Download Topic: Refraction of Light

Topic: Refraction of Light
New words:
normal 法线
boundary 界面
incident ray
入射光线
reflected ray
反射光线
refracted ray
折射光线
the angle of incidence
入射角
the angle of reflection
反射角
the angle of refraction
the reflection of light
折射角
光的反射
the refraction of light 光的折射
The Law of Reflection
反射定律
The Law of Refraction
折射定律
Snell’s law 斯涅耳定律
the refractive index
绝对折射率
the relative refractive index
Speed of light 光速
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相对折射率
T: At first, boys and girls, let’s discuss a question: What will
happen if a ray approaches a mirror?
S: The light will be reflected.
T: Which law does the reflection of light follow?
S: It follows The Law of Reflection(反射定律).
T: Yes, when a light ray is incident upon a reflecting surface,
the angle of reflection is equal to the angle of incidence. The
incident ray, the reflected ray, and the normal all lie in the
same plane. （当光线射到界面上，反射角等于入射角；入射光线、反
射 光 线 与 法 线 在 同 一 个 平 面 内 。） We call it “the law of
reflection”.
angle of angle
incident ray
of
reflected ray
incidenc reflection
e
mirror
T: If the light ray did not approach to a mirror but to a surface
of water, please guess what will happen?
S: Some of the light ray is reflected back into air, the other is
traveling into water.
T: The light ray in the water will be near or away from the normal
or the direction will not change?
Students discuss for a moment.
Teacher do the experiment:
A bundle of laser incident from air into water.
Student observer the experiment and answer .
Teacher use computer show the three light rays and say:
The path of light ray bends when traveling from one material to
another, such as from air to water, if the angle of incidence
is not zero degree. We call this appearance Refraction.
Incident ray
1
i
Reflected ray
boundary
水
2
2
r
normal
Refracted ray
the definition of Refraction:
The change in direction of light at the boundary between two
materials is call refraction.
angle of incidence
Incident ray
1
normal
i
Reflected ray
boundary
水
2
r
angle of refraction
normal
Refracted ray
T: The angle between incident ray and normal is called “angle of
incidence”. The angle between refracted ray and normal is called
“angle of refraction”. From the experiment and the model of light
we also can find the position of incident ray and refraction ray:
The incident ray、the refracted ray and the normal all lie in
the same plane. The normal lies between the incident and
refracted ray.(入射光线、折射光线和法线在同一平面内，且入射
光线折射光线分居在法线两侧。)
experiment:show the direction of the ray in changing
(1) If the angle of incidence is zero, the angle of refraction
is zero too.
(2) If the angle of incidence is increase, the angle of
refraction is increase too.
(3) If the angle of incidence is decrease, the angle of
refraction is decrease too.
T: But the angle of refraction is always smaller than the angle
of incidence when the ray travels from air into water If we
repeat the experiment use glass or other transparent material,
we can acquire the same conclusion.
Using computer show the light model while saying:
3
1 i
1 i
2
2
Conclusion: The ray in the transparent material is always
nearer to the normal than the ray in the air, if the incident
angle is not zero degree.
T: a ray travels from air to water; we can draw the model of light.
If a ray of light travel from water to air and the second incident
ray is against the first refracted ray, what is the direction
of the refracted ray in air?
S: It will be along the incident ray in the air.
air
water
i
air
water
r
i
r
Conclusion:
The reversible quality of the light.(光路是可逆的。)
Watch two pictures:
T: What do you find from the pictures?
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S: The person’s legs look shorter than normal one’s and the
pencil is broken.
T: Why?
S: The direction of the light ray changes when it travels from water
to air.
T: can you draw a model of light to describe the second picture?
Ask students draw it on the paper and show one of them by computer
and comment it with students.
Then teacher draws two pictures using computer quickly:
Conclusion: The object is looked shallower than it actually is,
when it is under the water.
T: We know the reflection of light follows the law of reflection.
Have you thought which law the refracted ray follows when it bends?
Physicists investigated it for a long time, until 1621,the Dutch
physicist Willebrand Snell found the relationship of the refracted
ray and incident ray. He measured the incited and refracted angles
when light traveled from air to glass. Let’s repeat the experiment,
and record the values. Can you find the conclusion from the values?
Do the experiment and show the values one by one by
computer:
Incited angle 1
Refracted angle 2
100
200
6.70
13.30
1/2
1.50
1.50
5
sin1/sin2
1.49
1.49
300
400
500
600
700
800
19.60
25.20
