Light and Optics
What is light?
It's a kind of energy called "electromagnetic (EM)
radiation" (but this kind of radiation is not harmful,
except for an occasional sunburn).
There are other kinds of EM radiation too (radio
waves, microwaves, x-rays, etc.), but light is the part
WE can see, the part that makes the rainbow.
How does light travel?
FAST and STRAIGHT.
• How FAST? About 186,000 miles per second
[300,000 kilometers per second], so light
from the sun takes about 8 minutes to go 93
million miles [149 million kilometers] to
earth. Does this seem SLOW? Well, if you
could DRIVE to the sun at 60 mph [100 kph],
it would take you 177 years to get there! In
one second, light can go around the earth 7
times!
• Light travels VERY FAST – around
300,000 kilometres per second.
At this speed it can go around
the world 7 times in one
second.
How STRAIGHT? Perfectly
straight, until something bends it.
The straight paths of light are
called LIGHT RAYS.
Properties of Light
• Light travels in straight lines:
Laser
• Light travels much faster than sound. For
example:
1) Thunder and lightning
start at the same time, but
we will see the lightning
first.
2) When a starting pistol is
fired we see the smoke first
and then hear the bang.
There are three ways to control light.
Block it
with something opaque that creates a shadow
Reflect it
by changing its path with a mirror
Bend it
Change its direction by making it pass into another transparent material of
different density, like glass or water. This is called REFRACTION, and it's
how lenses work.
• Shadows
Shadows are places where light is “blocked”:
Rays of light
Law of Reflection
The incident ray
strikes the mirror.
The reflected ray
bounces off.
The angle of
incidence equals
the angle of
reflection.
• The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at the same
angle it hits it.
Reflection
• In specular reflection each incident ray bounces off in a single
direction.
• A surface that is not shiny creates diffuse reflection.
• In diffuse reflection, a single ray of light scatters into many
directions.
• We see things because they reflect
light into our eyes:
Homework
Reflection
• Images appear in mirrors
because of how light is
reflected by mirrors.
• The incident ray follows the
light falling onto the mirror.
• The reflected ray follows the
light bouncing off the mirror.
Refraction
• Light rays may bend as they
cross a boundary from one
material to another, like from
air to water.
• This bending of light rays is
known as refraction.
• The light rays from the straw
are refracted (or bent) when
they cross from water back
into air before reaching your
eyes.
Refraction
Refraction is when waves ____ __ or slow down due to travelling
in a different _________. A medium is something that waves will
travel through. When a pen is placed in water it looks like this:
In this case the light rays are slowed down by the water and are
_____, causing the pen to look odd. The two mediums in this
example are ______ and _______.
Words – speed up, water, air, bent
Reflection and Refraction
• A lens is an optical device
that is used to bend light in a
specific way.
• A converging lens bends
light so that the light rays
come together to a point.
• A diverging lens bends light
so it spreads light apart
instead of coming together.
Reflection and Refraction
• Mirrors reflect light and allow us to see ourselves.
• A prism is another optical device that can cause light to change
directions.
• A prism is a solid piece of glass with flat polished surfaces.
The fine print:
There are actually other ways to bend or
deflect light, including diffraction gratings
and holographic lenses. These depend on the
wave nature of light, and are a little more
difficult to explain. Scientists have also
found that gravity can bend light, but it takes
a very large object with strong gravity such as
a star to bend light very much, so it's not an
effect you see every day!
Visible light waves are the only electromagnetic
waves we can see. We see these waves as the
colors of the rainbow. Each color has a
different wavelength. Red has the longest
wavelength and violet has the shortest
wavelength. When all the waves are seen
together, they make white light.
Spectrum Wavelengths
When white light shines through a prism, the white light is broken apart into
the colors of the visible light spectrum. Water vapor in the atmosphere can
also break apart wavelengths creating a rainbow.
Dispersion and prisms
• The variation in refractive
index with color is called
dispersion.
• A rainbow is an example of
dispersion in nature.
• Tiny rain droplets act as
prisms separating the colors
in the white light rays from
the sun.
Mirrors, Lenses, and Images
We see a world of images created on the retina of the
eye by the lens in the front of the eye.
Mirrors, Lenses, and Images
• Objects are real physical
things that give off or
reflect light rays.
• Images are “pictures” of
objects that are formed
in space where light
rays meet.
Mirrors, Lenses, and Images
• The most common image we see every day is our own
reflection in a mirror.
• The image in a mirror is called a virtual image because the light
rays do not actually come together.
 The virtual image in a flat
mirror is created by the
eye and brain.
Mirrors, Lenses, and Images
• Light rays that enter a converging lens parallel to its axis bend to
meet at a point called the focal point.
• The distance from the center of the lens to the focal point is
called the focal length.
• The optical axis usually goes through the center of the lens.
The image formed by a lens
• A lens can form a virtual image just as a mirror does.
• Rays from the same point on an object are bent by the lens so
that they appear to come from a much larger object.
The image formed by a lens
• A converging lens can also form a real image.
• In a real image, light rays from the object actually come back
together.
Technological Revolution in Optics
Communication by
photons
METRO
WAN
TRANSOCEANIC
V
WAN
METRO
Telephony/data/internet
Massive optical data
storage
CD/DVD
Blu-ray disc (25GB)
Precision laser
machining
Laser cutting
Laser writing on
human hair
Photolithography for manufacture
of computer chips
Medical laser therapy &
optical imaging
Corrective laser eye surgery
3-D laser imaging of cell
Optics has an important place in history
Optics, light & vision
has been vital for
human survival
Telescope observations forged our
understanding of the Universe
Microscopes revealed a microuniverse
Today, Optics
remains a key
scientific diagnostic
technique (e.g.
imaging).
A new revolution in
Optics has emerged
with the birth of the
laser, fibre optics,
integration of optics
and electronics, etc..
Fundamentals of Optics
REFLECTION
REFRACTION
Mirror
i
r
IMAGING
Refractive index boundary
1
Imaging Lens
ho
n1
2
n2
F’
O
hI
F
s’
s
r=i
DIFFRACTION
Snell’s Law
f
1 1 1
 
