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Light and Geometric Optics
• Optics is the science
dealing with light and
vision.
• Optical means ‘relating
to the eye’
Light
• Light is radiant energy
that travels (radiates)
outward in all directions
from the object giving
off the light.
• Light energy is the form
of radiant energy that
can be detected by the
unaided human eye.
Sources of Light
• All objects that we see
are sources of light,
even if they do not
produce light
themselves.
• Objects that produce
light are said to be
luminous.
• Sun, burning candle, light
bulb, flash of lightning.
Non-Luminous Objects
• Most objects do not produce light on their
own. Objects are able to be seen because light
reflects (bounces off) them to our eyes.
• Coloured liquids and stained glass windows
allow light to pass through them and this
allows us to see the objects.
• Objects that we see because light reflects
from them or passes through them are called
non-luminous sources of light.
• Non-luminous objects
are sometimes referred
to as illuminated
objects.
Luminous Sources of Light
• Luminous Sources of
Light can be both
natural and artificial.
• Natural sources of light
are those that produce
light without human
intervention.
• Sun, Northern Lights,
glow of red-hot lava
• Artificial sources of light
are those that are
produced and
controlled by humans.
• Car headlights, neon
signs, flashlights,
televisions
• Some sources of light,
such as burning wood,
may be classified as
both natural and
artificial.
How Light is Produced
• Light is produced when other forms of energy
(heat or chemical) are converted into light
energy.
• The four most common ways in which
luminous objects produce light are:
•
•
•
•
Incandescence
Passing an electric current through a gas
Fluorescence and Phosphorescence
Light Emitting Diode (LED)
Incandescence
• When steel is heated to
temperatures over 2000
degrees Celsius, it gives
off white light.
• A substance that gives
off light because it has
been heated to a high
temperature is called
incandescent.
• Some substances are
heated to
incandescence when
they are rubbed
(friction).
• Substances can be heated to
incandescence by electricity.
• The light from an old
household light bulb is
produced by heating a fine coil
of tungsten wire (filament)
until the temperature rises and
gives off a bright light.
• These old bulbs are called
incandescent bulbs.
• Toasters and ovens also
become incandescent when
heated.
Did you know that…
• The space inside an
incandescent light bulb
is filled with nitrogen or
argon.
• If the space was filled
with oxygen, the gas
would react with the
hot filament.
• Substances may also be
heated to
incandescence using
chemical energy.
• When wood, wax,
kerosene, or oil is
burned, energy is given
off in the form of light
and heat because of a
chemical reaction.
Passing an Electric Current through a
Gas
• When an electric current is passed through a
gas, light is sometimes produced.
• A natural occurrence of this is lightning
produced during a thunderstorm.
• Another natural example of this is the Aurora
Borealis (Northern Lights).
• An artificial way of producing light by passing
an electric current through a gas is Neon
Lights. Other gases are used to produce
different colours.
Sodium vapour lamps produce
an intense light and is often
used to illuminate streets and
highways.
Mercury vapour lamps are
very bright and are used to
light hockey rinks and sports
stadiums.
Fluorescence
• Some objects give off
light when invisible
ultraviolet light is shone
on them.
• Substances that
produce visible light
when exposed to
ultraviolet light are
called fluorescent.
Stamp collectors
shine UV light onto
stamps to check for
special fluorescent
markings.
A Fluorescent Light Bulb
• A fluorescent light tube consists of a long,
cylindrical tube that filled with mercury vapour.
• The inside of the tube is coated with a
fluorescent powder. Electrodes are placed at
either end of the tube.
• When an electric current is passed through the
vapour, UV light is produced. The UV light hits the
powder which then produces visible light.
• A fluorescent tube is much more efficient than
a regular incandescent light bulb.
• About 20% of the electrical energy used in a
fluorescent light is converted to visible light,
but only 5% of the electrical energy used in an
incandescent bulb is converted to visible light.
• If you place you hand near a fluorescent tube,
it feels cool. If you place you hand near an
incandescent bulb, it feels warm.
• When a fluorescent tube is switched off, no
more UV light is produced and the coating
stops giving off light.
• Some fluorescent substances though,
continue to give off light for hours after the
energy source has been removed. These
substances are called phosphorescent.
• Luminous dials on watches, glow in the dark objects
and similar items are made of phosphorescent
materials.
