Transcript Students will be assessed on their ability to

Light and electromagnetic waves
4th form IGCSE
Textbook: Chapters 13 & 12
Students will be assessed on their ability to:
3.14 recall that light waves are transverse waves which can be
reflected, refracted and diffracted
3.15 recall that the angle of incidence equals the angle of reflection
3.16 construct ray diagrams to illustrate the formation of a virtual
image in a plane mirror
3.17 describe experiments to investigate the refraction of light, using
rectangular blocks, semicircular blocks and triangular prisms
3.18 recall and use the relationship between refractive index, angle of
incidence and angle of refraction:
sin i
n
sin r
3.19 describe an experiment to determine the refractive index of glass,
using a glass block
3.20 describe the role of total internal reflection in transmitting
information along optical fibres and in prisms
3.21 recall the meaning of critical angle c
3.22 recall and use the relationship between critical angle and
refractive index:
1
sin c 
n
Reminder: light is...
• Energy
– Not matter...
• A wave
– Doesn’t need a
medium to travel
– Travels at c in a
vacuum
Reminder: reflection
• Light that is not absorbed or
transmitted is reflected
• Angle of incidence=angle of
reflection
• Find images by drawing ray
diagrams
• Smooth, shiny objects give
specular reflections (like mirrors)
• Rough ones give diffuse
reflections
Reflection
Virtual ripple tank
Refraction
• Refraction is the change in wave
speed when a wave passes from
one medium to another.
• This may be accompanied by a
change in direction.
• Refraction + curved surfaces →
lenses
Photo:
George Silk
(1962)
Air to glass
Air
• Passing into a denser
medium:
Glass
Light moves slowly in glass
– Wave slows down
– Bends towards normal
Glass to air
• Passing into a less dense
medium:
Glass
– Wave speeds up
– Bends away from normal
Light moves faster in air
Air
Investigating refraction
• Shine light ray through a glass
block and measure where it
goes
– Record incident angle and
refracted angle
Air
i
Glass
r
• Why might it be useful to use a
semi-circular block?
– If a ray travels along a radius, it
always strikes the curved surface
along the normal
– Only one change of direction
Air
Glass
Refraction
• Why the change
in direction?
• Section of
wavefront in
second medium
travels more
slowly
• Also change in
wavelength
Apparent depth
• Refraction causes
things underwater to
appear to be at a
shallower depth
when viewed from
the air
– Difficult to spear fish!
Prisms
• There is refraction at
each surface, as the ray
enters and leaves the
block
• The speed of light in
glass depends slightly on
the wavelength
– Dispersion
– “blue bends best”
– So which travels faster in
glass, red or blue light?
Investigation results
90
Air
i
r
Glass
incident angle (degrees)
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
refracted angle (degrees)
1
0.9
• Not a straight line...
0.8
0.7
Refractive gradient  sin i  n
sin r
index
Sin(i)
0.6
0.5
0.4
0.3
0.2
0.1
• This one is...
0
0
0.1
0.2
0.3
0.4
0.5
sin(r)
0.6
0.7
0.8
0.9
1
Refractive Index
• Refractive index of a material is defined as
sin i
n
sin r
Note: going from air into the material
• A ratio, so no units
• n is never smaller than 1 – be careful!
• The denser a material is, the higher its
refractive index
– for air, n is very close to 1
Example: A light ray is incident on a diamond of refractive
index 2.4 at an angle of 60° to the normal. What angle does it
travel at in the diamond, relative to the normal?
Going into a less dense material
• Which way does the light bend?
– Note there is always a weak reflection
• What happens as we increase i?
– When r=90°, i is called the critical
angle, c
– If i>c, we get total internal reflection
i
c
r
• No light is transmitted,
• All the energy is reflected
• Angle of incidence = angle of reflection
Note: this only happens going from a denser to a less dense material, not the other way around
Critical angle
• The critical angle is the angle of incidence for
which the refracted ray runs along the
boundary
– Angle of refraction = 90°
sin i
n
, sin 90  1, so
sin r
1
sin c 
n
Example: calculate the critical angle for water, n=1.33.
Describe what you would see looking at a light at the bottom
of a pool.
Uses of TIR
• To avoid multiple
reflections sometimes
associated with silvered
mirrors
– Reflective coating is on
the back of the glass, but
some light reflects from
the front surface.
– This can result in multiple
images
– Problem is avoided if we
use prisms to reflect
Uses of TIR
• A prism can be used as a retroreflector, eg for a bicycle.
– Why is this better than a mirror?
There are even some on the moon!
(Used to measure distance from Earth)
Uses of TIR
• In binoculars
– Fold the path of light
to make instrument
more compact
– Produce a noninverted image
• Would usually be
upside down or L-R
inverted otherwise!
Uses of TIR
• Optical fibres
– A thin strand made of two types of glass:
• a denser core surrounded by a less dense cladding
– Rays undergo multiple TIR and are guided through
the fibre, even around corners
• A “light pipe”; very useful for communications
Optical fibres
• Different types of fibre for different jobs
– For general illumination can use thick
fibres
– Telecoms fibre is very thin, to allow higher
speed data transmission
• Cladding diameter is same as a human hair,
and the core diameter is only 6% of that!
