EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 3-3

Light Waves

Edexcel IGCSE Physics pages 107 to 117 Content applying to Triple Science only is shown in

red type

on the next slide and is indicated on subsequent slides by ‘

TRIPLE ONLY

’ June 17 th 2012

Edexcel Specification

Section 3: Waves

d) Light and sound understand that light waves are transverse waves which can be reflected, refracted and diffracted use 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 mirror describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms know and use the relationship:

n = sin i / sin r

describe an experiment to determine the refractive index of glass, using a glass block describe the role of total internal reflection in transmitting information along optical fibres and in prisms explain the meaning of critical angle

c

know and use the relationship:

n = 1 / sin c

Red type: Triple Science Only

Reflection of light

Law of Reflection

The angle of incidence (i) is equal to the angle of reflection (r) incident ray angle of incidence, i normal

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.

angle of reflection, r reflected ray mirror

The image formed by a plane mirror

object image normals construction lines (virtual light rays) plane mirror

The image produced by the plane mirror is:

1. The same size as the object 2. The same distance behind the mirror as the object is in front 3. Upright (the same way up as the object) 4. Back-to-front compared with the object (lateral inversion) 5. Virtual

Virtual images

VIRTUAL 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:

point of reflection.

way up as the object.

normal virtual

WORD SELECTION:

size reflection real equal ninety

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 experiment

Typical results:

angle of incidence / °

0

angle of refraction / °

0 15 10 30 45 60 75 19 28 35 40

deviation / °

0 5 11 17 25 35 No deviation occurs when the angle of incidence is zero.

Increasing the angle of incidence increases the deviation.

Refraction of light at a plane surface

(a) Less to more optical dense transition (e.g. air to glass) AIR GLASS

normal angle of incidence angle of refraction 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)

angle of refraction normal angle of incidence

WATER AIR

Light bends

AWAY FROM

the normal.

The angle of refraction is

GREATER

than the angle of incidence.

Why a pool appears shallow

normals observer

AIR WATER

image object at the bottom of a pool

C

Complete the paths of the

RED

light rays:

A B E D F

The refraction equation

When a light ray passes from one medium to another:

i n = sin i sin r where: i

is the angle of incidence in the first medium

r

is the angle of refraction in the second medium

n

is a constant number called the refractive index .

r

An experiment to find the refractive index (

n

) of glass

1.

2.

3.

4.

5.

6.

Set up the equipment as shown in the diagram opposite For an initial angle of incidence,

i

of 30º 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 10º and 80º.

Calculate the average value of

n.

Question 1

Calculate the refractive index when light passes from air to glass if the angle of incidence is 30 ° and the angle of refraction 19º.

n = sin i / sin r

= sin (30º) / sin (19º) = 0.500 / 0.326

refractive index , n = 1.53

Question 2

Calculate 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 r

1.50 = sin (50º) / sin (

r

)

becomes: sin (

r

) = sin (50º) / 1.50

= 0.766 / 1.50

sin (

r

) = 0.511

angle of refraction = 30.7º

Question 3

Calculate 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 r

1.33 = sin (

i

) / sin 20º

becomes: sin (

i

) = 1.33 x sin (20º) = 1.33 x 0.342

sin (

i

) = 0.455

angle of incidence = 27.1º

medium 1 medium 2 air glass water air air water air glass diamond unknown

n

1.33

0.67

1.13

2.40

1.53

i

50 o 30 o 59.8

o 50 o 50 o

r

35.2

o 48.6

o 50 o 18.6

o 30 o

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.

white light prism spectrum

Choose appropriate words to fill in the gaps below:

wave also usually changes.

WORD SELECTION:

towards deviation most dispersion direction perspex speed colours

Total internal reflection

critical angle Total internal reflection occurs when: 1. 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.

GLASS AIR NORMAL Angle of incidence equal to the critical angle : NO Refraction and : : PARTIAL reflection

Critical angle equation

critical angle The critical angle is the angle of incidence in the denser medium that results in an angle of refraction of 90º

n = 1 sin c where: n

is the refractive index of the denser medium (glass in the example opposite).

c

is the critical angle .

GLASS AIR NORMAL angle of refraction = 90º

Question 1

Calculate the critical angle of glass to air if the refractive index of glass is 1.5

n

=

1

/

sin c

= 1.0 / 1.5

= 0.67

critical angle for glass, c = 42 °

Question 2

Calculate the critical angle of water to air if the refractive index of glass is 1.3

n

=

1

/

sin c

= 1.0 / 1.3

= 0.75

critical angle for water, c = 49 °

Question 3

Calculate 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 c

becomes:

sin c = 1 / n sin 30

° = 1.3 /

n

0.5 = 1.3 /

n

= 1.3 / 0.5

maximum refractive index, n = 2.6

Uses of total internal reflection

1. Prismatic periscope Glass and perspex both have critical angles of about 42º .

In each prism the light strikes the glass-air interface at an incidence angle of 45º Total internal reflection therefore occurs and the light ray is deviated by 90º 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.

A bicycle rear reflector contains many tiny red perspex prisms

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.

core cladding

Optical fibre communication

Optical 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 Endoscope

The 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.

TRIPLE ONLY

Diffraction of light

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

Sea wave diffraction

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.

Online Simulations

Law 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 Virtual image formation - eChalk Height of mirror and image seen - eChalk - NTNU Image formed by a plane mirror Reflection and images from two mirrors at 90 degrees to each other - NTNU - NTNU Lens / mirror effect on a beam of light - NTNU Multiple reflections from two plane mirrors - NTNU Bending Light rainbow. 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 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 Prism - NTNU - Does not show TIR effect - non dispersive reflections and refractions - NTNU Prism/Lens NTNU - non dispersive refraction and reflections 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 Dispersion of light using a prism angle can be changed - Freezeway.com - NTNU - prism apex Prism showing light dispersion for different colours Explore Science Prism - multishape prism and single light ray - no extra reflections - netfirms Wave Effects diffraction. - 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 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

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Light Waves

Notes questions from pages 107 to 117

Draw 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.