Download Physics in Everyday Life

9/16/2013
What is Light?
Physics in Everyday Life
• Light is one of the things we see around us all
the time
• The nature of light has been a topic of
discussion for centuries
Physics of Light and Sight
• The Sun is our
main source of
light.
Andrew Robinson
Thermal Sources of Light
Observable Optical Effects
• A range of optical effects
can be seen in the
atmosphere.
• Objects which are hot or
burning may give off
light
Jasper National Park, Alberta
The Nature of Light
Huygen’s Traité de la Lumiere
• The eminent Dutch scientist, Christiaan
Huygens developed a theory that light was a
wave
He was a contemporary of
Isaac Newton, and they
disagreed about many
things…
Published in 1690
Translation into English
only in 1912
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Newton’s Opticks
• Newton postulated
that light was a
particle: “Corpuscular
Theory”
Notice that this 4th
edition from 1704 is
printed in English, not
Latin
Light as a Particle
• The idea of light as a wave was the standard
model, until 1905, with the development of
“Modern Physics”
• Einstein proposed that some observable
phenomena can only be observed if light is a
particle, but with wave-like properties
• This is known as “Wave-Particle Duality” and
is one of the very challenging concepts in
Physics!
What is a Wave?
• A wave is a disturbance of something, which
can carry energy from place to place.
– Light and sound are examples
Light as a Wave
• Huygen’s Wave Theory was superior to
Newton’s Corpuscular Theory and was able to
explain many observable phenomena, and so
was the favoured theory of most scientists in
Europe
– Except England, where Newton’s prestige was so
great that nobody dared disagree with his ideas…
Physical Optics
• The optics we are going to discuss here is
called Physical Optics.
• It assumes that light travels in straight lines in
a single medium
• Light may change direction if it encounters a
boundary with a transmission medium with
different optical properties
• To explain all of the phenomena discussed,
light has to be treated as a wave.
The Mexican Wave
• People stand up and sit down in their seats in
sequence. The wave moves around the
stadium.
• Huygens did not know what the disturbance
was, but he knew that it had the
characteristics of a wave
– We now know that light is a disturbance of electric
and magnetic fields. But that’s a story for another
day.
http://angel.elte.hu/wave/index.cgi?m=models
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The Wavelength of a Wave
• The distance between two corresponding
parts of the wave
• Peak to peak height
Change the Wavelength
• This wave has a shorter wavelength than the
previous example
wavelength
The Frequency of the Wave
• The other characteristic of a wave is the
Frequency.
• How many wave cycles go past a fixed point per
second?
• Frequency is measured in Hertz (which you know
from tuning your radio to your favorite station)
– The radio waves have frequencies which are very high.
1 Megahertz is 1 million Hertz
The Speed of the Wave
• The speed of the wave is related to both
wavelength and frequency
𝑆𝑝𝑒𝑒𝑑 = 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 × 𝑊𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
• In Physics the speed of light, which is given
the special symbol, c, is extremely important
because it is constant everywhere
• 1 Hertz (Hz) = 1 wave cycle per second passing a
fixed point
What Type of Wave?
• Light is a form of Electro-Magnetic Radiation
• Electric and Magnetic Fields oscillate
• They are produced by oscillating charges
• Electromagnetic
radiation forms a
continuous
distribution of
wavelengths
• Visible Light is a very
small part of this
• Visible light is
“special” to us
because our eyes can
detect that range of
wavelengths
http://www.walter-fendt.de/ph14e/emwave.htm
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Wavelength of Light and Colour
• The property which defines the colour of the
light is the wavelength
• Violet light has a wavelength of 400
nanometres - 0.0000004 metres
• Red light has a wavelength of 700 nanometres
– 0.0000007 metres
Short
Wavelength
Long
Wavelength
Other Parts of the Electromagnetic
Spectrum
• Infra-red – “Beyond the red part of the visible
spectrum” - heat
Other Parts of the Electromagnetic
Spectrum
• Radio waves – used in radio and TV
• High frequency, long wavelength
White Light
• When we see white light, we are adding
together all of the contributions from all of
the visible wavelengths (Additive
Combination).
