Light Waves

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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 17th 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)
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.
incident ray
normal
angle of
incidence, i
angle of
reflection, r
reflected ray
mirror
The image formed by a plane mirror
image
object
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:
reflection
The law of reflection states that the angle of __________
is
equal to the angle of incidence.
always _______
normal
Both angles are measured relative to the _________,
a line
ninety degrees to the reflecting surface at the
that is at _______
point of reflection.
A plane mirror forms a _______
virtual image which is unlike a
real image in that it cannot be cast onto a screen. The
_____
image in a plane mirror is also the same ______
size and the same
way up as the object.
WORD SELECTION:
normal
virtual
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 / °
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.
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
Complete the paths of the RED light rays:
A
B
D
C
E
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. Set up the equipment as shown in
the diagram opposite
2. For an initial angle of incidence, i
of 30º trace the path of the light
ray.
3. Measure the angle refraction, r.
4. Calculate the refractive index using
the formula: n = sin (i) / sin (r).
5. Repeat for a range of angles
between 10º and 80º.
6. 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
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
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:
Refraction occurs when a wave changes ______
speed as it crosses
direction of the
the boundary between two regions. The _________
wave also usually changes.
towards the normal when they pass from
Light rays deviate ________
perspex
less dense air to more dense _________.
The greater the
deviation
angle of incidence the greater is the _________.
colours of light deviate by different amounts. Violet
Different ______
deviates the _____.
most A prism can be used to split the colours of
dispersion
white light into a spectrum. This is called _________.
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
Angle
Angle
of of
incidence
of
incidence
incidence
greater
equal
lessthan
to
thethe
critical
than
critical
theangle:
critical
angle: angle:
NORefraction
Refraction
Refraction
atand
and
90º and
TOTAL
PARTIAL
PARTIAL
INTERNAL
reflection
reflection
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
GLASS
AIR
where:
n is the refractive index of
the denser medium (glass in
the example opposite).
c is the critical angle.
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 - 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 Waves
Notes questions from pages 107 to 117
1.
2.
3.
4.
5.
6.
7.
8.
9.
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.
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