2 Reflection

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Transcript 2 Reflection

Chapter 2 :
REFLECTION
Introductory Experiment
S = light source
Incident ray
Reflected ray
boundary
Refracted ray
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Boundary : surface separating two transparent
media.
Reflection : sudden change of direction
experienced by light rays at the boundary while
remaining in the same medium.
Refraction : sudden change of direction
experienced by light rays when they cross the
boundary.
reflection
refraction
1. The laws of reflection
1.1. Experiments.

Consider a light ray hitting a plane (flat)
mirror (M) at point I (the point of
incidence).
 Diagram and vocabulary
N
S
i
O
r
M
I
S : light source
O : observer
I : point of incidence
M : reflecting surface (mirror)
SI : incident ray
IO : reflected ray
NI : normal at the point of incidence
i : angle of incidence
r : angle of reflection
SI and NI define the plane of incidence
1.2. The laws of reflection
By trying different values of i, we can
demonstrate experimentally that :
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the incident ray, the normal at the point of
incidence and the reflected ray are all situated in
the same plane
the angles of incidence and of reflection are equal
i=r
Laws of reflection (click me)
This is what
happens. The
incident ray, the
normal at the point
of incidence and the
reflected ray are all
situated in the
same plane.
This is what does
NOT happen. The
incident ray, the
normal at the point
of incidence and the
reflected ray are
NOT all situated in
the same plane.
2. Images formed by a plane (flat)
mirror
2.1. To find the image of a point,
draw a ray diagram

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the mirror
the observer’s eye is
at point O
the point object at A
A
O
To locate the image of the object use its
symmetry with respect to the mirror.
A
O
A’

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Draw a broken line from the image to the eye.
I is located at the point
where the line OA’ and
the mirror intersect.
Draw a solid line
(representing the
reflected ray of light) over
the portion IO of the line.
No real ray travels
between A’ and I.
A’
A
I
O
Draw a heavy line from A to I to represent the
incident ray.

Add arrows to
indicate the direction
of the light rays.
A’
A
I
O
The eye which receives the reflected rays ‘sees’
the image of the object at A'.


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A' is the image of A.
A’
A
The image is an ‘optical
illusion’ : this image
cannot be captured on a
screen. This type of
image is said to be a
O
‘virtual image’.
I

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An image is ‘real’ when the light rays
actually come together at the image. Real
images can be formed on a screen e.g.:
images formed on a screen when a film is
projected.
An image is ‘virtual’ if the light rays DO
NOT actually come together at the image.
A virtual image cannot be formed on a
screen.
2.2. How to find the image of an extended
object (as opposed to a simple point)
You must find the image of each point in the object
A
B
A'
B'
AB = object
A'B' = image of the object
Observer
A
B
A'
B'

The image formed by a plane mirror is:
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virtual (it cannot be formed on a screen)
symmetric with respect to the mirror (not
superposable on the object)
the same size as the object
2.3. Exercises
a) Construct the image of the letter M in the
following diagram:
mirror
observer
object
virtual
image
b) Among the letters of the alphabet, which ones
would have an image in a vertical mirror that
would be identical in shape to the letters
themselves?
ABCDEFGHIJKL
MNOPQRSTUVW
XYZ
The writing on this
ambulance is
reversed because…
… the image in a
plane mirror is also
reversed, so this is
what you would see in
the rear-view mirror:
3. Reflection in curved mirrors
An image formed behind
a convex mirror is
smaller than the object
An image formed behind a concave
mirror is bigger than the object
optical bench concave mirror
concave mirror
optical bench divergent mirror
convex mirror