VCE Physics

1
VCE Physics

• Unit 2
• Topic 2
• Wave Like Properties of Light

View physics as a system of thinking about the
world rather than information that can be dumped
into your brain without integrating it into your
own belief systems.
2
Unit Outline

• Describe transverse waves in terms of
• amplitude
• wavelength
• period and frequency
• Calculate wavelength, frequency, period and speed
  of travel of light waves, v f? ?/T
• Investigate and analyse the behaviour of light
  using ray diagrams including
• reflection, i r
• refraction, Snells Law
• total internal reflection, critical angle
• (any form of image location is not required)
• Describe light using a wave model and a particle
  model.
• Explain polarization of visible light and its
  relation to a transverse wave model
• Compare the wave model and the particle model of
  light in terms of whether they adequately
  describe reflection and refraction.
• Identify visible light as a particular region of
  the spectrum of electromagnetic radiation and
  that all light travels at the speed of light in a
  vacuum, c.
• Explain the colour components of white light as
  different frequencies of light combining to
  appear white.
• Explain colour dispersion in prisms and lenses in
  terms of refraction of the components of white
  light as they pass from one medium to another
• Identify and apply safe and responsible practices
  when working with light sources and optical
  devices

3
Chapter 1 - Waves

• This chapter covers the following topics
• Wave Behaviour
• Wave types Transverse and Longitudinal
• Electromagnetic Radiation
• The Medium
• Polarisation

4
1.0 Wave Behaviour
There are many types of waves all of which have
one common feature- They TRANSFER ENERGY from
one place to another.
Some waves (eg. Sound,
or Water Waves)
need a MEDIUM through which to travel. The MEDIUM
(eg. air, water), although disturbed by the
passage of the waves, does NOT suffer any
PERMANENT DISTORTION due to the waves movement
through it.
Other waves, eg light waves, microwaves
and X rays
dont require a medium and are so called self
perpetuating waves.
In cases where waves require a medium, it is
important to note that the waves does not drag
the medium along with it. The sea does not build
up along the shore as the waves break !!!!!
5
1.1 Wave Types - Transverse
There are two basic types of waves TRANSVERSE
WAVES. LONGITUDINAL WAVES.
1. TRANSVERSE WAVES are characterised by having
the individual particles of the medium through
which the wave travels, moving at right angles to
the direction of motion of the wave.
Notice the medium does not move along with the
wave. Pick a spot and follow its motion.
6
1.2 Wave Types -Longitudinal
2. LONGITUDINAL WAVES are characterised by
having the individual particles which make up the
medium through which the wave travels, moving
parallel to the direction of motion of the wave.
Pick a point in the medium and follow its
progress. Note that it does not move along with
the wave, but it is only displaced from its
initial position, returning to its original
position after the wave has passed.
7
1.3 Wave Types Electromagnetic Radiation

• Light is a form of ENERGY.
• It is described as ELECTRO - MAGNETIC RADIATION
  (EMR).
• EMR is a self propagating wave consisting of
  mutually perpendicular, varying ELECTRIC and
  MAGNETIC FIELDS.

Direction of Electromagnetic Wave Movement

• EMR travels through a vacuum at 300,000 kms-1,
  (3.0 x 108 ms-1) and only slightly slower in
  other mediums eg., air, water or glass

• In a single uniform medium (eg air or glass or
  water or plastic), the EM waves travel IN A
  STRAIGHT LINE.

8
1.4 The Medium

• A MEDIUM is any Transparent or Translucent
  material which allows light to pass through it.
• Light and, in fact, all E-M waves, DO NOT require
  a medium for travel.
• In the absence of a medium (ie. travelling
  through a vacuum), Light, and all E-M waves,
  travel at 300,000 km/s (3.0 x 108 ms-1).
• However, when travelling through a medium, Light
  and all E-M waves will travel at speeds less than
  300,000 km/s (3.0 x 108 ms-1).
• In a single, uniform medium (eg. air, plastic,
  glass, water) Light, and all E-M waves, travel IN
  STRAIGHT LINES.

