1© Manhattan Press (H.K.) Ltd. Reflection Refraction Refraction 12.1 Reflection and refraction Total internal reflection Total internal reflection

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• ReflectionReflection• • RefractionRefraction

12.1 Reflection and 12.1 Reflection and refractionrefraction

• • Total internal reflectionTotal internal reflection


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



Reflection and refraction

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Obey the Laws of Reflection

Obey the Laws of Refraction



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’



Reflection

2. Plane mirrors



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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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




Refraction


n1 sinθ1 = n2 sinθ2

n = n2/n1

Note: For air, n 1.00.



Refraction


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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



Refraction

2. Examples of refraction

(a) Real depth and apparent depth

depthApparent depth Real

n



Refraction


(b) Refraction by rectangular glass block

2

2121

2

21

cossin

sincos

sin )(nt displaceme Lateral

aa

ABBCd



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



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




Examples of total internal reflection

(a) Reflecting prism




Examples of total internal reflection

(b) Optical fibre

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Example 2Example 2


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Ray

A ray is a parallel beam of negligible thickness.

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It is a good way to use a laser beam to demonstrate how the light behaves in reflection and refraction.

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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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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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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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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



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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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



Solution:Solution:

5978482

8483311sin

2

2

1

..

.c,.n

nc

c

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1© Manhattan Press (H.K.) Ltd. Reflection Refraction Refraction 12.1 Reflection and refraction Total internal reflection Total internal reflection