CFL bulbs
• Governments around the world recommend
that homes and businesses switch to compact
fluorescent light bulbs (CFLs).
• CFL bulbs last longer than an incandescent
bulb and use less energy.
• CFLs cost more than an incandescent bulb and
the mercury in the CFLs can cause
environmental pollution.
• All used fluorescent bulbs should be taken to a
hazardous waste collection site.
• CFLs were devised to
reduce the length of
regular fluorescent light
fixtures.
Light Emitting Diode (LED)
• The Light Emitting Diode is
a very efficient light
producing technology.
• An electric current passes
through an LED, it emits
light.
• LEDs save energy, last
longer and stay cooler
than Incandescent bulbs
of CFL bulbs.
Other ways that light is produced
• Chemiluminescence
• Bioluminescence
Chemiluminescence
• Light can be the result
from the energy
released in chemical
reactions. The products
of the chemical reaction
give off visible light.
This process is called
chemiluminescence.
Bioluminescence
• Some living things can make themselves
luminous using a chemical reaction similar to
chemiluminescence. This is called
bioluminescence.
• Many organisms that live deep in the ocean
use bioluminescence because so little sunlight
reaches far below the surface of the water.
• Fireflies, glow worms, some fish, some bacteria
Properties of Light
Light that is being transmitted through the air is
reflected from water, ice, and rocks.
The Transmission, Reflection, and
Absorption of Light
• Light can be affected in three different ways
when it strikes matter:
• Transmitted
• Reflected
• Absorbed
• A substance, such as air,
allows light to pass
though it as it the air
wasn’t even there.
• When light passes
through a substance,
light is transmitted
through the substance.
• Any substance that
transmits light is called
a medium
• Other substances cause
light to bounce or
reflect from them.
• Mirrors and white
paper reflect most of
the light that strikes
them.
• Some substances
absorb the light that
strikes them.
• A black surface absorbs
most of the light energy
hitting it. This light
energy is then turned
into heat energy which
is why dark surfaces feel
warm.
Transparent, Translucent, and Opaque
Substances
• All substances can be classified according to
how they transmit, reflect, or absorb light.
• A transparent substance is one that transmits
light readily.
• Objects can be seen clearly through a transparent
substance.
• Some examples of transparent substances are:
• window glass, air, clean water, and plastic food wrap.
• A translucent substance is
one that transmits some
of the light that hits it.
• Objects can be detected,
but not clearly seen
through a translucent
substance.
• Frosted glass
• An opaque
substance is one
that does not
transmit any
light.
• When light
strikes an
opaque object,
the light is
reflected or
absorbed.
• Rocks, wood,
and metals
More Properties of Light
• Light travels in straight lines (as long as
the medium in which the light is
travelling remains the same). This
property is called the linear motion of
light.
Laser
• We see things because they reflect
light into our eyes:
Homework
• Light is invisible when it travels through a
vacuum or a transparent substance.
• A vacuum is a region that contains no matter at all, not
even gases.
• Light can only be detected by the effects it
produces.
• On a clear night, the light beam from a flashlight
cannot be seen.
• On a foggy night, the light beam can be seen because
the water droplets in the air reflect some of the light to
you eyes.
The Light Ray Model of Light
• The Light Ray Model is
one of the simplest and
most successful models
ever developed.
• The Light Ray Model is
based on a single
property of light (linear
motion).
• This model has played a
role in the development
of many optical
instruments such as
telescopes and
cameras.
• Because light travels in straight lines, the
geometric principles related to straight lines
can be used to show what happens to light
when it strikes various objects.
• So many effects produced by light can be
explained using this model, which makes up
the branch of study called Geometric Optics.
• To use the Light Ray Model, two terms need to
be defined.
• The path taken by a fine beam of light through a
medium is called a light ray.
» The light ray is represented in a diagram by a single,
straight line.
» The direction of the ray of light is shown by an arrow
pointing in the direction of travel.
• A group of parallel light rays is called a light beam.
• A ray box is a device
used to produce beams
and rays of light for
experiments.
Shadows
Shadows are places where light is “blocked”
Rays of light
Shadow
• A shadow is a dark region that forms behind
an object. The object is being illuminated
brightly on the other side of the shadow.
• Shadows are formed when some or all of the
light falling on an object is absorbed or
reflected.
• Transparent and colourless substances do not produce
shadows.
