EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 3-3Light WavesEdexcel IGCSE Physics pages 107 to 117June 17th 2012Content applying to Triple Science only is shown in red type on the next slide and is indicated on subsequent slides by TRIPLE ONLY

Edexcel SpecificationSection 3: Wavesd) Light and soundunderstand that light waves are transverse waves which can be reflected, refracted and diffracteduse the law of reflection (the angle of incidence equals the angle of reflection)construct ray diagrams to illustrate the formation of a virtual image in a plane mirrordescribe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prismsknow and use the relationship: n = sin i / sin rdescribe an experiment to determine the refractive index of glass, using a glass blockdescribe the role of total internal reflection in transmitting information along optical fibres and in prismsexplain the meaning of critical angle cknow and use the relationship: n = 1 / sin cRed type: Triple Science Only

Reflection of light

Law of ReflectionThe angle of incidence (i) is equal to the angle of reflection (r)

Note: Both angles are measured with respect to the normal. This is a construction line that is perpendicular to the reflecting surface at the point of incidence.

The image formed by a plane mirrorThe image produced by the plane mirror is:The same size as the objectThe same distance behind the mirror as the object is in frontUpright (the same way up as the object)Back-to-front compared with the object (lateral inversion)Virtual

Virtual imagesVIRTUAL images are formed where light rays only appear to come from. A virtual image cannot be cast onto a screen.

The image formed by a projector is known as a REAL image because light rays travel to it.

Choose appropriate words to fill in the gaps below:The law of reflection states that the angle of __________ is always _______ to the angle of incidence. Both angles are measured relative to the _________, a line that is at _______ degrees to the reflecting surface at the point of reflection.A plane mirror forms a _______ image which is unlike a _____ image in that it cannot be cast onto a screen. The image in a plane mirror is also the same ______ and the same way up as the object.realvirtualreflectionsizenormalequalninetyWORD SELECTION:realvirtualreflectionsizenormalequalninety

Light Refraction

Refraction occurs when a wave changes speed as it passes from one region to another. This speed change usually causes the wave to change direction. Water waves slow down as they pass over from a deeper to a shallower region.Light slows down as it passes from air into glass, perspex or water.

Refraction experimentTypical results:No deviation occurs when the angle of incidence is zero.Increasing the angle of incidence increases the deviation.

angle of incidence / angle of refraction / deviation / 00015105301911452817603525754035

Refraction of light at a plane surface(a) Less to more optical dense transition (e.g. air to glass)Light bends TOWARDS the normal.The angle of refraction is LESS than the angle of incidence.

(b) More to less optical dense transition (e.g. water to air)Light bends AWAY FROM the normal.The angle of refraction is GREATER than the angle of incidence.

Why a pool appears shallow

Complete the paths of the RED light rays:

The refraction equationWhen a light ray passes from one medium to another:

n = sin i sin r

where:i is the angle of incidence in the first mediumr is the angle of refraction in the second mediumn is a constant number called the refractive index.

An experiment to find the refractive index (n) of glassSet up the equipment as shown in the diagram oppositeFor an initial angle of incidence, i of 30 trace the path of the light ray.Measure the angle refraction, r.Calculate the refractive index using the formula: n = sin (i) / sin (r).Repeat for a range of angles between 10 and 80.Calculate the average value of n.

Question 1Calculate the refractive index when light passes from air to glass if the angle of incidence is 30 and the angle of refraction 19.n = sin i / sin r= sin (30) / sin (19) = 0.500 / 0.326refractive index , n = 1.53

Question 2Calculate the angle of refraction when light passes from air to perspex if the angle of incidence is 50 and the refractive index, n = 1.50.n = sin i / sin r1.50 = sin (50) / sin (r ) becomes: sin (r ) = sin (50) / 1.50= 0.766 / 1.50sin (r ) = 0.511angle of refraction = 30.7

Question 3Calculate the angle of incidence when light passes from air to water if the angle of refraction is 20 and the refractive index, n = 1.33.n = sin i / sin r1.33 = sin (i) / sin 20becomes: sin (i) = 1.33 x sin (20) = 1.33 x 0.342sin (i) = 0.455angle of incidence = 27.1

Complete: Answers

medium 1medium 2nirairwater1.3350o35.2oglassair0.6730o48.6owaterglass1.1359.8o50oairdiamond2.4050o18.6oairunknown1.5350o30o

Dispersion Dispersion occurs when a prism splits the colours of white light into the spectrum.

This occurs because the refractive index of the glass or perspex of the prism varies with the colours of the spectrum that make up white light.

Violet has the greatest refractive index and therefore deviates the most.Red has the lowest and deviates the least.

Choose appropriate words to fill in the gaps below:Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes.Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________.Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________.mostspeeddirectiondeviationcoloursperspextowardsWORD SELECTION:dispersionmostspeeddirectiondeviationcoloursperspextowardsdispersion

Total internal reflectionTotal internal reflection occurs when:

Light is incident on a boundary between optically more to less dense substance (for example glass to air).

2. The angle of incidence is greater than the critical angle, c for the interface.

Angle of incidence greater than the critical angle:NO Refraction and TOTAL INTERNAL REFLECTIONAngle of incidence less than the critical angle: Refraction and PARTIAL reflectionAngle of incidence equal to the critical angle: Refraction at 90 and PARTIAL reflection

Critical angle equationThe critical angle is the angle of incidence in the denser medium that results in an angle of refraction of 90

n = 1 sin c

where:n is the refractive index of the denser medium (glass in the example opposite).c is the critical angle.

