Transcript Light Part 2 - WordPress.com

Refraction 12.2-12.3 pg 222 – 236
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1) Principles of Refraction
2) Snell’s Law & Refractive Index
3) Total Internal Reflection
4) Optical Fibres
• What is this? Who has this
at home?
• A new generation of
Internet connection –
based on optical fibres.
• We will study the Physics
of optical fibres in this
topic.
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You might have learned that
(1) Light travels in a straight line
(2) Speed of light is a constant
In this topic, you will learn that both (1) and (2)
are INCORRECT
Moral of the story: keep your mind open,
always be willing to question your teachers,
and question what you already think you
know.
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Definition: refraction is the bending of light as
light passes from one optical medium to
another.
Similar to reflection, there is an angle of
incidence (i) and an angle of refraction (r)
(applet)
Can you use refraction to explain why white
light forms a rainbow when it passes through a
prism?
Learn Sine and Arc Sine!
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Sine and Arc Sine will be needed to do
calculations for the topic of refraction
As previously mentioned, there is an angle of
reflection (i) an angle of refraction (r)
Snell’s Law is defined as
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(Sin i)/(Sin r) = constant
[verify Snell’s law using applet]
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Snell’s Law is very closely related to the
concept of “refractive index”
Symbol for refractive index is n (no units)
Refractive Index is a measure of how fast (or
how slow) light passes through a medium
There are 3 different ways to define refractive
index:
First way: n = (speed of light in vacuum) /
(speed of light in medium)
The first definition is also the “official”
definition (in your syllabus)
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Hence, n for vacuum or air = 1
n for any other medium >1
Second way: n = (Sin i) / (Sin r)
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IMPORTANT light MUST be coming from air
(vacuum) and going into the medium (e.g. glass,
water, etc.)
Recall Snell’s Law: (Sin i)/(Sin r) = constant
Can you see how they are related?
The third way will be discussed in the next
sub-topic.
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Refractive Index (n) is a property of a material.
The same material will always have the same n.
[Not in syllabus]
Material
Refractive Index (n)
Vacuum
1 (by definition)
Air
1.00027
Water
1.3330
Plastic
1.4 to 1.6
Glass
1.5 to 1.9
Diamond
2.419
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Magic Trick – Disappearing Coin
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http://www.youtube.com/watch?v=Slas3ik9Lpo
Disappearing Glass & Disappearing Gel
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https://www.youtube.com/watch?v=qH1S83Bkttw
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If the glass block has a refractive index of 1.5,
what is angle x? (hint: need to use arc sine)
60°
x
30°
60°
x
r
Step 1: Draw Normal!
Step 2: i = 90 – 60 = 30°
Step 3: Check: light going from air
to medium?
Ans: Yes!
Step 4: Sin i / Sin r = n
[IMPORTANT: presentation of
working]
Sin (30) / Sin r = 1.5
Sin r = Sin (30)/1.5
Sin r = (0.5)/1.5 = 0.3333 (4 sf)
r = Sin-1 (0.3333)
r = 19.5°
(final answer?)
No!! Qn ask for x not r!!
r = 19.47° (2 dp)
x = 90 – r = 90 – 19.47
= 70.5° (1 dp)
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Always draw normal and measure i and r
against normal!
Light entering at 90° doesn’t bend
Light bends towards the normal when it is
traveling from lower to higher n
Light bends away from the normal when it
traveling from higher to lower n
If you need to find n, but light is travelling
from medium to air, use (sin r)/(sin i) instead
of (sin i)/(sin r) [Qn: How come can we do
this?]
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Sometimes the question uses the term “more
dense” or “less dense”
This density refers to Optical Density, it is not
to be confused with Physical Density (i.e. Mass
/ Volume)
More dense means higher n, less dense means
lower n
When “less dense” think air
When “more dense” think water or glass
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1) Straight surface Single Ray
2) Curve surface Single Ray
3) Multiple ray
These case studies can also apply to refraction!
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Just like previously in refraction, draw a
tangent, and consider angles i and r using the
tangent.