9
Introduction
1. A common feature of all waves is that A
They all transfer energy from one place to
another B They all require a medium for
propagation C They carry the medium along with
them D They permanently distort the medium
through which they travel 2. There are two main
types of waves. They are known as A Transverse
and Long B Travelling and Longitudinal C
Square and Perpendicular D Transverse and
Longitudinal 3. Light is a form of EMR, which
means light is A Electric Magnetic
Readings B Electromagnetic Radiation C
Electron Miniature Rendition D Electromagnetic
Readings
10
Introduction 2
4. Light travelling through a translucent
material suffers a 50 reduction in its speed.
Its speed through the material is A 3.0 x 108
ms-1 B 1.5 x 104 ms-1 C 1.5 x 108 ms-1 D
3.0 x 104 ms-1 5. Our ability to reach out and
grasp an object in our hand depends upon which
one or more of the following properties of
light A Light is made up of a series of colours
added together B Light travels in straight
lines C Light travels at 3.0 x 108 ms-1 D
Light is a form of radiation
11
1.5 Polarisation
We now know that light is an Electromagnetic Wave
made up of mutually perpendicular, varying
electric and magnetic fields. The diagram on the
previous slide showed only ONE pair of Electric
and Magnetic fields. In reality, the are many
pairs of Electric and Magnetic fields, each
perpendicular, spread around the line of the
direction of propagation.
E
For clarity, only the Electric Fields are shown
Suppose a light globe is giving out light rays.
We will focus on one direction of propagation
only.
Little or no light emerges
The polarising filter (simply called a POLARIOD),
only allows light parallel to the polarising axis
to pass through.
A second filter with its axis at 90o to the first
will block most, if not all, light from passing
to the eye.
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Chapter 2

• This chapter covers the following topics
• Amplitude
• Frequency
• Period
• Wavelength
• Speed
• Rays and Shadows

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2.0 Wave Properties. Amplitude

• Amplitude is a measure of the size of a
  disturbance above or below a mean or average
  value.

14
2.1 Wave Properties. Frequency

• Frequency (symbol f ) is most generally defined
  as the number of events which occur during a time
  interval.
• In terms of Light Waves it represents the number
  of complete light waves passing a given point in
  a given time.
• In the SI system, frequency is defined as the
  number of events or cycles per second.
• The UNIT for frequency is the HERTZ (Hz), where 1
  Hz 1 cycle per second

15
2.2 Wave Properties. Period

• Period (symbol T) is defined as the time it takes
  for one event to occur.
• It is the time it takes for one complete light
  wave to pass a given point.
• Period is the measure of a time interval, thus
  has the unit seconds (s).
• Period and frequency are the inverse of one
  another thus

Period (T) 0.02 s f 1/T
1/0.02 50 Hz
Thus, a wave of period 0.02 s has a frequency of
50 Hz
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2.3 Wave Properties. Wavelength

• Wavelength, (symbol ?, Greek Letter LAMBDA), is a
  measure of the distance between two adjacent
  points on a wave undergoing similar motions.
• Thus the distance between two adjacent
  compressions or two adjacent rarefactions would
  be 1 wavelength.
• Wavelength is a distance measure, hence the unit
  for ? is metres (m).

Longitudinal Wave
17
2.4 Wave Properties.Wave Speed

• Wave Speed (symbol v) is a measure of how
  quickly a wavetrain is moving.
• The wave speed is dependent on the frequency and
  wavelength of the wave train.
• The relation is summarised in the so called
  WAVE EQUATION,
• v f?

where v Speed (ms-1),
f Frequency (Hz) ?
Wavelength (m).

• This is a most important equation used in many
  areas of the course.

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2.5 Wave Properties. Shadows
When an object is illuminated it casts a shadow.
The shadow may have strictly defined edges, a so
called, sharp shadow, or it may have ill
defined edges, a so called fuzzy shadow.
The factor which decides what type of shadow is
produced is the SIZE of the light source.
POINT SOURCES produce sharp shadows.
EXTENDED SOURCES produce fuzzy shadows.
Umbra Region of Full Shadow Penumbra Region
of Partial Shadow (causes the fuzzy edges)
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Wave Formula
6. A wave travelling at 4.52 x 103 ms-1 has a
measured frequency of 2.45 x 104 Hz, Calculate
its wavelength.
v f? ? ? v/f (4.52 x 103)/( 2.45 x 104 )
0.18 m
7. A wave of frequency 100 Hz will have a period
of ?
T 1/f 1/100 0.01 s
8. A ray of red light travelling through a vacuum
at a speed of 3.0 x 108 ms-1 has a wavelength of
450 nm. Calculate (a) its frequency and (b) its
period.
(a) v f? ? f v/? (3.0 x 108)/(450 x 10-9)
6.67 x 1014 Hz (b) T 1/f 1/(6.67 x 1014)
1.5 x 10-15 s
9. What is the meaning of the terms umbra and
penumbra ?
Both terms refer to shadows. Umbra is full
shadow, penumbra is partial shadow
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Chapter 3