• Translucent objects may transmit some light, but they
also absorb or reflect the striking light.
• Opaque objects do not transmit any light so all of the
light is absorbed or reflected. Very dark shadows form
behind opaque objects.
•This figure is of a ray
box and two screens.
It shows a group of
light rays (light beam)
travelling from the box
to the screens.
•Although a light
beam is actually made
up of a large number
of rays, only a few are
drawn to represent
the beam.
•This diagram is the
same as the previous
diagram showing only
a few light rays.
•This is an even more
simplified version
showing only the
outer limits of the
beam.
Umbra and Penumbra
• A very dark shadow,
where there is no light
is called an umbra.
• Umbra have sharply
defined edges.
• There is a lighter
shadow with edges that
are not as sharp as an
umbra, called a
penumbra.
Light Ray Model Explains the
Formation of Shadows
• Consider the shadow produced by a point
light source. The light rays are given off in all
directions from the light source.
• Some of the light rays strike an opaque object
and are absorbed, forming an umbra behind
the object.
• The other light rays continue to travel in
straight lines from the light source as they
were not stopped by an opaque object.
Notice that to define a shadow, it is only necessary to draw the light rays
travelling from the point source to the outside edges of the opaque object .
The shape of the umbra is the same as the opaque object.
The size of the umbra depends on the distance between the light source and the
opaque object. If the object is far from the light, a small shadow will be produced.
When the object is closer to the point source, the shadow will be large.
• A different kind of
shadow is formed if a
large light source is
used. The light is no
longer coming from
one single point.
• The larger light
sources are called
extended light
sources.
• When an extended light source is used, the
resulting shadow does not have sharply
defined edges like the shadow of a point
source.
• Instead, a lighter area of a shadow is formed
(penumbra) because light from the extended
source overlaps at the outside edges of the
opaque object
Penumbra
Umbra
Penumbra
Reflection of Light
Regular Reflection of Light.
• The reflection of light from smooth, shiny
surfaces, such as mirrors, is called regular
reflection.
• Light that travels towards a mirror is called the
incident light.
• Light that is reflected from a mirror is called
the reflected light.
• The study of regular reflection requires
measuring the angles at which the incident
light strikes and the reflected line leaves the
mirror.
• The use of a normal is used to measure the
angles from all mirror surfaces. A normal is
drawn at right angles to the reflecting surface.
• The angle of incidence is the angle between
the incident ray and the normal.
• The angle of reflection is the angle between
the normal and the reflection ray.
Two Laws of Reflection
• The Light Ray Model can be used to explain
how light reflects from a smooth, flat surface.
• The First Law of Reflection states that the
angle of incidence equals the angle of
reflection.
• The Second Law of Reflection states that the
incident ray, the refection ray and the normal
are all on the same plane.
Clear vs. Diffuse Reflection
Smooth, shiny surfaces
have a clear reflection:
Rough, dull surfaces have
a diffuse reflection.
Diffuse reflection is when
light is scattered in
different directions
Diffuse Reflection
• Most objects that reflect light have rough, dull
surfaces. What happens when parallel rays of
light strike a rough surface?
• Because the surface is not smooth, the light
rays are scattered in many directions. This is
called diffuse reflection.
• The laws of reflection still apply to rough
surfaces!
Plane Mirrors
• Most objects that you can see are nonluminous (you can see them because they
reflect light to your eyes).
• Most non-luminous objects have rough
surfaces and will reflect light in a manner that
reveals their shape, colour, and texture.
• Some non-luminous objects reflect light is
such a way that an image is formed. These are
called mirrors.
• Polished metal and calm water are also mirrors.
• A flat reflecting surface is called a plane.
• Plane means two dimensional surface, just like
in math!
Characteristics of Images
(used to study and compare images)
Characteristic
Possible Description
Size
•Smaller than the object viewed
•Larger than the object viewed
•Same size as the object viewed
Attitude
•Upright (right side up)
•Inverted (upside down)
Location
•Appears closer to the plane than the object
•Appears further to the plane than the object
•Appears to be on the opposite side of the plane than the
object
Type
•Real image (can be placed onto a screen)
•Virtual image (can only be seen by looking through an optical
device)
The image of an object reflected by a plane mirror appears
to be at the same distance behind the mirror’s surface as
the object is itself in front.