s s f
n1sin1=n2sin2
INTERFERENCE
m
Linear polarised
a
Elliptically polarised
b

 cos( ) 
E  E0 
 cos(t )
 sin( ) 
double-slits
Finite no. of waves
s
s
POLARISATION
Aperture
beam spread
Continuum of waves
I
screen
EM-theory
E
E 0  cos(t ) 


2  cos(t   / 4) 
Our Eyes
Vision Correction with Lenses
No correction needed
a. Normal eye
b. Myopia (nearsightedness) Corrected with concave lens
c. Hyperopia (farsightedness) Corrected with convex lens
A light review:
• Our primary source of light is the sun.
• Light travels in straight lines at a speed of
186,000miles per second.
• Light waves travel faster than sound waves.
• Light energy from the sun travels through space ,
reaches earth, and some of it turns to heat energy
and warms the earth’s air.
• Light from the sun also travels to the cells of green
plants (producers) and is stored as energy.
• When light reaches an object, it is absorbed,
reflected, or passes through it.
*Electromagnetic Radiation
• Electromagnetic radiation can be described in terms of
a stream of photons. Each photon is traveling in a
wave-like pattern, moving at the speed of light and
carrying some amount of energy.
• The only difference amongst radio waves, visible light,
and gamma-rays is the amount of energy of the
photons. Radio waves have photons with low energies.
Microwaves have a little more energy than radio
waves. Infrared has still more energy.
• As we move down the chart we see visible, ultraviolet,
X-rays, and gamma-rays whose photon energies
gradually increase.
• Gamma and Cosmic rays have the highest energy
waves.
*Slide info from NSTA 2004 conference
Remember radio waves are long…and gamma rays are small
Radio-TV -Microwave- Infrared - VISIBLE -Ultraviolet -X-rays - Gamma- Cosmic
Essential Knowledge
Properties of a wavelength
Vocabulary
• Waves: a disturbance which carries energy and that
travels away from the starting point.
• Wavelength: the distance between one crest of a
wave and the next crest (distance between
compressions); red is longest and violet is shortest
• Peak: the highest point of a light wave
• Trough: the lowest point of a light wave
• Frequency: the number of waves (light or sound)
produced in a given unit of time, such as a second
Vocabulary
• Reflection: the bouncing of light from a
surface
• Refraction: the bending of light as it passes
through one material to another
Vocabulary
• Transparent: materials that allow all light to
pass through
• Translucent: letting light through but
scattering it
• Opaque: materials that do not let light
through
• Transmit: to send (as in sound or light). It also
means light passing through an object.
Galileo Galilei
•
•
•
•
Italian
Lived from 1564-1642
Developed the refracting telescope
Developed the first known example of the
microscope
Robert Hooke
• English
• Lived from 1635-1703
• Devised the compound microscope and
illumination system
• Discovered plant cells
• Examined fossils with a microscope
Isaac Newton
• English
• Lived from 1643-1727
• Discovered that white light was not a simple
entity, but splits into a range of colors
• When he passed white light through a glass
prism, he noted that a spectrum of light was
formed; particle theory of light
• Reflecting telescope