• This chapter covers the following topics
• Electromagnetic Spectrum
• Colour
• Colour Mixing
• Transparent Materials
• Opaque Materials

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3.0 The Electromagnetic Spectrum
The ELECTROMAGNETIC SPECTRUM encompasses all
ELECTROMAGNETIC RADIATION of which VISIBLE LIGHT
is but a small part.
In a vacuum, all EM Radiation travels at the same
speed v 3.0 x 108 ms-1
1 nm 10-9 m
?RED 7.5 x 10-7 m 750 nm
?VIOLET 4.5 x 10-7 m 450 nm
Notice how small the range of wavelengths is for
the visible region of the E-M Spectrum.
This small range of wavelengths give us our
ability to see the world. Imagine how much more
complex the world be if our eyes were able to
see all the wavelengths of the electromagnetic
spectrum.
22

3.1 Colour
The fact that white light is made up of a
mixture of colours was first discovered by Isaac
NEWTON (1642 - 1727). When Newton passed white
light through a triangular prism, it was broken
up into its constituent colours. This process is
called DISPERSION
This break up occurs because each colour has a
slightly different wavelength and when passed
through the prism suffers a slightly different
change in direction.
Colours associated with definite wavelengths are
called SPECTRAL COLOURS.
The colours and their wavelengths
RED 650 ORANGE 600 YELLOW 580 GREEN 550 BLUE
500 INDIGO 470 VIOLET 450
From both the diagram and the table, it should be
obvious that the shorter the wavelength, the
greater the change in direction.
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Electromagnetic Radiation
10. Arrange the following examples of EMR from
shortest to longest wavelength Cosmic rays, Radio
waves, Visible light, Gamma rays, Ultraviolet,
Microwaves, Infrared, X Rays, TV.
Cosmic rays, Gamma rays, X Rays, Ultraviolet,
Visible light, Infrared, Microwaves, TV, Radio
waves.
11. Why can a prism split white light into is
component colours ?
The reason that this break up occurs is because
each colour has a slightly different wavelength
(or frequency which ever term you want to use)
and because of this each suffers a slightly
different change in direction while passing
through the prism.
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3.2 Colour Mixing
Just as light can be broken up into individual
colours, so individual colours of light can be
recombined to produce white light.
When the various colours of LIGHT are mixed
together we can achieve all colours of the
rainbow, but more importantly, we can produce
white light. This is an ADDITIVE PROCESS.
However, when paints or pigments are added
together , each pigment absorbs, or subtracts,
certain colours, so this is a SUBTRACTIVE
PROCESS. Thus when every pigment colours is
added, all colour will have been absorbed leaving
only black.
25
3.3 Colour Transparent Materials
There are 3 basic types of materials in the
world. TRANSPARENT objects transmit most light
reflecting and absorbing very little.
TRANSLUCENT objects transmit some light (usually
distorted) while reflecting and absorbing more of
the light than transparent materials. OPAQUE
objects transmit no light and reflect and /or
absorb all light.
TRANSPARENT OBJECTS which are coloured (usually
called FILTERS) transmit their colour whilst at
the same time absorbing all other
colours. Examples of White Light interacting with
various filters are shown below.
No Light Emerges
26
3.4 ColourOpaque Materials
Examples of White Light interacting with various
OPAQUE MATERILALS are shown below.
No reflected Light
Opaque objects appear coloured because they
reflect their colour while, at the same time,
absorbing all other colours.
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Colour
12. How is the process of adding light different
from the process of adding pigments ?
When the various colours of light are mixed
together we can achieve all colours of the
rainbow, but more importantly, we can produce
white light. This is an ADDITIVE PROCESS.
However, when paints or pigments are added
together, each pigment absorbs, or subtracts
certain colours, so this is a SUBTRACTIVE
PROCESS.
13. As far as light is concerned there are 3
types of materials in the world Draw a line
between the term and its meaning
Term Meaning
Transparent materials transmit some light (usually distorted) while reflecting and absorbing more of the light than other materials.
Translucent materials transmit no light and reflect and/or absorb all light.
Opaque materials transmit most light reflecting and absorbing very little.
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Colour and Filters
14. Why is a blue tee shirt blue to an observer ?
Blue tee shirts are blue because the blue fabric
absorbs all colours except blue which it
reflects.
15. Describe how you could use 2 simple filters
to block out all visible light
Shine light firstly through a blue filter leaving
only blue light, then shine that onto a red
filter, no light will emerge from the red filter.
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Chapter 4