• Combining red, blue and green light
gives us white light
• Our eyes have red, blue and green
sensors in the retina, which send a
signal to the brain, which interprets
them as a colour
Other Parts of the Electromagnetic
Spectrum
• Microwaves – used for communications, radar, and
microwave ovens
Other Parts of the Electromagnetic
Spectrum
• Ultra violet “beyond violet light” – sun tan, sun burn
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Other Parts of the Electromagnetic
Spectrum
Energy Carried by the Radiation
• The HIGH frequency
radiation carries more
energy and is more
dangerous
• X-rays – medicine, dentistry
Light Travelling in a Straight Line
• In many applications, we can assume that light
travels in a straight line
• This is known as Geometric Optics.
• Light can change direction when it encounters
a mirror or a transparent material
– Most common optical instruments, and the eye,
can be described by this assumption.
• The LOW frequency
radiation carries less
energy and is less
dangerous
Speed of Light
• The speed of light is extremely fast, and is
extremely difficult to measure.
• Many scientists have made attempts to
measure it over the centuries, starting with
Galileo.
• The speed of light in a vacuum is:
𝑐 = 299,792,458 𝑚𝑒𝑡𝑟𝑒𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑
• or 1,080,000,000 km/hour!
Light is Fast, But Not That Fast
Speed of Light in a Solid
• Even though light moves at high speed, it still
takes over 8 minutes for light to reach us from
the sun
• Light slows down when it encounters a
transparent medium or object
• Air slows down light very slightly, but only by
about 88,000 metres per second which is not
much compared with 299,792,458 metres per
second.
We are now
seeing the sun as
it was 8 minutes
ago
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Light in Water
Light in Glass
• While light is travelling through water, it slows
down to about three quarters of c
Speed c
Speed ¾ c
air
water
Speed c
air
• The speed of light in glass depends on the
chemical composition of the glass
– It can be measured very precisely using a very
small piece of glass, and so is very useful in
forensic science, where it can identify slivers of
glass associated with a crime scene
• Generally, the light slows down to about two
thirds of c while in the glass
• When it comes out of the water, it moves at c
again
Reflection
• If light hits a perfectly flat reflecting surface (a
mirror), it is reflected at the same angle with
which it strikes the surface
• Pied Avocet near Oosterend, Texel island, the
Netherlands.
• Still water acts as a mirror surface
Reflective Surfaces
• Polished metals
• Mirrors
• Gloss Paint
•
•
•
•
Paper
Wood
Unpolished metals
Flat or matt Paint
Highly reflective surfaces
Diffuse reflecting surfaces
Reflection and Transmission
• When light reaches a boundary
between two transparent
materials, some light is
reflected, and some is
transmitted
• The reflected light still obeys
the law of reflection
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Reflection in a Glass Display Case
• Most light is transmitted through the glass
• Some is reflected
Artifacts in the National
Museum of Malaysia
(Wikimedia)
Refraction
• If light hits a surface of a transparent medium,
then some of the light passes into the new
medium.
• It slows down in the medium, and this causes
a change in wavelength and a change in
direction
Air
Water
• Huygens was the one who worked out why
the wave changed directions
• The frequency of the wave stays the same, but
if the speed changes, then the wavelength
must change too
Prism
• This change in direction of light can be used to
split light into the colours of the spectrum
• We use a specially shaped piece of glass to do
this
• You can easily see the refraction effect by
putting a straw in water in a glass
• You have different speeds of light in the air,
the water and the glass, so you see
discontinuous images of the straw
• The prism splits up white light because each
colour has a slightly different wavelength, and
travels at a slightly different speed when in
the glass
– The speed differences are known as dispersion,
and only occur when light passes into a
transparent medium
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Rainbows
• This dispersion effect is the cause
of many atmospheric optical
phenomena, such as rainbows.
• During and after rain, there may
be sufficient small droplets of
water suspended in the
atmosphere to disperse the light
coming from the sun.
• Each droplet acts like a tiny
prism.