Question 1Calculate the critical angle of glass to air if the refractive index of glass is 1.5n = 1 / sin c= 1.0 / 1.5= 0.67 critical angle for glass, c = 42

Question 2Calculate the critical angle of water to air if the refractive index of glass is 1.3n = 1 / sin c= 1.0 / 1.3= 0.75 critical angle for water, c = 49

Question 3Calculate the maximum refractive index of a medium if light is to escape from it into water (refractive index = 1.3) at all angles below 30.n = 1 / sin cbecomes:sin c = 1 / nsin 30 = 1.3 / n0.5 = 1.3 / n = 1.3 / 0.5maximum refractive index, n = 2.6

Uses of total internal reflection1. Prismatic periscope

Glass and perspex both have critical angles of about 42.

In each prism the light strikes the glass-air interface at an incidence angle of 45

Total internal reflection therefore occurs and the light ray is deviated by 90 in each prism.

2. Reflectors

The reflector is made up of many small perspex prisms arranged so that light undergoes total internal reflection twice.

The overall result is that the light is returned in the direction from which it originally came.

The reflector will be seen to be lit up from the point of view of the light source for example the driver of a car with its headlights on.

3. Optical fibres

Optical fibre consists of two concentric layers of different types of glass, core and cladding.

Light entering the inner core always strikes the boundary of the two glasses at an angle that is greater than the critical angle.

Optical fibre communicationOptical fibres can be used to transmit information using visible light or infra-red radiation. The light cannot escape from the fibre, it is continually reflected internally by the fibre.

Compared with microwaves and radio waves optical fibres:can carry far more information due to the higher frequency of light and infra-red.are more secure because the signals stay within the fibres.

The fastest broadband uses optical fibres.

The EndoscopeThe medical endoscope contains two bundles of fibres. One set of fibres transmits light into a body cavity and the other is used to return an image for observation.

Diffraction of lightDiffraction occurs when waves spread out after passing through a gap or round an obstacle.TRIPLE ONLY

Diffraction becomes more significant when the size of the gap or obstacle is reduced compared with the wavelength of the wave.

The wavelength of light is about 0.0005mm.

Therefore light diffraction is only noticeable with very small apertures or where an image is highly magnified.TRIPLE ONLY

Online SimulationsLaw of Reflection - NTNU - features a movable plane mirror Reflection in a plane mirror - eChalk Lateral inversion demo showing law of reflection - Freezeway.com Very simple ray reflection - Freezeway.com Reflection at a plane mirror with a protractor - Freezeway.com Reflection at a plane mirror ray diagram - Freezeway.com Balloon blasting game - eChalk Height of mirror and image seen - NTNU Virtual image formation - eChalk Image formed by a plane mirror - NTNU Reflection and images from two mirrors at 90 degrees to each other - NTNU Lens / mirror effect on a beam of light - NTNU Multiple reflections from two plane mirrors - NTNU Bending Light PhET - Explore bending of light between two mediums with different indices of refraction. See how changing from air to water to glass changes the bending angle. Play with prisms of different shapes and make rainbow. Refraction through a rectangular block - Freezeway.com Refraction through a rectangular block with some protractors - Freezeway.com Light Refraction - Fendt Refraction explained - Fendt Reflection & Refraction at a boundary related to wave speed - NTNU Refraction animation - NTNU - Does not show TIR effect Prism - non dispersive reflections and refractions - NTNU Prism/Lens - non dispersive refraction and reflections - NTNU Refraction by a semicircular block - Freezeway.com Refraction through a semicircular block with protractors - Freezeway.com Light moving from water to air or vice-versa - NTNU Where is the fish? - refraction by water - NTNU The appearance of an object under water / ray diagram - NTNU How a fish sees the world - NTNU Fibre optic reflection - NTNU Dispersion with the effect of filters - Freezeway.com Dispersion of light using a prism - NTNU - prism apex angle can be changed Prism showing light dispersion for different colours - Explore Science Prism - multishape prism and single light ray - no extra reflections - netfirms Wave Effects - PhET - Make waves with a dripping faucet, audio speaker, or laser! Add a second source or a pair of slits to create an interference pattern. Also shows diffraction. Diffraction at a single slit - Fendt Single slit diffraction - wavelength adjustable - NTNU Diffraction from a single slit - netfirms Diffraction around an obstacle - netfirms BBC AQA GCSE Bitesize Revision: Optical fibres

Diffraction past a barrier - netfirms Resolution from two circular apertures - NTNU

Light WavesNotes questions from pages 107 to 117Draw a diagram illustrating the law of reflection. (see page 107)With the aid of a diagram explain how a plane mirror forms an image. also list the properties of this image. (see page 108)(a) What is refraction? (b) Draw a labelled diagram showing how a light ray travels through a rectangular glass block. (see page 109)(a) State the equation relating incident and refraction angles. (b) Calculate the angle of refraction with glass (n = 1.5) if the angle of incidence is 55. (see page 110)(a) Explain what is meant by total internal reflection and critical angle. (b) state the equation for critical angle and calculate the value of this angle for a substance of refractive index 1.4. (see pages 111 and 112)With the aid of diagrams explain how total internal reflection is used in (a) prismatic periscopes and (b) optical fibres. (see pages 112 to 114)

(a) What is meant by dispersion? (b) How is it caused? (c) Draw a diagram (on colour) showing how a prism can disperse white light. (see page 115)Answer the questions on pages 116 and 117.Verify that you can do all of the items listed in the end of chapter checklist on page 116.

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