• This chapter covers the following topics
• Reflection
• Refraction
• Index of Refraction
• Critical Angle
• Total Internal Reflection
• Optical Fibres
• Interference Patterns

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4.0 The Properties of Light - Reflection
Before studying the Laws of Reflection the idea
of a NORMAL needs to be introduced.
The NORMAL is an imaginary line drawn at Right
Angles to the Reflecting Surface. It is used to
define the angles of incidence and reflection.
Reflecting Surface
THE LAWS OF REFLECTION 1. Angle of Incidence
Angle of Reflection
Incident Ray
Reflected Ray
2. The Incident Ray, the Reflected Ray and the
Normal are COPLANER (all lie in the one plane)
and all lie on the same side of the reflecting
surface.
Reflecting Surface
i Angle of Incidence
r Angle of Reflection
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Reflection
16. What is a Normal ?
The Normal is an imaginary line perpendicular to
the reflecting or refracting surface.
17. Two plane mirrors are set up as shown a ray
incident on the lower mirror reflects onto the
upper one. Determine the values of the angles
marked W, X, Y and Z.
W 600 X 600 Y 300 Z 600
32
4.1 The Properties of Light - Refraction
Refraction is the changing of the direction of
travel of a light ray which generally occurs when
light passes from one medium to another (eg when
passing from air to glass). The reason for the
change in direction is that THE SPEED OF LIGHT
changes when passing into the new medium. The
speed is inversely proportional to the density of
the medium, ie. The slower the speed, the denser
the medium.
Note If the incident ray is directed in along
the Normal, it will NOT change its direction when
it crosses the boundary.
For other directions a change in direction WILL
occur.
Since DensityGLASS gt DensityAIR
it can be seen that in travelling from a less
dense to a more dense medium the light ray is
refracted TOWARD the Normal.
i Angle of Incidence
r Angle of Refraction
33
4.2 The Properties of Light - The Laws of
Refraction
THE LAWS OF REFRACTION. The first law of
refraction is also known as SNELLS LAW 1.
Snells Law. For a given pair of media, the ratio
of the Sine of the angle of Incidence to the Sine
of the angle of Refraction is a constant
(n). Mathematically Sin i/Sin r n n is
called the Index of Refraction or more simply
the Refractive Index
i Angle of Incidence
r Angle of Refraction
2. The Incident Ray, the Refracted Ray and the
Normal to the refracting surface at the point of
incidence are COPLANER and the Incident and
Refracted Ray are on opposite sides of the
refracting surface.
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4.3 The Properties of Light - The Index of
Refraction (1)
ABSOLUTE REFRACTIVE INDEX. If one of the media
involved in the Refraction process is a VACUUM,
the constant, n, in Snells Law becomes THE
ABSOLUTE REFRACTIVE INDEX. Values of the
absolute refractive index for various materials
are
Note the closeness of the values for Vacuum and
Air. In all cases in this course, nAIR can be
taken to be equal to nVACUUM 1.00
RELATIVE REFRACTIVE INDEX If neither of the
mediums involved in a refraction process is a
vacuum, each will have its own Absolute
Refractive Index and the constant (n) in Snells
Law must reflect that fact.
In general for two mediums with Absolute
Refractive Indexes n1 and n2, Snells Law
becomes n1Sin i n2 Sin r ? Sin
i/Sin r n2/n1 The ratio n2/n1 is usually quoted
as n12 and is called the RELATIVE REFRACTIVE
INDEX for light travelling from medium 1 to
medium 2
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4.4 The Properties of Light - Index of
Refraction (2)
A block of Plastic
is floated on water
An example may be useful in understanding
Relative Refractive Index
A beam of light passes through the plastic
and enters the water.
In this situation the Relative Refractive Index
(RRI) for light travelling from Plastic to
Water, nPW nW/nP 1.33/1.50 0.89
RRIs less than 1 mean the light is travelling
into a less dense medium.
If the position is reversed and the light travels
from the water into the plastic.
The Relative Refractive Index for light
