# ch3 Reflection ```Ch.3 Reflection (反射)
Reflection of Light (光的反射)
normal
angle of angle of
incidence reflection
incident ray
reflected ray
plane mirror
Incident Rays
The light ray moving towards the mirror
(入射線)
Reflected Rays
The light ray moving out of the mirror
(反射線)
Normal
The line perpendicular (垂直) to the mirror
(法線)
Angle of incidence
The angle between the incident ray and the normal.
(入射角)
Angle of reflection
The angle between the reflected rays and the normal.
(反射角)
3
Laws of reflection (反射定律)
Laws of reflection:
1. The angle of
reflection r is
equal to the angle
of incidence i.
i = r
normal
incident
ray
i
r
vertical
plane
reflected
ray
plane mirror
Laws of reflection:
normal
2. The incident ray,
the reflected ray
and the normal
all lie in the same
plane.
incident
ray
vertical
plane
reflected
ray
plane mirror
When the angle of incidence equals to zero
(i = 0) , the angle of reflection is also zero
(r = 0).
i=0
r=0
air
mirror
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7
40&deg;
60&deg;
40&deg;
60&deg;
30&deg;
45&deg;
30&deg;
45&deg;
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5.3 Experiment (實驗)

9
i /
r /
0
30
45
60
75
Conclusion (結論)
The angle of incidence is ____________ the
angle of reflection.
10
i /
r /
0
0
30
30
45
45
60
60
75
75
Conclusion (結論)
The angle of incidence is _equal_ the angle
of reflection.
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Regular reflection (單向反射)
Will the reflected rays
remain parallel (平行)
after reflection?
For flat (平) and smooth (滑) surface,
parallel reflected rays
parallel incident rays
i
r
i r
i
r
plane mirror
 Each light ray obeys the laws of reflection.
same i
same r
 The reflected rays are therefore parallel.
Regular reflection (單向反射) occurs at:
plane mirror
The following reflecting surfaces can form clear
images due to regular reflection.
metal surface
a calm water surface
Diffuse reflection (漫反射)
For rough (粗糙) surface,
parallel incident rays
i1 r1
i2
i3
r2
reflected rays are not
parallel
r3
rough surface
Each light ray obeys the laws of reflection.
different i
different r
Diffuse reflection occurs at a rough
surface (e.g. a piece of paper).
e.g. a piece of paper
Lateral inversion (橫向倒置)
Mirror images are laterally inverted.
the object
image formed by the
plane mirror
The laterally inverted words on an
ambulance (救護車)
An image formed by a plane mirror is
1. same
size as the object,
2. laterally
3.
inverted. (橫向倒置)
same distance
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Uses of plane mirrors (平面鏡的應用)
rear-view mirrors (倒後鏡)
23
Explain why the Chinese characters 救護車 are printed
in the special way as shown.
24
So that the driver of the car in front of the
ambulance can see the Chinese characters

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Periscope (潛望鏡)
 A periscope consists of two mirrors
 The mirrors make an angle of 45 with the
line joining them.
Periscopes are used in a double-deckers (雙層巴士)
Periscope (潛

submarine
(潛水艇).
28
Plane mirrors used in interior design (室內設計).
29
Plane mirror used by dentist (牙醫).
30
In Figure 5.4, draw ONE light ray from the head of
the object to the eye after reflection from the two
mirrors.
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