Refraction – Learning Outcomes - PowerPoint Presentation

Slide1

Refraction – Learning Outcomes

Define refractive index.Demonstrate refraction.State the Laws of Refraction.Solve problems about refraction.HL: Solve problems about refractive index in terms of relative speeds.Give examples of refraction in nature.Define critical angle and total internal reflection.Demonstrate total internal reflection.

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Refraction – Learning Outcomes

Solve problems about total internal reflection.Give uses and natural occurrences of refraction.Discuss transmission of light in optical fibres.Give uses of optical fibres.2Slide3

Refraction

Refraction is the bending of light as it passes from one medium to another.3Slide4

To Demonstrate Refraction

Aim a narrowed beam from a ray box at the side of a block of glass.Vary the angle of incidence and note that the angle of refraction increases with angle of incidence.Note that the ray exiting the block is parallel to the incident ray.

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Laws of Refraction

The incident ray, the normal at the point of incidence, and the refracted ray all lie in the same plane.The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant.

The second law is also called “Snell’s Law” which we must verify experimentally.

The constant in the second law is the refractive index between the two media,

x

n

y

, i.e. the ratio of the absolute refractive indices.

Formula:

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Refractive Index

The refractive index of a medium is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light travels from a vacuum into that medium.

Material

Refractive Index

Vacuum

1 (by definition)

Air

1.0003

Water

1.33

Glass

~1.5 (varies with glass)

Diamond

2.4

Germanium

4.1

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Snell’s Law

e.g. A ray of light enters glass from air. The angle of incidence is 30o and the angle of refraction is 19o. What is the refractive index of the glass?e.g. A ray of light enters water from air. If the angle of incidence is 40o, find the angle of refraction if the refractive index of water is 1.33.e.g. Light enters water from glass. If the angle of incidence is 40

o

and the angle of refraction is 46.3

o

, what is the refractive index between glass and water?

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Depth

Due to refraction, objects immersed in a fluid will appear to be closer to the surface than they really are.This is given by:Formula:

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Depth

e.g. Sorcha draws a mark on a sheet of paper and places a glass block with thickness 8 cm over it. When viewed from above the glass, the mark appears to be 5.33 cm from the surface. What is the refractive index of the glass?e.g. A pool of water is 12 m deep. If the bottom of the pool is viewed from the air, how deep does it appear? nwater = 1.33

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Refraction in Nature – Bears

Bears have to recognise refraction when fishing – the fish appears to be higher up than it really is.10

Bear by Joseph Smit – public domain

Real fish by unknown artist – public domainSlide11

HL: Speed of Light

Light travels at different speeds in different media. The ratio of speeds between two media is the refractive index between them.Formula:

For any medium, it follows that:

Formula:

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HL: Speed of Light

e.g. The refractive index of water is 1.33. If the speed of light in air is

, what is the speed of light in water?

e.g. Light enters glass from air. The angle of incidence is 35

o

and the angle of refraction is 22

o

. If the speed of light in glass is

, calculate the speed of light in air.

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Total Internal Reflection

When light travels from a denser to a rarer medium, the critical angle, C is the angle of incidence which gives an angle of refraction of 90o.Total internal reflection (TIR) occurs when light travelling from a denser to a rarer medium is incident at an angle greater than the critical angle.

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To Demonstrate TIR

Aim a narrowed beam from a ray box at a semi-circular slab of glassAim the beam so that it is incident on the flat face of the slab internally.Starting with a small angle of incidence, increase this angle.When the critical angle is reached, the refracted ray skims along the flat face of the glass.

For higher angles of incidence, the refracted ray changes to a totally internally reflected ray.

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To Demonstrate TIR

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Total Internal Reflection

Given

, we can set up the equation for TIR.

If the rarer medium is a vacuum, then:

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Total Internal Reflection

e.g. The critical angle for a certain medium is 50o. Find its refractive index.e.g. The refractive index of glass is 1.5. What is the critical angle of glass?17Slide18

TIR in Nature – Mirages

The refractive index of air changes with temperature.On hot days, light from the sky can bend away from a road towards your eye, creating a “puddle” image.18Slide19

TIR in Nature – Mirages

19By Brocken Inaglory from Wikipedia – CC-BY-SA-3.0Slide20

TIR in Nature – Snell’s Window

Looking up while underwater, only light from within a certain radius will reach you – the rest is totally internally reflected from underwater. This is called Snell’s window.20Slide21

TIR in Nature – Snell’s Window

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by Jayme

Pastoric

– public domainSlide22

Uses

Prism reflectors are used in road signs to ensure that light from headlights reflects back at the driver.Safety reflectors on bikes and cars use the same effect.Many modern devices use round reflectors, which work on the same principle.

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Uses – Optical Fibres

Optical fibres are thin transparent glass rods that can transmit light via total internal reflection.23Slide24

Uses – Optical Fibres

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by

Timwether

– CC-BY-SA-3.0Slide25

Optical Fibres

Optical fibres are used to transmit telephone, television, and internet signals as pulses of light.It is better than the old copper cables in pretty much every way – lower loss, lower size, lower interference.They are also used in medicine as flexible cameras, called endoscopes.25