Sec 3 Light - Refraction Slides

7/30/2019 Sec 3 Light - Refraction Slides

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Refraction of light


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Objectives

Recall and use the terms for refraction, includingnormal, angle of incidence, angle of refraction

recall and apply the relationship sin i / sin r =constant to new situations or to solve relatedproblems

define refractive index of a medium in terms of theratio of speed of light in vacuum and in themedium

explain the terms critical angle and total internalreflection


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Word Splash

Use the following words and create a mindmap onRefraction!

bendingspeed of light

optical density

refractive index

vacuumair medium

apparent depth

real depth

towards

away

normal

different

r

in

sin

sin


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Refraction of light

Light source

Glass block

from: http://www.physics.brown.edu/physics/demopages/Demo/optics/demo/6a4210.htm


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What is Refraction?

Refraction is the bending of light as it travelsfrom one transparent medium into another.

This is due to a change in the speed of light

Medium: the substance that light passesthrough

E.g. air, water, glass, plastic, diamond

Optical density measure of the material toenable transmission of light

The more optical denser the medium, the slower

light travels


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When does refraction occur?

Refraction occurs when

1. there is a change in speed of the light wave,

and

2. the light wave approaches the boundary atan angle other than 90 to the boundary.


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Law of Refraction

The incident ray, the normal and the refractedray all lie in the same plane.

For 2 particular media, the ratio of the sine of

the angle of incidence to the sine of the angleof refraction is a constant i.e.

Also known as Snells Law

constantsin

sin

r

i

Willebrord Snell (1580 1626)

from: http://www.iesfelanitx.org/departaments/fisica-quimica/retrats/snell.jpg


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Refractive index

Refractive index, n, of a medium is the ratio of the

speed of light in vacuum and the speed of light in

that medium.

The bigger the value ofn, the more optically denserthe substance.

Refractive index affects how much light bends.

v

cn

mediuminlightofspeedvacuuminlightofspeed


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Refractive index

The ratio is constant, however it differs for

each medium.

For the special case oflight passing from a

vacuum (or in practice, air)into a given medium,the constant ratio is equal to the refractive index

of the medium.

r

i

sin

sin

vacuum

medium

i

rr

in

sin

sin


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When light travels from an optically less dense to a

denser medium, light bends towards the normal

When light travels from an optically denser to a less

dense medium, light bends away from the normal

normal

Incident ray

i

r

refracted ray

water

air

iis greater than r

normal

refracted ray

r

i

incident ray

water

air

iis smaller than r


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Example 1

The diagram shows a ray of light incident on thesurface of a liquid and its subsequent path. What

is the refractive index of the liquid?

air

liquid

59

40

i = 59 and r = 40,

rsinisinn

331

40sin

59sinn

.


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Example 2

air

crown glass

i

If light is incident upon crown glass (n = 1.52) atan angle of 40, what is the angle of refraction?

Also, complete the drawing of the ray diagram.

rsinisinn

rsin

40sin521

.

1.52

40sinrsin

0.25

)52.1

40sin(sin 1

r

r


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Reversibility of light

The principle of reversibility states that lightwill follow exactly the same path if its

direction of travel is reversed.

air

crown glass

40

25

air

crown glass

40

25


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Example 3

A ray of light emerges from water (n= 1.33) to air.Calculate the angle of refraction, r.

air

water

r

35

Applying reversibility of light,

use ras the angle of incidence and

35 as the angle of refraction.

i.e. solve the question as though light

is passing from air to water.

rsin

isinn

7.49

)35sin33.1(sin

35sin33.1sin35sin

sin33.1

1

r

r

r

r


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Apparent depth

Due to refraction, the depth of water looksshallower.

This perceived depth is called apparent depth.

real

depth

apparent

depth


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You can use apparent depth to calculate therefractive index of a medium.

depthapparentdepthrealn


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Distortion of objects

Due to refraction, the stick appears bentunderwater.


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Distortion of objects


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Multiple images

How manygoldfish are there?


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Investigating Total Internal

Reflection

Calculate the angle of refraction of the following lightrays. Sketch the refracted ray. The refractive index

of glass is 1.50.

30

airglass

41.81 41.82

Applet: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=49


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A light ray enters a glass block:

When the angle of incidence is zero at theglass/air boundary, the ray emerges without

deviation.

glass

air

incident ray

undeviated ray

Semi-circular

glass block

i = 0


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A light ray exits a glass block:

When the angle of incidence is less than thecritical angle, there will be refracted ray and

also a weak internal reflection.

Strong

refracted

ray

Weak, internally

reflected ray

i

i c


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A light ray exits a glass block:

When the angle of incidence is equal to thecritical angle, the angle of refraction in the

less dense medium is 90.

90

c c

Strong

refracted

ray

Weak, internally

reflected ray

i = c


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Critical Angle :

Critical angle is defined as the angle of incidence inthe optically denser medium for which the angle of

refraction in the less dense medium is 90.

cn

c

n

rin

sin

sin

sin

sinsin

1

90

90

c

n: refractive index

c: critical angle


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When the angle of incidence is greater

than the critical angle, the ray is reflected

totally within the block.

This is known as total internal reflection.

strong totally

internally

reflected ray

i c

i

C diti f t t l i t l


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Conditions for total internal

reflection to occur:

The light ray travels from an optically densermedium to an optically less dense medium.

The angle of incidence at the point ofincidence is greater than the critical angle of

the optically denser medium.

video: http://www.youtube.com/watch?v=5Q4Cl6sDxu8


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Example:

A right angled prism is made of glass ofrefractive index 1.5. A ray of light enters the

prism.

45

(a) Calculate the critical angle of the prism

(b) Complete the path of the ray until it emerges into

the air again.


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Applications of total internal reflection

1. Periscopes

- Can be used to look over high obstacles, eg. wall

- Consists of 2 prisms positioned at an angle of 45o.

2. Camera


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Optical Fibre

consists of a central glass core of high refractiveindex glass or plastic, coated with a thin layer ofglass of lower refractive index.

a light ray entering the pipe is totally internallyreflected.

light will travel the whole length of the light pipe withrepeated total internal reflections at the surfacesand then emerges from the other end of the pipewithout loss of intensity.

can be used in endoscope to enable doctors to seeorgans inside the human body and also used intelecommunications. video: http://www.youtube.com/watch?v=UPzSuh7BPKM


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Ad t f i ti l


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Advantages of using optical

fibres:

They are lighter. They are becoming cheaper to manufacture

than wires.

They can carry more information over longdistances than copper wire.

They will experience less signal loss as

compared to wires during transmission.

Documents

Sec 3 Light - Refraction Slides