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© Manhattan Press (H.K.) Ltd. 1
• ReflectionReflection• • RefractionRefraction
12.1 Reflection and 12.1 Reflection and refractionrefraction
• • Total internal reflectionTotal internal reflection
© Manhattan Press (H.K.) Ltd. 2
12.1 Reflection and refraction (SB p. 198)
Reflection and refraction
Light – travels in straight lines (diffraction and interference of light are not prominent)
Light incidents on glass, it is reflected and refracted
incident ray reflected ray
refracted rayGo to
More to Know 1More to Know 1
© Manhattan Press (H.K.) Ltd. 3
12.1 Reflection and refraction (SB p. 198)
Reflection and refraction
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More to Know 2More to Know 2
Obey the Laws of Reflection
Obey the Laws of Refraction
© Manhattan Press (H.K.) Ltd. 4
12.1 Reflection and refraction (SB p. 199)
Reflection
1. Laws of Reflection
Laws of Reflection:1. The incident ray, the normal to the surface and the reflected ray are alllie in one plane.2. The angle of reflection is equal to the angle of incidence. i.e. θ1 = θ1’
© Manhattan Press (H.K.) Ltd. 5
12.1 Reflection and refraction (SB p. 199)
Reflection
2. Plane mirrors
© Manhattan Press (H.K.) Ltd. 6
12.1 Reflection and refraction (SB p. 200)
Reflection
2. Plane mirrors
Note:
1. The properties of the image:• virtual (image cannot be formed on screen)• erect• same size as the object• laterally inverted
2. The image distance (v) is equal to the object distance (u).
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More to Know 3More to Know 3
© Manhattan Press (H.K.) Ltd. 7
12.1 Reflection and refraction (SB p. 200)
Refraction
1. Laws of Refraction
Laws of Refraction:(a) The incident ray, the normal and the refracted ray at the point of incidence are all lie in one plane.(b) At the interface between any two different media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for any particular wavelength of the ray. This is named as Snell’s Law and defined as:
1
2
2
1 )(constant sinsin
nn
n Go to
More to Know 4More to Know 4
© Manhattan Press (H.K.) Ltd. 8
12.1 Reflection and refraction (SB p. 200)
Refraction
1. Laws of Refraction
n1 sinθ1 = n2 sinθ2
n = n2/n1
Note: For air, n 1.00.
© Manhattan Press (H.K.) Ltd. 9
12.1 Reflection and refraction (SB p. 201)
Refraction
1. Laws of Refraction
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More to Know 5More to Know 5
In vacuum - light travels at c
Other media – light travels at lower speed
medium in thelight of Speedin vacuumlight of Speed
)(index refractive Absolute n
higher n, greater bending of refracted ray
© Manhattan Press (H.K.) Ltd. 10
12.1 Reflection and refraction (SB p. 201)
Refraction
2. Examples of refraction
(a) Real depth and apparent depth
depthApparent depth Real
n
© Manhattan Press (H.K.) Ltd. 11
12.1 Reflection and refraction (SB p. 202)
Refraction
2. Examples of refraction
(b) Refraction by rectangular glass block
2
2121
2
21
cossin
sincos
sin )(nt displaceme Lateral
aa
ABBCd
© Manhattan Press (H.K.) Ltd. 12
12.1 Reflection and refraction (SB p. 203)
Refraction
2. Examples of refraction
(c) Refraction by prism
2sin
2sin
sinsin
:Law sSnell'By 22
when deviation Minimum
)( angle Refracting
)(deviation of Angle
min
2
12
2min
1
2211
min
22
2121
A
DA
n
A,DA
','DD
'A''D
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Example 1Example 1
© Manhattan Press (H.K.) Ltd. 13
12.1 Reflection and refraction (SB p. 204)
Total internal reflection
1 increases, 2 is larger
1 to finite value (critical angle),
2 =90o
1 > c, all light are reflected
11
2
211sin
90sinsin
nnn
c
ncn
© Manhattan Press (H.K.) Ltd. 14
12.1 Reflection and refraction (SB p. 205)
Total internal reflection
Examples of total internal reflection
(a) Reflecting prism
© Manhattan Press (H.K.) Ltd. 15
12.1 Reflection and refraction (SB p. 206)
Total internal reflection
Examples of total internal reflection
(b) Optical fibre
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Example 2Example 2
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End
© Manhattan Press (H.K.) Ltd. 17
Ray
A ray is a parallel beam of negligible thickness.
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12.1 Reflection and refraction (SB p. 198)
© Manhattan Press (H.K.) Ltd. 18
It is a good way to use a laser beam to demonstrate how the light behaves in reflection and refraction.
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12.1 Reflection and refraction (SB p. 198)
© Manhattan Press (H.K.) Ltd. 19
Moving of object in front of mirror
When an object moves towards a fixed mirror at a speed v, its image moves at the same speed v towards the mirror too.
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12.1 Reflection and refraction (SB p. 200)
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Frequency of light in refraction
Only the speed and the wavelength of the light changes when it travels from one medium to another. Its frequency remains unchanged.
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12.1 Reflection and refraction (SB p. 200)
© Manhattan Press (H.K.) Ltd. 21
Refractive index and frequency
The refractive index varies with the frequency of light. The higher the frequency of light, the greater the refractive index.
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12.1 Reflection and refraction (SB p. 201)
© Manhattan Press (H.K.) Ltd. 22
Q: Q: Light incidents on a 5 mm thick of glass plate with refractive index 1.48 at angle of incidence of 50°. What is the lateral displacement x of the light ray after transmitting through the glass?(The refractive index of air is about 1.00.)
Solution
12.1 Reflection and refraction (SB p. 203)
© Manhattan Press (H.K.) Ltd. 23
Solution:Solution:
mm 88123150sin231cos
5 4From
231 sin48150sin001 1From
4sincos
3From
sin
and ,In
3cos
cos then , Let
(2), and (1) From2sinsin
1sinsin :givethey
air, toglass of interface theand glass toair of interface at the applied is Law sSnell'
2
2
212
21
21
22
31
3322
2211
...
x,
...,
............dx,rxBACxBCABC
...dr,rdrAB
.............nn.............nn
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12.1 Reflection and refraction (SB p. 204)
© Manhattan Press (H.K.) Ltd. 24
Q: Q: If you look up from the bottom of a swimming pool, you can see the objects above water subtended within a cone as shown in the figure. Find the angle θ. Given that the refractive index of water is 1.33.
Solution
12.1 Reflection and refraction (SB p. 206)
© Manhattan Press (H.K.) Ltd. 25
Solution:Solution:
5978482
8483311sin
2
2
1
..
.c,.n
nc
c
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12.1 Reflection and refraction (SB p. 206)