Special case: semi-circular glass block [applet]
Math rule: if a line passes through the center of
a circle, it will always be perpendicular to the
tangent
Note: Semi-circular glass block is a possible lab
experiment for light
Note: the terms “real depth” and “apparent
depth” are not in syllabus
But you need to know that the image is more
shallow than the object
Eye
Image
Object
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[Not in syllabus]
Photographers: What is a fish eye lens?
Why is it called a “fish eye lens”?
Qn: There is a device inside your home (esp if
you live in HDB flat) which is also a fish eye
lens. What is it?
Ans: Doorviewer
[Not in syllabus]
Source: idigitaldarwin.wordpress.com
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1) Draw a ray diagram of light going from glass
to air, n = 1.5, i = 45°
What happened?
2) Find the angle of incidence when r = 90°
(What did you realize about sin 90?)
There is a special name for this angle, it is
called the critical angle, or c
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Definition: the angle of incidence in an
optically denser medium for which the angle
of refraction in the optically less dense
medium is 90°
Third definition of refractive index (n):
n = 1 / Sin c
Question: what happens when i > c?
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Important: Something which only occurs when
light is going from more dense (i.e. higher n) to
less dense (lower n)
TIR never occurs when light is going from less
dense (lower n) to more dense (higher n)
Definition: The complete reflection of a light
ray inside an optically denser medium at its
boundary with an optically less dense
medium
[applet demo]
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TIR is a case of reflection! That means
angle of incidence = angle of reflection
applies
When ray is going from higher n to lower
n, always check for TIR!
Remember the two conditions for TIR:
From more dense medium to less dense
medium
 Angle of incidence > Critical Angle
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If the glass block has n = 1.5, complete the ray
diagram below:
75°
49.9°
75°
40.1°
n = sin i / sin r
1.5 = sin 75 / sin r
r = 40.09°
[What happens at the top
surface?]
[Check for TIR!]
n = 1/sin c
c = 41.8
[Is i>c? Yes]
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Periscopes and Binoculars can be constructed
either using mirrors (reflection) or using prisms
(total internal reflection)
Prisms are preferred and is the better method.
Qn: Why?
Ans: There is double reflection in mirrors due
to thickness of glass (remember your lab expt)
Important: Textbook Pg 234 has an error!
Reward for someone who can spot and explain
the error.
Source: sensorcentral.com
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(see textbook Pg 235)
Higher carrying capacity
Less signal degradation
Lightweight
Lower cost
Can be used as endoscopes for medical
purposes
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McGyver overcomes the light sensors trap
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https://www.youtube.com/watch?v=O50bOf_Z2cg
The Engineer Guy: How Fiber Optic Cables
work
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http://www.youtube.com/watch?v=0MwMkBET_5
I
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[Not in syllabus]
Qn) Why does light bend when it goes into a
different medium? (i.e. what is the actual
explanation behind Snell’s law?)
Ans: speed of light changes when medium is
changed
Qn) Why does a change in speed result in the
light being bent?
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[Not in syllabus]
Recall that the n of diamond is very high
There are many ways to describe & valuate a
diamond.
One way is called the “brilliance” of the
diamond
What is “brilliance” and how is it related to
refraction?
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3 definitions to memorize in this subtopic!
Refraction - is the bending of light as light
passes from one optical medium to another.
Total Internal Reflection - The complete
reflection of a light ray inside an optically
denser medium at its boundary with an
optically less dense medium
Critical Angle - the angle of incidence in an
optically denser medium for which the angle
of refraction in the optically less dense
medium is 90°
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Snell’s Law: (Sin i)/(Sin r) = constant
3 equations for refractive index (n):
n = (speed of light in vacuum) / (speed of light
in medium)
n = (Sin i) / (Sin r), provided light coming from
air and going into the medium
n = 1 / Sin c, where c is critical angle
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Refraction as bending of light due to change in
medium
Less dense to more dense – bend towards the
normal
More dense to less dense – bend away from the
normal
Use of Snell’s Law, Sine and Arc Sine to solve
refraction problems
Critical Angle & Total Internal Reflection
Optical Fibres – how it works and its
advantages
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If you think you have not understood:
(a) Principle of Refraction / Snell’s Law – put a
colored paper clip on your exit card
(b) Total Internal Reflection / Critical Angle /
Optical Fiber – put a gold paper clip on your
exit card