travelling from Water to Plastic, nWP nP/nW
1.50/1.33 1.13
RRIs greater than 1 mean the light is travelling
into a more dense medium.
36
Refraction
18. State Snells Law in mathematical terms
Sin i/Sin r a constant
19. Calculate the angle of refraction for light
passing from air (n 1.00) at an incident angle
of 460 into diamond (n 2.42)
nair Sin i ndiamondSin r ? r Sin-1 (1.00 Sin
46/2.42) 17.30
20. What is the difference between an absolute
refractive index and a relative refractive index ?
If one of the media involved in the refraction
process is a vacuum, the constant, n, in Snells
Law is called the ABSOLUTE REFRACTIVE INDEX. If
neither medium is a vacuum the constant, n, in
Snells Law is called the RELATIVE REFRACTIVE
INDEX
21. A beam of light passed from perspex into
oleic acid with an angle of refraction of 720 .
What was the beams incident angle ? noleic acid
1.46 n perspex 1.50. Draw a clear, fully
labelled diagram of this situation.
nperspex Sin i noleicSin r ? i Sin-1(1.46 Sin72/1.5) 680
37
4.5 The Properties of Light - Critical Angle
When light travels from a more dense to a less
dense medium (eg. from water to air), it refracts
AWAY from the Normal.
As the angle of incidence (i) increases
eventually a value of i will be reached which
produces and angle of refraction of 900.
This angle of incidence is called the CRITICAL
ANGLE (iC)
The size of the critical angle for this pair of
media can be calculated from Snells Law nW
Sin iC nA Sin r 1.33 Sin iC 1.00 Sin 900
Sin iC 1/(1.33) ? iC 48.80
38
4.6 The Properties of Light - Total Internal
Reflection
If the angle of incidence increases beyond the
Critical Angle, the light ray will no longer
leave the denser medium. The surface of the
denser medium will act like a plane mirror
reflecting the beam back with an angle of
reflection equal to the angle of incidence.
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4.7 The Properties of Light Optical Fibres
Optical Fibres are the basis of modern high
speed, high volume communication systems allowing
telephone systems, Cable Television and
interconnected computer systems to operate in an
efficient and timely manner. Optical fibres rely
on the Total Internal Reflection of a laser beam
to transfer information (usually in digital form)
from one place to another.
Each time the laser beam reflects off the inner
wall of the optical fibre, it suffers attenuation
(loses some of its energy), thus, at intervals of
about 10 to 20 km along optical fibre, repeater
stations are needed to boost the power of the
laser beam before it is transmitted further along
the fibre.
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4.8 The Properties of Light - Double Slit
Interference
When light of a single wavelength is passed
through a pair of closely spaced, narrow slits,
an interference pattern is produced.
This pattern has a series of equally spaced
coloured and black bands spread across the screen
onto which it is projected. The width of the
coloured bands and their spacing depends on the
wavelength of the light used. Short wavelength,
BLUE light produces a pattern with narrow blue
bands which are closely spaced. Long wavelength,
RED light produces a pattern with wider red bands
which are spread farther apart.
This experiment is known as Youngs Double Slit
Experiment
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Critical Angle
22. Define critical angle.
Critical Angle that angle of incidence that
produces an angle of refraction of 900
 23. Determine the critical angle (ic) for light
travelling from crown glass (refractive index
1.52) into air.
ic Sin-1(1/1.52) 41.10
24. What happens to light rays which approach a
boundary with a lower refractive index material
at an angle greater than the critical angle for
that combination ?
The rays are totally internally reflected
 25. What are Optical Fibres and what is the
basis of their operation ?
Optical Fibres are flexible glass (or plastic)
tubes through which laser light can travel. Their
basis of operation is total internal reflection
42
Chapter 5

• This chapter covers the following topics
• Image Formation
• Plane Mirrors
• Curved Mirrors
• Lenses
• Ray Tracing