Drawing Images formed by a Plane
Mirror
1. Draw a line representing the mirror and
shade one side of it. The shade represents
the reflective coating of the mirror. Draw the
object in front of the mirror.
2. Draw the normals from the mirror surface to
significant points, such as the top and bottom
of the object. The number of other points
depends on the object.
3. Measure the length of each normal drawn
from the mirror to the object. Extend the
normal an equal length behind the mirror
surface. Complete the image by connecting
the ends of the normals behind the mirror.
Application of a Plane Mirror
• A Periscope!
• It is an instrument in which plane mirrors are used to
fold light so that the image of an object can be brought
down to a lower level.
• It is used for observing enemy movements from
trenches without any danger of being seen. Sailors on
submarines use periscopes to see things above the
water level.
Curved Mirrors
• Curved Mirrors have as many different uses as
plane mirrors.
• Curved mirrors for this class are spherical
mirrors because they have the same curvature
as a sphere.
• There are two basic curved mirrors.
• Convex mirrors bulge outwards in the middle.
• Concave mirrors bulge inwards in the middle.
To help identify the two spherical mirrors, think of the word “cave” in concave. The
surface of a concave mirror is curved in like the opening into cave!
Curved Mirror Terminology
Center and Radius of Curvature
• The center of the
sphere from which the
curved mirrors are from
is called the center of
curvature (C).
• The distance from the
center of the curvature
to the surface of the
mirror is called the
radius of curvature.
Vertex and Principle Axis
• The center of the mirror
is called the Vertex (V).
Vertex
• The line that passes
through the center of
curvature and the
vertex is called the
principal axis (P).
V
• Any other line that passes from the center of
curvature (c) to the surface of the mirror is
called a secondary axis.
• Secondary axis meet the surface of the mirror
at 90 degree angles.
• The radius of the circle is an example os a secondary
axis.
• The secondary axis and the principle axis are normals to
the surface of the mirror.
Secondary axis, also
known as a radius
Focal Point
• The point that is half way between the C and
the V is called the Focal Point (F).
• The length between the C and F is called the
focal length.
Concave Mirrors
Concave Mirror
When parallel lines shine along the P. Axis
of a concave mirror, reflecting rays meet at
the Focal point (F).
Two Rules of Reflection for
Concave Mirrors
• Light always reflects according to the law of
reflection, regardless of whether the reflection
occurs off a flat surface or a curved surface.
• Using reflection laws enables us to determine the
image location for an object.
• The image location is where all reflected light
appears to diverge from. To determine this location
we need to know how light reflects off a mirror.
• Any incident ray traveling parallel to the
principal axis on the way to the mirror will
pass through the focal point upon reflection.
• Any incident ray passing through the focal
point on the way to the mirror will travel
parallel to the principal axis upon reflection.
Step by Step Method for Drawing
Ray Diagrams (Concave)
• Pick a point on the top of the object and draw
two incident rays traveling towards the mirror.
• Using a straight edge, accurately draw one ray so that it
passes exactly through the focal point on the way to the
mirror.
• Draw the second ray such that it travels exactly parallel
to the principal axis. Place arrowheads upon the rays to
indicate their direction of travel.
• Once these incident rays strike the mirror,
reflect them according to the two rules of
reflection for concave mirrors.
• The ray that passes through the focal point on the way
to the mirror will reflect and travel parallel to the
principal axis. Use a straight edge to accurately draw its
path.
• The ray which traveled parallel to the principal axis on
the way to the mirror will reflect and travel through the
focal point. Place arrowheads upon the rays to indicate
their direction of travel. Extend the rays past their point
of intersection.
• Mark the image of the top of the object
• The image point of the top of the object is the point
where the two reflected rays intersect.
Image of Object Beyond C
• When the object is
located beyond C, the
image will always be
located somewhere in
between C and F.
• The image will be an
inverted image. That is
to say, if the object is
right-side up, then the
image is upside down.
• The image is reduced in size
• The image is a real image. Light rays actually
converge at the image location.
• If a sheet of paper was placed at the image location,
the actual replica of the object would appear projected
upon the sheet of paper.
What if the object is right at C?
• When the object is
located at C, the image
will also be located C.
• The image will be
inverted (a right-side-up
object results in an
upside-down image).
• The image dimensions
are equal to the object •Finally, the image is a real
image. Light rays actually
dimensions.
converge at the image
location.
•The image of the object
could be projected upon
a sheet of paper
What if the object is between C
and F?