– A bundle of fibres can give an image from
inaccessible places
Fibrescopes
Rainbows
Sun
• The result of TIR in water
droplets
– Need the Sun behind you
– Different wavelengths
refract at different angles,
leading to dispersion
• Rainbows are bows
because each wavelength
refracts at a different
angle
Sun
o
raindrops 42 40'
40o 35'
rainbow
Students will be assessed on their ability to:
3.14 recall that light waves are transverse waves which can be
reflected, refracted and diffracted
3.15 recall that the angle of incidence equals the angle of reflection
3.16 construct ray diagrams to illustrate the formation of a virtual
image in a plane mirror
3.17 describe experiments to investigate the refraction of light, using
rectangular blocks, semicircular blocks and triangular prisms
3.18 recall and use the relationship between refractive index, angle of
incidence and angle of refraction:
sin i
n
sin r
3.19 describe an experiment to determine the refractive index of glass,
using a glass block
3.20 describe the role of total internal reflection in transmitting
information along optical fibres and in prisms
3.21 recall the meaning of critical angle c
3.22 recall and use the relationship between critical angle and
refractive index:
1
sin c 
n
Students will be assessed on their ability to:
3.10 understand that light is part of a continuous electromagnetic spectrum
which includes radio, microwave, infrared, visible, ultraviolet, x-ray and
gamma ray radiations and that all these waves travel at the same speed in
free space
3.11 recall the order of the electromagnetic spectrum in decreasing
wavelength and increasing frequency, including the colours of the visible
spectrum
3.12 recall some of the uses of electromagnetic radiations, including:
• radio waves: broadcasting and communications
• microwaves: cooking and satellite transmissions
• infrared: heaters and night vision equipment
• visible light: optical fibres and photography
• ultraviolet: fluorescent lamps
• x-rays: observing the internal structure of objects and materials and medical applications
• gamma rays: sterilising food and medical equipment
3.13 recall the detrimental effects of excessive exposure of the human body
to electromagnetic waves, including:
• microwaves: internal heating of body tissue
infra-red: skin burns
• ultraviolet: damage to surface cells and blindness
• gamma rays: cancer, mutation.
What is light?
• An electromagnetic wave
• But what’s one of those?
– A periodic variation in the electric and magnetic
field
– No matter is involved!
electric
field
Magnetic
field
Travel
direction
There is more than we can see...
• Visible light is just a small portion of a whole
range of different types of electromagnetic waves
• Can you name some of the other types?
• Can you say what they are useful for?
• What do they have in common?
• What are the differences between them?
Electromagnetic spectrum
Increasing wavelength
• All EM waves:
– are vibrations of an electric and magnetic field
– Transfer energy and require no medium
– Can be reflected, absorbed or transmitted
• (and also refracted or diffracted)
– travel at the same speed in free space
• about 30,000,000 ms-1
You need to learn the spectrum
indigo
Indigo was included by
Newton just because
he liked the number 7!
blue
gamma rays
X-ray
UV
green
Visible light
infrared
Remember this beard!
Richard III can be your friend
yellow
microwaves
orange
radio UHF (TV)
radio VHF (radio)
radio (AM radio)
radio VLF
red
Increasing frequency/Decreasing wavelength
violet
Uses of EM waves
Increasing wavelength
• First let’s look at the waves with less energy
than visible light…
• i.e. longer wavelength
Radiowaves
• Used for communications
– TV
– Radio
• Radio waves produced by
moving electrons at
transmitter
• Induce current in receiver
aerial
Microwaves
• Microwaves are high frequency radio waves
• Uses include:
– Cooking
– Communications
The atmosphere is a selective filter
• Only some wavelength can pass through it
• Need to choose the right frequency if you want to
communicate with a satellite
• Microwaves are preferable to radio waves
because:
• They spread out less
• They can carry data faster
Microwave cookers
• Microwaves’ varying electric field
causes water and fat molecules
to rotate rapidly
• This causes heating
Energy delivered  intensity  time
• Microwaves cannot pass through
metal
– Metal case and grid in door prevent
them from escaping
superheating
Infrared radiation
• All objects emit infra-red radiation
– The hotter they are, the more they emit
– Can be used for imaging and diagnostics
• Also used for
– heating and cooking
– Communications*
* Although the
exam board say
visible light is
used with optical
fibres – they are
wrong!
Infrared imaging
• The colours are
“artificial” but
show temperature
differences
• Which picture
shows before the
ball was bounced?
What happened to
the ball when it
was bounced?
Annoying woman video
Fart video(L6 folder)
Infrared imaging
• Also finds military
and security
applications.
• Emergency services
can use IR cameras
to see through
smoke.
Visible light
• It’s what we see!
– Lots of uses...
?
Electromagnetic spectrum
Increasing wavelength
• Now let’s look at those waves with more
energy than visible light…
Ultraviolet
• Invisible!