• An individual droplet behaves like this:
Refraction
Reflection
from the
inside of
the drop
Second Refraction
• We see a rainbow because there are many
droplets, all refracting the light, but at
different heights and positions
• So we see the rainbow with the red at the top
and the violet at the bottom
An observer a long way
from the droplets sees
the red light from the
upper droplets, and the
purple light from the
lower droplets
Secondary Rainbows
• If conditions are
favourable,
sometimes we can
see a second
rainbow above the
first, with the
colours reversed
• In the secondary rainbow, light moves on a
different path
• This time there are two
reflections inside the
droplet.
• The light emerges at a
steeper angle than for the
primary rainbow, so the
observer sees reversed
colours.
• The intensity is less because
this double reflection is less
likely
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Light Bending
• Sometimes, the light does not move in straight
lines because it passes through zones where it
is refracted and reflected many times, which
“bends” it.
• Clouds of ice particles or water droplets are
usually responsible
• This gives rise to a range of optical
phenomena
• The sun dog can occur in cloud formations too
Sun-Dogs
• Images of the Sun which have been bent by
clouds of suspended ice crystal
Fargo,
North
Dakota
Solar Halo
• More refraction from ice crystals.
View from the
Brocken, Harz,
Germany
Sun dog over Stonehenge
Mirages
• Sometimes layers of air are arranged in such a
way that they refract light to different degrees
• This gives rise to Mirages
Cool air bends light more
The Inferior Mirage
• Images from above are seen on the ground
This type of
mirage is not
that stable, as
the warm air
tends to rise.
Warm air bends light
less
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Designing Optics
Lens Design
• We use a “Ray diagram” to show the direction
that we expect rays to move in, when they
pass through a lens, or reflect from a mirror
• Lenses are designed to either
– make parallel rays diverge (diverging lens) or
– Make parallel rays converge (converging lens)
• The glass or plastic is shaped to make the
places where the convergence or divergence a
fixed position
object
– controlled refraction
Principle axis
F focal point of lens
lens
• Converging Lens
Image Formation and the Brain
• Your brain takes images from the eye and
interpolates back in a straight line
• This means that some images appear to come
from a point in space where there is actually
no image or object. These images are known
as virtual images
• Diverging Lens
The eye is tricked into
believing that the rays
from the lens all come
from the focal point
F
Real Image
• For a real image, the rays from the object pass
through the image point.
• This means that a
real image can
always be
projected onto a
screen (camera,
projector, glasses)
Image: Wikimedia
The image from a
magnifying glass is a
good example
Virtual Image
• Light rays can be traced back to a point from
which they appear to diverge.
– Light rays appear to come
from that point, even though
they don’t really.
– A virtual image cannot be
projected onto a screen
– An image from a mirror, or a
magnifying glass are examples
of a virtual image
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The Camera
The Projector
2f
2f
• Object more than twice the focal length away
from the lens
• Image is real, inverted and smaller than object
The Magnifying Glass
• Image is upright,
enlarged and virtual
2f
2f
– Example:
Magnifying glass
f
• Object is between f and 2f
• Image is real, inverted, larger than object
Diverging Lens
• Regardless of the position of the object the
image formed is always
• Virtual
• Upright
• Smaller than
the object
2f
2f
f
f
• Converging lens –
object closer than f
Convex Mirror
• A convex mirror curves
away from the observer.
• The rays diverge from a
focal point that appears
to be behind the mirror
• Convex mirrors give a bigger field of view than
a plane mirror
• However, they also make objects seem smaller
and further away than they really are
“Objects in the mirror are
closer than they appear”
Required to be engraved
on car side mirrors in US,
Canada, India, Australia
Image: Wikimedia
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Concave Mirror
• Curves towards the viewer
• Rays are focused at the focal point in front of
the mirror
• Concave mirrors produce a magnified image
• Mirrors for cosmetics, shaving and use in
dentistry are concave
Satellite dishes do the same
thing with radio waves
Multiple Stage Optics
• Jodrell Bank radio
telescope
• The performance of one lens or mirror is often
limited
• By combining more than one lens and or
mirrors, other more powerful optical devices
can be made.