43
5.0 Image Formation
Images formed by mirrors or lenses are
characterised by a number of properties which
need definition.
1. IMAGE TYPE There are 2 types of images formed
by mirrors and lenses (a) REAL IMAGES, these
actually exist and can be projected onto a
screen. (b) VIRTUAL IMAGES, these do not actually
exist and CANNOT be projected onto a screen.
2. IMAGE ORIENTATION When compared to the
object, the image may be (a) upright or erect,
ie. in the same orientation as the object, or (b)
inverted or upside down, ie. opposite in
orientation to the object.
3. IMAGE SIZE When compared to the object the
image may be larger than, equal in size to, or
smaller than the object.
4. LATERAL INVERSION This occurs when images are
laterally transposed ie. Right becomes left and
visa versa.
44
5.1 Images - Plane Mirrors
The eye expects light to travel in straight
lines. In order to see the top of her head, the
lady needs a ray to travel along the path shown.
In order to see her chin she needs a ray to
travel as shown.
The expectation that light travels in straight
lines means she sees her image inside the
mirror, as shown.

• The image produced by a plane mirror has the
  following properties
• Virtual
• Upright
• Laterally Inverted
• Equal in size to the object
• As far behind the mirror as the object is in
  front.

45
Image Formation
26. Images produced by mirrors and lenses are
classed as either real or virtual, upright or
inverted, and larger or smaller or the same size
as the object. Write definitions for the
underlined words.
Real the light rays actually meet to produce an
image that can be projected onto a screen Virtual
- the light rays do not actually meet and the
image cannot be projected onto a screen Upright
the image is in the same orientation as the
object Inverted the image is upside down when
compared to the object
27. How far behind a plane mirror is the image of
an object formed ?
As far behind the mirror as the object is in
front of it.
46
5.2 Curved Mirrors
There are two kinds of curved mirrors 1. CONCAVE
MIRRORS (Converging Mirrors ). These mirrors
force parallel incoming rays together at one
point after reflection.
2. CONVEX MIRRORS (Diverging Mirrors). These
mirrors force parallel incoming rays to diverge
from an apparent meeting point behind the mirror.
47
5.3 Curved Mirrors - Properties and Definitions
Before proceeding with image determination, a
number of terms associated with curved mirrors
need to be defined.
A Aperture of Mirror (diameter of reflecting
surface)
O Pole of the Mirror (the centre of the
reflecting surface)
C Centre of Curvature (centre of the sphere of
which the mirror is a part)
R Radius of Curvature (radius of sphere of
which the mirror is a part)
PA Principal Axis (a line joining the centre of
curvature and the pole of the mirror).
f Focal Point (point at which rays, initially
parallel to the principal axis, meet after
reflection).
F Focal Length (distance from pole to focal
point).
48
5.4 Ray Tracing Concave Mirrors
In order to determine the position and type of
image produced by a concave mirror, a number of
standard light rays can be used. The three most
important are illustrated below
Ray 1 - A ray, initially parallel to the
Principal Axis, will, after reflection pass
through the Focal Point.
Ray 2 - A ray initially directed toward the pole
of the mirror, will, after reflection, leave the
mirror such that angle i angle r
Ray 3 - A ray, initially directed through the
Focal Point, will, after reflection, leave the
mirror parallel to the Principal Axis.
Generally, only two of the three standard rays
ever need to be used to locate the position of an
image.
49
5.5 Images - Concave Mirrors
Image type, size and orientation for concave
mirrors may be found by using the ray tracing
technique.

• The Image produced is
• REAL
• INVERTED
• SMALLER than the Object

• When the object is placed 2f from the mirror, the
  image is
• REAL
• INVERTED
• SAME SIZE as the object

• When the object is put inside the focal length,
  the image is
• VIRTUAL
• UPRIGHT
• LARGER than the Object