• Produced in mercury vapour
tubes
• Can cause substances to
fluoresce
– Convert UV to visible light
Fluorescent tubes
• Electric current excites mercury vapour in the
tube
• Excited Mercury emits UV light
• UV light is absorbed by the white fluorescent
coating, some of the energy is dispersed, the
rest is emitted as visible light - fluorescence
Uses of UV
• Fluorescence
– Eg forensics or security
•
•
•
•
Curing glue
Vitamin D production
photolithography
Sun bed (not recommended)
Uses of x rays
• X-rays are transparent to soft materials but
absorbed by dense materials
• Used to produce shadow pictures of bones in our
bodies or of objects in aircraft passengers’ luggage
• Looking for cracks and faults in buildings or machinery
Gamma Rays
• Shortest wavelength
• Highest frequency
• Most energy carried in
each chunk of waves
(quantum or photon)
• Go through almost
anything!
• Stopped by several
centimetres of lead or
metres of concrete
Uses of gamma rays
• Medical Imaging (gamma camera)
• Treating cancer
– radiotherapy
• Sterilisation (seal then irradiate, g)
– Food
– Surgical instruments
The Gamma Camera
Injecting a weak shortlived gamma source into
the blood lets a gamma
camera image the blood
pattern in a body.
Hopefully most of the
gamma escapes without
doing too much damage!
Absorbing radiation
• When a material absorbs electromagnetic radiation it
gains energy.
• Absorbed radiation can:
Increasing energy
– Induce electric currents in metals
• e.g. radio waves absorbed by an aerial
– Heat up an object, by increasing the vibration of its particles
– Cause chemical changes
• e.g. photosynthesis or photocells in the eye
– Cause ionisation, breaking up molecules or atoms.
Ionising and non-ionising radiation
Increasing photon energy
gamma rays
X-ray
Ionising
UV
Visible light
infrared
microwaves
radio UHF (TV)
radio VHF (radio)
radio (AM radio)
radio VLF
Non-ionising
EM radiation and living things
• The health risk posed by radiation depends on
the type and intensity of the radiation.
• In general, exposure to very high levels of nonionising radiation can be harmful, but at lower
doses they are relatively “safe”.
• Ionising radiation can damage or kill cells,
cause mutations and cancer.
• There is no “safe” dose, but the risk increases
with length and strength of exposure.
Effects of radiation on tissue
gamma rays
X-ray
Ionising. Can be damaging –
cancer, genetic mutation
UV
Visible light
infrared
microwaves
radio UHF (TV)
radio VHF (radio)
radio (AM radio)
radio VLF
Heating. Can be damaging –
burns, chemical changes
No detectable effect?
The atmosphere is a selective filter
• It absorbs and so protects us from ionising
radiation from space (UV, x-ray, gamma)
• It allows through IR, visible and a little UV, and
radio waves (satellite communications)
Protecting tissue from radiation
Risks and Benefits
• Exposure to ionising radiation carries risks but can
also have benefits
Risks and Benefits
• What are the risks and
benefits of sunbathing?
–
–
–
–
–
–
Burns and skin aging
Skin cancer
Cataracts
Skin colour (?)
Vitamin D production
Makes people happy
• What precautions might you
take?
–
–
–
–
Sun block
Limit exposure
Protective clothes
sunglasses
• On balance, is sunbathing a
good idea?
Skin Cancer
• Ultraviolet (UV)
photons harm the DNA
molecules of living
organisms in different
ways.
• In one common
damage event,
adjacent bases bond
with each other,
instead of across the
“ladder.”
• This makes a bulge,
and the distorted DNA
molecule does not
function properly.
Microwaves and Mobiles
• Microwave ovens and mobile phones both use
microwaves, but:
– Wavelength of microwave ovens is ~12 cm
– Wavelength of mobiles is ~16-33 cm
• There is no evidence that phone use is harmful in the
short term
• Phone use does cause
measureable
brain heating, but no more
than
moderate exercise
• Precautionary principle is
recommended, meanwhile we
are all guinea pigs.
People at risk from radiation
•
•
•
•
•
Hospital radiologists
Nuclear workers
Pilots and flight crew
Some miners
Industrial radiation
workers
• Exposure is regularly
monitored
• Exposure is kept as low
as reasonably
achievable
• Exposure time is
adjusted to ensure
annual dose is within
accepted levels
EM weapons
demo
Students will be assessed on their ability to:
3.10 understand that light is part of a continuous electromagnetic spectrum
which includes radio, microwave, infrared, visible, ultraviolet, x-ray and
gamma ray radiations and that all these waves travel at the same speed in
free space
3.11 recall the order of the electromagnetic spectrum in decreasing
wavelength and increasing frequency, including the colours of the visible
spectrum
3.12 recall some of the uses of electromagnetic radiations, including:
• radio waves: broadcasting and communications
• microwaves: cooking and satellite transmissions
• infrared: heaters and night vision equipment
• visible light: optical fibres and photography
• ultraviolet: fluorescent lamps
• x-rays: observing the internal structure of objects and materials and medical applications
• gamma rays: sterilising food and medical equipment
3.13 recall the detrimental effects of excessive exposure of the human body
to electromagnetic waves, including:
• microwaves: internal heating of body tissue
infra-red: skin burns
• ultraviolet: damage to surface cells and blindness
• gamma rays: cancer, mutation.