EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 3-3 Light Waves

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EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 3-3Light
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 17th 2012
2
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
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Reflection of light
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Law of Reflection
The 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.
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The image formed by a plane mirror

• The image produced by the plane mirror is
• The same size as the object
• The same distance behind the mirror as the object
  is in front
• Upright (the same way up as the object)
• Back-to-front compared with the object (lateral
  inversion)
• Virtual

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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.
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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.
reflection
equal
normal
ninety
virtual
real
size
WORD SELECTION
real
virtual
reflection
size
normal
equal
ninety
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Light Refraction
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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.
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Refraction experiment
Typical results
angle of incidence / angle of refraction / deviation /
0 0 0
15 10 5
30 19 11
45 28 17
60 35 25
75 40 35
No deviation occurs when the angle of incidence
is zero. Increasing the angle of incidence
increases the deviation.
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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.
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(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.
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Why a pool appears shallow
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Complete the paths of the RED light rays
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The refraction equation

• When a light ray passes from one medium to
  another
• 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.

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An experiment to find the refractive index (n) of
glass

• 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.

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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

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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

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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

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Complete
Answers
medium 1 medium 2 n i r
air water 1.33 50o 35.2o
glass air 0.67 30o 48.6o
water glass 1.13 59.8o 50o
air diamond 2.40 50o 18.6o
air unknown 1.53 50o 30o
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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.

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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 _________.
speed
direction
towards
perspex
deviation
colours
most
dispersion
WORD SELECTION
most
speed
direction
deviation
colours
perspex
towards
dispersion
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Total internal reflection

• Total 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
REFLECTION
Angle of incidence less than the critical angle
Refraction and PARTIAL reflection
Angle of incidence equal to the critical angle
Refraction at 90º and PARTIAL reflection
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Critical angle equation

• 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.

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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

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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

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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

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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.

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• 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.

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• 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.

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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.

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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.

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Diffraction of light
TRIPLE ONLY

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

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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.

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Online Simulations
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

• 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 - 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

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Light WavesNotes questions from pages 107 to 117

1. Draw a diagram illustrating the law of
   reflection. (see page 107)
2. With the aid of a diagram explain how a plane
   mirror forms an image. also list the properties
   of this image. (see page 108)
3. (a) What is refraction? (b) Draw a labelled
   diagram showing how a light ray travels through a
   rectangular glass block. (see page 109)
4. (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)
5. (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)
6. 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)
7. (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)
8. Answer the questions on pages 116 and 117.
9. Verify that you can do all of the items listed in
   the end of chapter checklist on page 116.