The sensitive radio
receiver goes at the focal
point
Mike Peel; Jodrell Bank Centre for
Astrophysics, University of Manchester
Periscope
Two mirrors
Telescopes
 =45o

=45o
 =45o
• Refractor Telescope
• Converging lens near the
object (objective lens)
• Diverging lens as the
eyepiece
• High magnification
 =45o
Used in submarines to observe ships while the submarine is under water. Image: US
Navy
Yerkes 40 inch refractor
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Microscopes
• Newtonian Reflector
• The compound microscope has two lenses to
increase magnification to up to x1000
Replica of Newton’s original
• Newton used a spherical primary mirror – so the
performance wasn’t as good as the refractor
telescopes of the day.
• When parabolic mirrors were used (much later), the
design proved to be superior to refractors.
The Human Eye
• The human eye is an optical device which
bends light to form a focussed image on the
retina, at the back of the eyeball
• Focussing on far objects is done with relaxed
ciliary muscles
• As we get older, the
ciliary muscles are
not able to tense as
much.
• We cannot form a
good image close up
(the near point gets
further away)
• We need reading
glasses
21st century version
1896 version
Accommodation
• The lens can
change shape to
allow us to focus on
objects far away or
nearby.
• Focussing on far
objects is done
with relaxed ciliary
muscles
Rods and Cones
• The rods and cone cells on the retina are there
to sense light falling on them
• The rod cells are
for imaging in low
light conditions,
but do not give
colour
information
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Cones
• The cone cells need more light to function, but
they are capable of detecting a range of
different colours
• We have three
types of cone
cells in our eye,
which look for
red, green and
blue light
respectively
Why are Tennis Balls Green?
Sensitivity to Colour
•
•
•
•
The cone cells are sensitive to
Short wavelengths (blue light)
Medium wavelengths (green light)
Long wavelengths (red light)
Colour Blindness
• Colour blindness occurs if one of more types
of cone cells are absent or not working
83 visible if you have normal vision
37 visible unless you are
protanopic where you lack the L
cones, which see the redorange-yellow part of the
spectrum (1% of human males)
• Someone who is deuteranopic
might not see this number (49).
Note that the 9 may be difficult
to discern even with normal
vision.
• Deuteranopia is the loss of the
M medium wavelength cones,
affecting red – orange – yellow
again, slightly differently to the
protanopes
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• Someone who is tritanopic
might not see this number
(56). Image may not be visible
on LCD or with excessive
screen glare.
• Tritanopes lack the L (Long
wavelength) cones, so they
have difficulty distinguishing
blues and violets
Correcting Near Sight
• Myopia
– light focussed in front
of the retina
– Eye bends the light too
much
• Hyperopia
– Light focussed behind
the retina
– Eye doesn’t bend the
light enough
Correcting Far Sight
• To correct
hyperopia, put a
converging lens in
front of the eye
• This assists the
eye by increasing
the total angle
through which
the light is bent
F
• To correct myopia
we put a diverging
lens in front of the
eye
• The optometrist has
to work out the
focal length of the
lens which gives you
corrected vision
Defects of the Eye
Focal length
Astigmatism
• Astigmatic vision is
where you have a
different focal length in
the horizontal and
vertical directions
• In this example
vertical lines are
focussed correctly,
horizontal lines are
not
Thermal Source
• Objects that are hot will emit
light at all visible frequencies
• They also emit large quantities
of infra-red light too
• Incandescent light bulbs get
very hot and are using most of
the power to produce heat,
not visible light, so they are
not efficient
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Incandescent Light Bulbs
• Incandescent light bulbs use a
hot filament to produce light.
• They produce white light (as a
mixture of all colours)
• They produce more infra-red
(heat) radiation than visible light,
so they are not very efficient
Atomic Emission Lines
• Neon signs use neon (and other gases at low
pressure) to produce light. The light is only
one wavelength, so we see one colour
Compact Fluorescent Light Bulbs
• Usually have mercury vapour in them, which
emits several different wavelengths, but not a
continuous spectrum
• They also don’t emit
as much infra red
radiation, so they
are much more
efficient
Bioluminescence
• Fireflies
A molecule called Luciferin is
responsible for the emission
of the green-yellow light
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