• When the object is
located in front of C, the
image will be located
beyond the C.
• The image will be
inverted (a right-side-up
object results in an
upside-down image).
• The image dimensions
are larger than the
object dimensions.
•The image is a real
image. Light rays actually
converge at the image
location.
•The image of the
object could be
projected upon a
sheet of paper.
What if the image is between F and
the mirror?
• When the object is located beyond the focal
point, the image will always be located
somewhere on the opposite side of the mirror.
• The image will be an upright image. If the
object is right-side up, then the image will also
be right-side up.
• In this case, the image is magnified
• The image location can only be found by
extending the reflected rays backwards
beyond the mirror.
• The point of their intersection is the virtual
image location. It would appear to any
observer as though light from the object were
diverging from this location.
What is the image is right on F?
• When the object is located at the focal point,
no image is formed.
• Light rays from the same point on the object
will reflect off the mirror. They will neither
converge nor diverge.
• After reflecting, the light rays are traveling
parallel to each other and do not result in the
formation of an image
Convex Mirrors
• When parallel lines shine along a principle axis
on a convex mirror, the reflected rays diverge
from one another.
• If you extend the reflected rays to behind the mirror,
they would meet up at a point called the virtual focal
point.
Two rules of Reflection for Convex
Mirrors
• Any incident ray traveling parallel to the
principal axis on the way to a convex mirror
will reflect so that its extension will pass
through the focal point.
• Any incident ray traveling towards a convex
mirror such that its extension passes through
the focal point will reflect and travel parallel to
the principal axis.
Step by Step Method for Drawing
for Ray Diagrams (Convex)
• Pick a point on the top of the object and draw
two incident rays traveling towards the mirror.
• Using a ruler, draw one ray so that it travels towards the
focal point on the opposite side of the mirror
• Draw the second ray so that it travels exactly parallel to
the principal axis. Place arrowheads upon the rays to
indicate their direction of travel.
• Once these incident rays strike the mirror,
reflect them according to the two rules of
reflection for convex mirrors.
• The ray that travels towards the focal point will reflect
and travel parallel to the principal axis. Use a straight
edge to accurately draw its path.
• The ray that traveled parallel to the principal axis on the
way to the mirror will reflect and travel in the direction
of the focal point. Align a straight edge with the point
of incidence and the focal point, and draw the second
reflected ray.
• Locate and mark the image of the top of the
object.
• The top of the image is the point where the two
reflected rays intersect. Since the two reflected rays are
diverging, they must be extended behind the mirror in
order to intersect.
• Using a straight edge, extend each of the rays using
dashed lines. Draw the extensions until they intersect.
• The point of intersection is the top of the image.
• All images produced by convex mirrors
produce images with the following
characteristics:
• located behind the convex mirror
• a virtual image
• an upright image
Refraction
• Reflection occurs when light rays bounce off
object.
• Using the laws of reflection, the direction in
which reflected light travels can be predicted
and the location of an image can be predicted.
• But what happens when light moves from air
into a different medium?
As you look through the side of the glass at the portion of the pencil located above
the water's surface, light travels directly from the pencil to your eye. Since this light
does not change medium, it will not refract.
As you look at the portion of the pencil which was submerged in the water, light
travels from water to air. This light ray changes medium and subsequently
undergoes refraction. As a result, the image of the pencil appears to be broken.
The portion of the pencil which is submerged in water also appears to be wider
than the portion of the pencil which is not submerged.
• Refraction is the bending of light when it
travels from one medium to another.
• Light bends because it changes speed when it
moves between materials with different
densities.
• Light travels more slowly in thicker/denser
material.
• The bending of light makes an object’s image
appear to be in a different position from
where the object really is.
The Direction of Bending
• Refraction is the bending of the path of a light
wave as it passes from one material into
another material.
• The refraction occurs at the boundary and is
caused by a change in the speed of the light
wave upon crossing the boundary.
• The tendency of a ray of light to bend one
direction or another is dependent upon
whether the light wave speeds up or slows
down upon crossing the boundary.
• The direction which the path of a light wave
bends depends on whether the light wave is
traveling from a more dense (slow) medium to
a less dense (fast) medium or from a less
dense medium to a more dense medium.
• As light travels through a given medium, it
travels in a straight line. However, when light
passes from one medium into a second
medium, the light path bends. Refraction
takes place. The refraction occurs only at the
boundary.
