝑣2 = 156,141,905
𝑚
𝑠
 What is its wavelength?

𝑛2
𝑛1
𝑣
𝜆
= 𝑣1 = 𝜆1
1.92

1.50
2
=
2
𝑚
𝑠
𝑚
156,141,905
𝑠
199,861,639
 𝜆2 = 507.8𝑛𝑚
=
650𝑛𝑚
𝜆2
Refraction MC
Total Internal Reflection Demos
 What is Total Internal Reflection?
 Water Tank
Bellringer
 What is the difference
between a reflected wave
and a refracted wave?
DO
STOP
WORK
Objectives
 Be able to answer all questions about reflection and
refraction of light
 Begin your journey through space and astronomy.
Lenses
 Two types of lenses
Convex – A lens that is thicker at the center than at
the edges.
2. Concave – A lens that is thinner at the center than at
the edges.
1.
Lenses
 Lenses use refraction to
bend light rays.
 The way they are curved
changes the amount that
they can bend the light
rays.
Convex Lens
 Why does this type of
lens focus light beams?
 The curved shape
changes the incident
and refracted angles.
Concave Lens
 Why does this type of
lens disperse light
beams?
 The curved shape change
the incident and
refracted angles.
Computer Demo
 https://phet.colorado.edu/en/simulation/bending-
light
RR Test
 Law of Reflection
 Two types of reflective surfaces
 Snell’s Law
 Calculate n values using velocity and c
 Ratios of index of refraction, velocity, and wavelength
of light at boundaries.