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Reflection
1. Introduction
When light travelling in one medium encounters a boundary with another
medium, part of the incident light changes direction and returns into the medium
from which it originated, which is called reflection. The following diagrams show
some common examples of reflection in daily life.
The reflection of Mount Hood in
Trillium Lake
(http://en.wikipedia.org/wiki/Reflectio
n_(physics))
Reflection in a soap bubble
(http://commons.wikimedia.org/wiki/
File:Reflection_in_a_soap_bubble_edi
t.jpg)
Window Reflection of Canal Houses on
a Clear Winter Day
http://goamsterdam.about.com/od/a
msterdamphotos/ig/AmsterdamReflection-Pictures/Canal-HouseWindow-Reflection.htm
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2. Ray diagram
In optics, we use a ray to represent the propagation of light.
a ray of light
a beam of light
Different types of light beam:
parallel light
divergent light
convergent light
3. Reflection
3.1 Two types of reflection
Diffuse Reflection
Reflection from any rough surface is known as diffuse reflection. When the
surface is rough, a beam of parallel light is reflected not as a parallel set but in
various directions as shown in the Fig.1.
Fig.1
(http://commons.wikimedia.org/wiki/File:Difracao.svg)
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Specular (regular) Reflection
Reflection of light from a smooth surface is called specular reflection. When a
beam of parallel light strikes a smooth surface, the reflected rays are also parallel
to each other as indicated in Fig.2 below.
Fig. 2
(http://www.screentekinc.com/glossary/glossary-specular_reflection.gif)
The difference between these two kinds of reflection explains why it is more
difficult to see while driving on a rainy night. If the road is wet, specular
reflection occurs at the smooth surface of the water and hence most of the light is
reflected away from the driver. When the road is dry, the rough surface of the
road reflects the light diffusely. Part of the light is reflected back towards the
driver, allowing the driver to see the highway clearly.
3.2 Principle of reflection
Consider a light ray travelling in the air and incident at an angle on a flat
smooth surface, as shown in Fig. 3.
Fig. 3
(http://electron9.phys.utk.edu/phys135d/modules/m10/images/Image1282.gif)
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 The incoming light is called the incident ray and the outgoing ray is called the
reflected ray.
 The normal is a line drawn perpendicular to the surface at the point where the
incident ray strikes.
 The incident and reflected rays make angles and , respectively, where the

angles are measured from the normal to the rays as indicated in Fig. 3.
and are called the angle of incidence and angle of reflection respectively.
 Experiment shows that the angle of reflection equals the angle of incidence:
This relationship is called the principle of reflection.
4. Mirror
4.1 Characteristics of image formed by plane mirrors
Mirror Image
http://wpcontent.answers.com/wikipedia/commons/thumb/5/52/Mirror.jpg/200pxMirror.jpg
What are the characteristics of image formed by plane mirrors?
 The image has the same size as the object. For example, when you look
into the mirror, your image has the same height as you.
 The image is upright. For example, when you stand upright in front of the
mirror, your image is also upright. (Your feet will not appear on top of your
head!)
 The image is virtual. It cannot be captured in a screen.
 The image distance (distance between the image and the mirror) equals
the object distance (distance between the object and the mirror).
 The image is laterally inverted. For example, when you raise your right
hand in front of the mirror, the image in the mirror raises the left hand.
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4.2 Ray diagram to illustrate reflection by plane mirrors
Consider the point source of light placed at O in Fig. 4, as a distance p in front
of a plane mirror. The distance p is called the object distance.
p
q
I
O
p=q
Fig. 4
mirror
Light rays leave the light source and strike the mirror. Light rays are reflected
and continue to diverge. However, if we extend the reflected light rays in the
opposite direction (behind mirror), they appear to converge at a point (light
appears to be given out from that point), I, behind the mirror. Point I is called
the image of the object at O. Images are classified as real or virtual. A real
image is formed when light rays pass through and converge at a point; a
virtual image is formed when the light rays do not pass through the image
point but appear to be given out from the point. Hence, the image formed
by plane mirror in Fig. 4 is virtual. Real images can be displayed at a screen (as
at a movie), but virtual images cannot be displayed on a screen.
Fig. 5 shows another ray diagram for images formed by plane mirrors. In this
case, instead of a point source, the object is represented by an arrow. To form
the image of the object, we have to choose two end points of the object and
find the images of these two points. (At least two rays are required to find one
image point.)
Object
Image of
Object
Mirror Fig. 5
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4.3 Image formed by curved mirrors
Concave Mirrors
A concave mirror has a reflecting surface that bulges inward.
A concave mirror tends to focus light towards a focal point. The image form by
concave mirrors depends on the object distance.
Effect on image of object's position relative to mirror focal point
Object's
position (S),
focal point
(F)
S<F

(Object
between 
focal point

and mirror)
Image
Diagram
Virtual
Upright
Magnified (larger)
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
S=F
(Object at
focal point)
the image is formed at infinity.
(Note that the reflected light rays are
parallel and do not meet the others.
In this way, no image is formed or
more properly the image is formed at
infinity.)
F < S < 2F 
(Object at 2x

focal point)

Real

Real

Inverted (vertically)

Same size
S = 2F
Inverted (vertically)
Magnified (larger)
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S > 2F

Real

Inverted (vertically)

Diminished (smaller)
(http://en.wikipedia.org/wiki/Concave_mirror)
image form on the inside of a spoon
(http://studio.nathanielguy.com/previews/a_spoon_full_of_sky_preview.jpg)
An important property of concave mirrors is that light rays passing through the
focal point are reflected as parallel lights (parallel to the principle axis). The
reflected light of a ray parallel to the principal axis will pass through the focal
point.
Convex Mirrors
A convex mirror is a curved mirror in which the reflecting surface bulges
outwards. The image is always virtual, diminished and upright.
Convex mirror lets motorists see around a corner. (http://en.wikipedia.org/wiki/Concave_mirror)
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5. Application of reflection
Plane Mirror
We see plane mirror in our everyday life. The following are two examples.
A Dance Studio
http://gotrhythmgardiner.com/db1/00073/gotrhythmgardiner.com/_uimages/DanceStudio2.JPG
A Dressing Room
http://www.mirrotek.com/blog/wp-content/uploads/2008/11/3vu1448b.jpg
Many optical instruments use plane mirrors. Submarines have periscopes which
help the crew to see what is above the water when the submarines are under
water. The working principle of periscope is shown in Fig.6.
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Fig. 6
A kaleidoscope makes use of two or more plane mirrors placed at an angle to see
another to form multiple images of small colourful objects.
Concave Mirror & Convex Mirror
Convex and concave mirrors and reflectors have many uses. Fig. 7 shows how a
concave reflector produces a parallel beam of light in a car headlamp.
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Fig. 7 concave mirror produces a parallel beam of light
The parallel beam of light allows people far away from the car to see the car
coming. It also allows the driver to see what is ahead in the highway.
Reflecting telescopes make use of mirrors to reflect light from the stars or other
celestial bodies. The working principle is shown on Fig. 8.
Fig.8
http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Gregorian_Telescope_Lightpath.s
vg/800px-Gregorian_Telescope_Lightpath.svg.png
Concave mirrors are also used by dentists to produce a magnified image of one’s
teeth when held close to the teeth.
Dentist Mirror
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Convex mirrors are usually used in shops for security and on roads or in car parks
to allow drivers to see around blind spots such as sharp corner.
Security Mirror
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