REFLECTION IN A MIRROR
• When light rays hit a smooth surface, it gets reflected.
object
image
A plane mirror has a flat surface.
What are the characteristics of an image formed
by a plane mirror?
CHARACTERISTICS OF IMAGE IN PLANE MIRROR
a
b
same size as the object
distance of object from the mirror
= distance of image from the mirror (a = b)
REFLECTION
• The image you see behind the mirror is due to
the many light rays that bounce off you, onto the
mirror, into your eyes.
• To locate the image, we draw ray diagrams.
RAY DIAGRAM (REFLECTION)
NO, the perpendicular line to
the surface, is also known as
the normal.
N
B
A
AO - incident ray
Angle i - angle of incidence.
r
i
OB - reflected ray
Angle r – angle of reflection.
incident
ray
reflected
ray
O
Angle of incidence = Angle of reflection
RAY DIAGRAM (REFLECTION)
• The incident ray AO strikes the mirror at point O.
• Incident ray AO makes an angle of incidence, i, with the normal.
A
incident ray
i
r
N
B
reflected ray
• The reflected ray OB travels into the boy’s eye.
• Ray OB makes an angle of reflection, r, with the normal.
• Angle i = angle r
O
HOW THE EYE SEES
1. Diverging light rays enter the eyes…
2. Image is
formed at back
of eye ball
3. Brain
processes
the image
RAY DIAGRAM (REFLECTION)
To locate the image…
mirror
object O
RAY DIAGRAM (REFLECTION)
Measure equal distance away from mirror…
mirror
image
object O
If the image position is already given, you can skip this step.
RAY DIAGRAM (REFLECTION)
Using only 1 ray from object to locate image…
mirror
image
object O
Note that the light rays are indicated using solid lines and arrows.
All other lines are dashed.
All arrows are directed towards the eye.
RAY DIAGRAM (REFLECTION)
If using 2 rays from object to locate image…
(more common method)
mirror
image
object O
Notice that the light rays do not converge at the eyes.
RAY DIAGRAM (REFLECTION)
If using 2 rays from object to locate image…
(more common method)
mirror
image
object O
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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?
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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

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
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

http://www.youtube.com/watch?v=0MwMkBET_5
I
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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