• Once the light has crossed the boundary
between the two media, it continues to travel
in a straight line. Only now, the direction of
that line is different than it was in the former
medium.
On the diagram, the direction of the students is represented by two arrows
known as rays. The direction of the students as they approach the boundary is
represented by an incident ray (drawn in blue). And the direction of the students
after they cross the boundary is represented by a refracted ray (drawn in red).
• Since the students change direction (refract),
the incident ray and the refracted ray do not
point in the same direction.
• See the perpendicular line drawn to the
boundary at the point where the incident ray
strikes the boundary?
• A line drawn perpendicular to the boundary at
the point of incidence is known as a normal
line.
• Observe that the refracted ray lies closer to
the normal line than the incident ray does. In
such an instance as this, we would say that
the path of the students has bent towards the
normal.
Light Traveling from a Fast to a
Slow Medium
• If a ray of light passes across the boundary
from a fast material into a slow material, then
the light ray will bend towards the normal
line.
• FST = Fast to Slow, Towards Normal
• If a ray of light passes across the boundary
from a material in which it travels fast into a
material in which travels slower, then the light
ray will bend towards the normal line.
• Now suppose that the each individual student
in the train of students speeds up once they
cross the masking tape.
• The first student to reach the boundary will
speed up and pull ahead of the other
students. When the second student reaches
the boundary, he/she will also speed up and
pull ahead of the other students.
• This continues for each student, causing the
line of students to now be traveling in a
direction further from the normal.
The refracted ray (in red) is further away from the normal then the incident ray
(in blue). In such an instance , the path of the students has bent away from
the normal.
Light Traveling from a Slow to a
Fast Medium
• If a ray of light passes across the boundary
from a slow material into a fast material, then
the light ray will bend away from the normal
line.
• SFA = Slow to Fast, Away From Normal
• If a ray of light passes across the boundary
from a material in which it travels slow into a
material in which travels faster, then the light
ray will bend away from the normal line.
Light and Colour
Colour
• Colour is the property
of light that allows us to
visually distinguish
between two objects
that have identical
shape, structure, and
size.
White Light
• The sun is the most important source of light.
The light given off by the sun is called white
light.
• In the 17th century, Sir Isaac Newton (an
English scientist) conducted a famous
experiment.
• He placed a prism in front of a beam of white
light and a band of colours emerged. Each
band refracted at a different angle, producing
a rainbow effect.
• Newton realized that colour was not added to
light, but already present in white light.
• Next, Newton passed the band of colours
through a reverse prism and this time only the
white beam appeared.
• He proposed that white light is the result of
mixing together all of the different colours of
light.
The Spectrum
• When white light is refracted into different
colours, the resulting pattern is called a
spectrum.
• For sunlight, the colours range from red
through orange, yellow, green, blue, indigo,
and violet. (ROY G BIV)
• The colours are always the same and always in
the same order.
• The separation of white light into its colours is
called dispersion
• The spectral colours can be mixed together
(recombined) into white light by using a prism,
lens, or concave mirror.
• When light strikes an
object, the light may be
reflected off the object,
absorbed by the object,
or transmitted through
the object.
• If light is shone at a pair
of blue jeans, the jeans
absorbs all of the colours
except blue. Only the blue
light is reflected or
transmitted.
• The colour seen when light strikes an opaque
object depends on which colours are reflected
and which ones are absorbed.
• The white paper used to write on is reflecting all
colours so all that is seen is white.
• The black ink is absorbing all of the colours so black is
seen.
Additive Primary Colours
• The three colours red, green, and blue are called
the additive primary colours.
• They are called additive because adding all three
together in proper amounts, will produce white
light.
• The three primary colours can also be combined
in differing intensities to produce any of the
spectral colours, as well as colours not in the
spectrum.
• No other combination of coloured lights can
produce red, green, or blue.
Additive Primary Colour
Additive Primary Colour
Additive Secondary Colour
Red
Green
Yellow
Red
Blue
Magenta
Blue
Green
Cyan
The light of two additive primary colours will produce a secondary colour.
• Television screens use
additive primary colours.
The screen contains many
groups of three tiny
phosphor dots.
• Each dot glows with a
different colour when it
receives energy – one dot
glows red, another green,
and the third glows re
•It is possible to obtain white light by adding two coloured lights together
•Any two colours that combine to produce white light are called
complementary colours.