50
5.6 Lens Types
There are two basic types of lenses 1. CONVEX
LENS (Converging Lens). These lenses cause
incoming parallel rays to be refracted to a
single point.
2. CONCAVE LENS. (Diverging Lens). These lenses
cause incoming parallel rays to diverge as if
they originated from a focal point.
51
5.7 Lenses - Properties Definitions
Before proceeding with image determination, a
number of terms associated with lenses need to be
defined.
A Aperture of Mirror (diameter of refracting
surface)
O Optical Centre of the lens (the very centre
of the lens)
C Centre of Curvature (centre of the sphere of
which the lens surface is a part)
R Radius of Curvature (radius of sphere of
which the lens surface is a part)
PA Principal Axis (a line joining the centre of
curvature and the optical centre of the lens).
f Focal Points (point at which rays, initially
parallel to the principal axis, meet, after
refraction). Note the lens has two focal points.
F Focal Length (distance from optical centre to
focal point(s)).
52
5.8 Ray Tracing - Lenses
In order to determine the position and type of
image produced by a concave mirror, a number of
standard light rays can be used. The three most
important are illustrated below
Ray 1 - A ray from the top of the object,
initially parallel to the principal axis, will,
after refraction through the lens, pass through
the focal point on the far side of the lens.
Ray 2 - A ray from the top of the object will
pass through the optical centre of the lens
undeviated.
Ray 3 - A ray from the top of the object passing
through the near focal point will, after
refraction, emerge from the lens parallel to the
principal axis.
Generally, only two of the three standard rays
are ever needed to locate the position of the
image.
53
5.9 Images - Convex Lenses
Image type, size and orientation for convex
lenses may be found by using the ray tracing
technique.

• The Image produced is
• REAL
• INVERTED
• SMALLER than the Object

• When the object is put inside the focal length,
  the image is
• VIRTUAL
• UPRIGHT
• LARGER than the Object

Rays projected backwards WILL meet.
These diverging rays will never meet.
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Ray Tracing
28. What is the process of determining where the
image of an object is in a curved mirror or lens
called ?
Ray Tracing
29. How many rays are needed to locate the image
of an object in a curved mirror or lens ?
2
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Chapter 6

• This chapter covers the following topics
• Conceptual Modelling
• Light as Particles
• Theories for Light

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6.0 Conceptual Modelling
When we observe strange or unusual behaviour in
people, we often try to explain what we see by
supposing the person is suffering some mental
problem.
They may have suffered a skull fracture resulting
in brain damage, have a congenital brain defect,
or have problems due to parental abuse in
childhood.
Conceptual modelling here refers to the
activity of constructing abstract models of
knowledge about the world of light
In attempting to find a explanation for what we
see, we model various concepts, in other
words, we try to find or develop a theory which
will fit all the behaviours we observe.
We have two possible models available to fit
lights observed behaviours.
(a) Light is a wave and all its properties and
behaviours can be explained by assuming light is
a wave.
(b) Light is a particle and all its properties
and behaviours can be explained by assuming light
is a particle.
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6.1 Light as Waves
By assuming light is a wave all the properties
and behaviours we have so far investigated can be
explained by using the known properties and
behaviours that waves, in general, exhibit.
For example, waves undergo reflection and follow
the laws of reflection exactly
Waves refract according to Snells Law

Waves produce interference patterns.
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6.2 Light as Particles
Just as waves can be reflected, so too can
particles. They also follow the laws of
reflection.
Particles can also undergo refraction obeying
Snells Law. (although speed predictions are
incorrect)
However, one property of light cannot be
explained by the wave model. In the
Photoelectric Effect certain metals eject
electrons when illuminated by particular colours
of light
Metal
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6.3 Comparing Models
Does our new found knowledge mean that the wave
model for light is now to be replaced by the
particle model ?
Well, no because the particle model has the same
limitations as the wave model, that is, it cannot
adequately explain ALL known properties of light.
So how useful is each theory in explaining light
properties ? The table summarises.
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6.4 Where to Now ?
This leaves us in the position of not having one
single model that can describe all lights known
behaviours. So we have developed a hybrid model
called the Wave Particle Duality, where light
is assumed to be made up of a stream of
individual particles called PHOTONS.
Photons are neither waves nor particles, having
properties similar to particles when travelling
through a vacuum and when in a gravitational
field, while also having properties similar to
waves when refracting and interfering.
Photons can be pictured as a series of individual
particles each of which displays some wave like
properties.
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Models for Light
30. What are the two theories which compete to
explain lights behaviour ?
The Wave Model and The Particle Model
31. For which particular light behaviour is the
particle model inadequate ?
Refraction
32. For which particular light behaviour is the
wave model inadequate ?
Photoelectric Effect
33. What is a Photon ?
A Photon is an attempt to explain lights
behaviour with a model that has both particle and
wave nature
34. What is the wave particle duality ?
The wave particle duality is the currently
accepted model for the explanation of lights
behaviour.
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