30.70
35.10
38.60
40.60
1.53
1.59
1.63
1.71
1.81
1.97
1.49
1.51
1.50
1.51
1.50
1.51
The 1 、2 and 1/2 show together, sin1/sin2 show finally.
T: From the table the ratio of the sins of angles of incidence and
refraction is constant.
sin  1
=constant
sin  2
Snell repeated his experiments with different materials and found
that the ratio was still constant. Snell discovered the
relationship so it is called Snell’s law or refraction law.
Snell’s law(斯涅耳定律)
A ray of light bends in such a way that the ratio of the sine
of the angle of incidence to the sine of the angle of refraction
is a constant.（入射角的正弦和折射角的正弦成正比。）
sin  1
=constant
sin  2
T: Snell found that it had different constant when the ray of light
traveled from air to different materials. For example, if the ray
traveled from air to water, the constant was 1.33; if the ray
traveled from air to glass, the constant is 1.50. So the ratio of
the sins depends on the character of the material. If the ratio
of one material is bigger than the other, its ability of bending
light is bigger too. So if a light ray passing from a vacuum into
a given material, this constant is called the index of refraction
for that material.
It can be written:
sin i
n=
sin r
In this equation i is the angle of incidence. r is the angle of
refraction and n is the index of refraction of the material.
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“n” of the material is bigger the ability of bending light
is bigger too.
Show index of refraction of some materials:
Material
Value
vacuum
1.00
air
1.0028
water
1.33
(4/3)
glass
1.50
(5/2)
You can find from the table the n of air approach to the n of
vacuum, so we always think that the index of refraction of air is
equal to 1.
Let’s do some exercises:
Example 1:
A ray of light strikes a glass block of refraction index
1.5 at an angle of incidence of 300, what is the angle of
refraction?
Solution:
1
2
nglass=1.5
nglass=
air=300
glass=?
sin  air
sin  glass
sin 30 0 0.5
singlass=
=
=0.33
1 .5
1.5
T:
glass=20.170
=200
Note:
(1)
The refractive index is talk about the light traveling
from vacuum to another material.
Please think about this problem:
A ray incited from air into glass, if the incident angle is
zero degree, what is the angle of refraction?
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S: the refractive angle is zero degree too.
T: Yes, The direction of the light doesn’t change. How much is
the refractive index now?
S: The refraction index of the glass is still equal to 1.5.
T: We must notice:
(2) The refractive index only depends on the properties of
two different materials. It doesn’t depend on the sini
and sinr or angle of incidence i and angle of refractive
r.
T: If the light ray travels from material 1 to material 2,we call
the index of refraction material 2 to material 1, such as a ray
travels from water into glass.
Material 1
1
water
n1
glass
n2
2
Material 2
The snell’s law can be write:
(3)
sin  1 n 2
=
sin  2 n1
n1 is the index of refraction of the material 1 and n2 is
the index of refraction of the second medium. Angles 1 and
2 are the angles of incidence and refraction, respectively.
The value of the ratio is called the relative refractive
index.
T: Why does the light bend when it travels in the different
materials?
Because the light has different speed in different material.
S0 the refractive index has another meaning:
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Refractive index =
speed of
speed of
light
light
in vacuum
in material
The formula is:
c
v
T: Because light has its highest speed in vacuum, the index of
refraction is always larger than 1.
You can read the table of refractive index, if the index is
larger than others; the speed of the light is smaller than others.
Example 2:
The refractive index of glass is 1.5 .How fast does
light travel in it?
Solution:
nglass=1.5 c=3.0108
vglass=?
n=
nglass=
c
v glass
Speed of light in glass:
c 3.0  10
vglass= =
=2.0108m/s
n
1.5
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T: If the light ray travels from material 1 to material 2, such
as a ray travels from water into glass. the speed of light in
water is v1, the speed in glass is v2.
Material 1
water

V1 n1
1
glass

V2
2
Material 2
n2
The snell’s law also can be write:
sin  1 n 2 v 2
= =
sin  2 n1 v1
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Example:
Continue with the example 1, the ray of light from the glass block
travels into air, what is the angle of refraction in the air?
300
20
0
i
20
0
300
nglass=1.5
nair=1.0
glass=200
air=?
sin  air n glass
=
sin  glass n air
sinair=
n glass
n air
singlass=
1 .5
sin 20 0 =0.5
1
air=300
From this example we can have a conclusion:
A parallel plate of transparent material does not alter the
direction of travel of a light beam.（经过均匀物质的两平行面，
光的传播方向不变。）
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