Additive Primary Colour
Complimentary Colour
Light Transmitted
Red
Cyan (Blue and Green)
White Light
Green
Magenta (Red and Blue)
White Light
Blue
Yellow (Red and Green)
White Light
How We See Colour
• The retina of the human eye contains two types
of cells that respond to light.
• Some cells look like tiny cylinders, called rods, and they
detect the presence of light.
• The other cells are cones because they look like cones.
These cells respond to colour.
» Some of the cones respond to green, some respond to red,
and the third type of cone responds to blue.
• Signals from the rods and all three type of cones
travel along the optic nerve to the brain. The
brain interprets the shape and colour of the
objects seen.
Colour Blindness
• Humans share the phenomenon of seeing in
full colour with such organisms as goldfish,
bees, and apes.
• Some people’s eyes have defective cone cells
so they have difficulty detecting some colours.
This condition is known as colour blindness.
• The term colour blindness is a bit misleading
since most people can actually see some
colour.
Normal Vision
A person who confuses red and
green
Subtractive Primary Colours
• Cyan, Magenta, and Yellow are subtractive
primary colours because some portion of
white light has been removed in order to
produce it.
• Cyan is made up of green and blue light. It is missing
the red.
• If cyan light and red light are shone onto a screen,
white light will be reflected.
• Cyan and red are complimentary colours because
together they form white light.
Subtractive Primary Colours
Light Colour
Includes
Missing
Cyan
Blue and Green
(B + G)
Red
(-R)
Magenta
Red and Blue
(R + B)
Green
(-G)
Yellow
Red and Green
(R + G)
Blue
(-B)
Using Colour Filters
• Colour filters subtract colour from the light
they transmit. This theory is called the
subtractive colour theory.
• When white light strikes a red filter, all of the
other colours are absorbed (subtracted) and
only the red colour is transmitted.
• Only blue light passes through a blue filter
• Only green light passes through a green filter
• Each subtractive colour filter absorbs one of
the primary colours.
• Yellow light is made up of green and red.
• A yellow filter will let green and red pass through, but
not blue.
Primary Colour Filter
Absorbed colour
Transmitted Colours
Magenta (made up of red
and blue)
green
Red and blue allowed
through
Yellow (made up of red
and green)
blue
Red and green allowed
through
Cyan (made up of green
and blue)
red
Blue and green allowed
through
Subtractive Primary Colour
Filter
Absorbed Colour
Transmitted colour
Magenta (made up of red
Green absorbed
and blue)
+
Yellow (made up of red and Blue absorbed
green)
Red transmitted through
Magenta (made up of red
and blue)
+
Cyan (made up of blue and
green)
Blue transmitted through
Green absorbed
Red absorbed
Yellow (made up of red and Blue absorbed
green)
+
Cyan (made up of blue and Red absorbed
green)
Green transmitted through
An Example
• If you place a magenta filter in front of a beam
of white light and then place a cyan filter in
front of the light transmitted, What light will
shin on to a screen.
• White light is made up of red, green, and blue light.
• The magenta filter will take out the green, leaving the
blue and red light.
• The cyan filter will take out the red, leaving only the
blue.
Try this one!
• If you have a beam of white light (red, green,
blue) and place a yellow filter in front. What
colour(s) will transmit?
• Minus the blue, so green and red still pass through
• Then place a red filter in front of the
transmitted beam. What colour(s) will
transmit onto a screen?
• Minus the green, so red will still pass through
Black
• Black is not a colour. Instead it is the absence
of coloured light.
• When a combination of filters is used, black
can be the result. All of the other colours have
been absorbed and none are being
transmitted.
Applications of Colour Filters
• Skiers and boarders where amber-coloured
glasses/goggles while on the hill.
• The yellow in the glasses will prevent the blue
in the bright sunlight from reflecting off the
snow and striking the eyes.
• This prevents a condition called snow
blindness, a temporary blindness caused by
the glare light reflected by snow.
Lenses
• Knowing how curved mirrors reflect light can
help you understand how lenses affect light
rays.
• A lens is a curved piece of transparent
material. Light refracts as it passes through a
lens, causing the light rays to bend.
Basic Lens Shapes
•A converging lens is a
lens that is thickest in the
middle and causes
incident parallel rays to
converge at a single
point.
•A diverging lens is a lens
that is thinnest in the
middle and causes
incident parallel rays to
spread apart after
refraction.
Diverging (Concave) Lenses
• A concave lens is thinner and flatter in the
middle than around the edges.
• Light passing through the thicker, more curved
areas of the lens will bend more than light
that passes through the flatter, thinner area in
the middle.
• Rays of light are spread out (diverged) after
passing through the lens.
Optical Center
Secondary Principal Focus
(F’)
Converging (Convex) Lenses
• A convex lens is thinker in the middle than
around the edges.
• This thicker middle causes the refracting light
rays to come together (converge)
Optical Center
Secondary Principal Focus
(F’)
Three Rules for a Converging Lens
1. A ray parallel to the principal axis is refracted
through the principal focus.
2. A ray through the secondary principal focus is
refracted parallel to the principal axis.
3. A ray through the optical center continues
straight through without being refracted.
Using Ray Diagrams with Converging
Lenses
• Draw the ray parallel to the principal axis.
Draw the refracted ray so that it passes
through the principal focus.
• Draw a ray from the top of the object through
the middle of the lens. This ray is undeviated.
Where the rays meet, that is where the image
is.
• You can also draw the incident ray
passing through F’ on the way to the
lens. The result will be a parallel ray to
the principal axis.
Locating Images – Object beyond 2F’
•Image is
•Smaller
•Inverted
•Between F and 2F
•Real
Locating Images – Object at 2F’
•Image is
•Same size
•Inverted
• At 2F
•Real
Locating Images – Object between 2F’
and F’
•Image is
•Larger
•Inverted
•Beyond 2F
•Real
Locating Images – Object AT F’
•Image is
•No clear image due to
emergent rays being
parallel
Locating Images – Between F’ and Lens
•Image is
•Larger
•Upright
•Behind the lens
•Virtual
Three Rules for a Diverging Lens
1. A ray parallel to the principal axis is refracted
as if it had come through the principal focus.
2. A ray that appears to pass through the
secondary principal focus is refracted parallel
to the principal axis.
3. A ray through the optical center continues
straight through without being refracted.
•A diverging lens always produces
the same image no matter where the
object is.
•Smaller
•Upright
•On the same side as the object
•Virtual
The Eye
The Human Eye
• The human eye is the optical instrument that
helps most of us learn about the external
world.
How We See Colour
• The retina of the human eye contains two types
of cells that respond to light.
• Some cells look like tiny cylinders, called rods, and they
detect the presence of light.
• The other cells are cones because they look like cones.
These cells respond to colour.
» Some of the cones respond to green, some respond to red,
and the third type of cone responds to blue.
• Signals from the rods and all three type of cones
travel along the optic nerve to the brain. The
brain interprets the shape and colour of the
objects seen.
• Most people believe that we see with our
eyes. In reality, the eye acts as a light
gathering instrument. We actually see with
our brain!
• The cornea-lens combination acts like a
converging lens and produces smaller, real,
inverted images on the retina.
• Impulses from the retina travel through the
optic nerve to the brain. The brain takes the
inverted image and flips it so that we see the
image as it appears.
• This is how the lens in a normal human eye
focuses light rays onto the retina.
• Light refracts through the lens onto a lightsensitive area at the back of the eye called the
retina.
• The image you see is formed on the retina.
Myopia – Nearsightedness
•For some people, the eye has a
longer shape.
•This means the imaged forms in
front of the retina. These people are
near sighted (have trouble seeing
far objects).
Hyperopia – Farsightedness
•For some people, the eye has a
shorter length so the image has not
formed by the time it reaches the
retina. The image forms behind the
retina.
•These people are far sighted and
have trouble seeing things close up.
Presbyopia
• Presbyopia is an age-related loss of flexibility
of the lens inside the eye.
• With the onset of presbyopia, you'll find you
need to hold books, magazines, newspapers,
menus and other reading materials farther
away in order to see the print clearly.
• Knowledge of how light behaves when it
travels through lenses helps eye specialists
correct vision problems.
• A convex lens in placed in front of the far
sighted eye to help bend the light rays to form
an image on the retina.
Eyes and Cameras
• There are many similarities between the
human eye and a camera.
• You know that you see objects when light is
reflected to your eye, refracted through your
lens, and focussed on your retina.
• In a camera, the lens refracts the light and the
film senses the light.