Physics
Light and Geometric Optics
Objectives
1.
2.
3.
4.
Light and colour in technology
Properties of light
Light through different media
Reflection and refraction of
light
5. Light and colour
What is Light?
• If you had an alien friend come
visit you on Earth, how would
you describe light? Explain
light? What is light?!
Various Types of Light
Emissions
1) Luminescence
is light produced using energy
sources other than heat; can
occur cooler temperatures
a)
b)
Electrons of atoms become
excited & unstable when it
absorbs energy
The electron will return to
normal when it releases this
“extra” energy as light (photon).
2) Chemiluminescence
• Light energy released from a chemical reaction
without the involvement of heat or a flame (a cold
system) eg: glow sticks
3) Bioluminescence
• Light is released by a chemical reaction;
occurs naturally in living things like fireflies & jelly fish
and glow worms
4) Incandescence
• Light energy is produced by heated object
 substance becomes hot and glows
eg: incandescent light bulbs
5) Fluorescence
• A light emitted by some substance
only when they are exposed to electromagnetic
radiation (eg: fluorescent paint)
6) Phosphorescence
• The ability to store energy from a light
source, and then emit it slowly over a long period
of time. (eg: phosphorescent paint)
eg: Painted with phosphorescent ink
7) Triboluminescence
• Light produced by friction
eg: eating wint-o-green lifesavers in the dark
8) Electric Discharge
• When an electric current passes through gases,
light is often produced
• Ex: Lightning, fluorescent bulbs
The Electromagnetic
Spectrum
• All forms of energy are organized & classified in this
spectrum by their size of wavelength ~ width of the wave
Longest – Radio waves
Shortest – Gamma Rays
• Where is visible light in the spectrum?
Where does Light Come From?
• Comes from an energy source…
• Light can move through a
vacuum (such as…) & through
different mediums (such as…)
Which comes first,
lightning or thunder?
• How fast does light travel?
• Speed of Light = 299 792 458 m/s
• For simplicity, we will use:
Speed of light = 3.0 x 108 m/s
Sound is a wave too!
Speed of Sound = 340 m/s
Sound moves slower than light!
•
Light moves 300 000 000 meters every
second in a vacuum
What is Light?
•
•
Light is a form of energy
that is visible to the human eye.
It can be described:
i)
ii)
As electromagnetic waves
As particles of light called
photons
Light is a wave that
travels in a straight line…
Light Waves
• Light waves contain both
electric and magnetic fields
• Because light has both
electric and magnetic fields,
it is also referred to as
electromagnetic radiation
What does it
Travel through?
Through space!
• EMR does not require
particles to travel like sound
waves do
Properties
of Waves
• Crest: highest point on a wave
• Trough: lowest point on a wave
• Wavelength ( λ - lambda): distance from one place in a wave
to the next similar. Ex: Distance from crest to crest.
• Amplitude: wave height from the rest position to the crest
OR the trough
• Frequency( ƒ ): the rate of repetition of a wave; measured
in hertz (Hz) = cycles per second. eg: 3 cycles /second
• Cycle: one full wavelength represents one complete cycle
Understanding the
Electromagnetic Spectrum
The shorter the wavelength,
the higher the energy.
Which
wave has
more
energy?
Electromagnetic Waves From the Sun
1.
2.
3.
4.
Which can we see?
Which can we feel?
Which is invisible to us?
Which is dangerous?
 Infrared (heat)
 Visible Light (ROYGBIV)
 Ultraviolet (UV)
Electromagnetic Waves From the Sun
• UVA and UVB rays can
penetrate our skin (shorter
waves, higher energy)
Best ways to avoid skin cancer:
1. Stay out• ofUVC
the sun/
beds
raystanning
are absorbed
by
2. Block
UVA/UVB
the atmosphere
before they
3. reach
No smoking
the ground (waves
Best way to get
= tanning
toowrinkles
long, low
energy)
• Summer Months - UV is
highest
• Midday - UV highest at 10
AM and 4 PM
• Snow, Sand and Water Reflection enhances UV
Components of Visible Light
ROYGBIV
• Wavelengths of the light we can see is =10-6 m or 400-700
nanometers (billionths of a meter)
• “Visible light” is made up of ROYGBIV – colours of the
rainbow
• Visible white light is filled with colour!
Colours and Visible Light
• Visible light waves have different sizes
• The varying wavelength’s (’s) gives us ROYGBIV
• Which colours have the least energy? The most energy?
Colours and Energy
• What colours are visible?
• Which part of the flame would be the
hottest?
Colour is Made in 2 ways…
1.
2.
Additive Colour Theory – white light is
made up of different colours (wavelengths) of light
Subtractive Colour Theory: coloured matter (pigment)
absorbs different colours (or wavelengths) of light.
1. Additive Colour Theory
Can split up
white light
through a prism
• White light is made up of a mixture of wavelengths of light –
each colour is a different wavelength: ROYGBIV. It can be
split up into separate colours and can be added together again.
• White light can also be composed
of the 3 primary colours:
RED, BLUE & GREEN
1. Additive Colour Theory
• Primary colours of light  red, blue & green
• Mixing 2 primary colours make Secondary colours:
yellow, cyan & magenta
Adding blue and red makes
magenta
Adding red and
green makes yellow
Adding blue and green
makes cyan (light blue)
Adding all three
makes white again
2. Subtractive Colour Theory
• We see colours based on the
absorption (or subtraction) &
reflection of certain wavelengths
• We see WHITE when ALL colours
are reflected
• We see BLACK when ALL colours
are absorbed (or subtracted)
2. Subtractive Colour Theory
• Applies to pigment (powder
used to colour substances
like paint & ink cartridges)
• Primary colours for pigment
are:
Cyan, Magenta, & Yellow
R
G
B
Which one is which?
A pair of magenta trousers would reflect magenta light
(which is made of red and blue wavelengths)
Magenta light
A white hat would reflect all seven colours:
White
light
Using coloured light
• If we look at a coloured object in coloured
light we see something different.
• For example, consider this gym uniform:
Shirt appears red
White
light
Shorts appear blue
• In different colours of light this uniform would look
different:
Red
light
Shirt looks red
Shorts look black
Shirt looks black
Blue
light
Shorts look blue
Some further examples:
Object
Red socks
Colour of light
Colour object
seems to be
Red
Red
Blue
Black
Green
Black
Red
Blue teddy
Blue
Green
Red
Green camel
Blue
Green
Red
Magenta book
Blue
Green
How Does a Ball Behave in
these different materials?
How would
the ball
Imagine
behave…?
throwing
a
ball
through the
air…
Imagine throwing
a ball at a brick
wall…
Imagine
throwing a
ball under
water…
How does Light Behave?
The ball & light behave similarly:
• Light travels the fastest when…
There’s no matter… in a vacuum
• Light travels at differing speeds when…
It passes through matter…different mediums…
 REFRACTION (light bending)
• Light bounces back when…
It cannot penetrate a surface
REFLECTION (light scattering)
When light hits an object, it can be…
1. Transmitted- pass through the object
2. Refracted- light bends as it is absorbed by the object
3. Reflected- light is scattered away from the object
Law of Reflection
• Reflection from a plane mirror:
Normal
Incident ray
Angle of
incidence (θ i)
Angle of
reflection (θ r)
Reflected ray
Mirror
Mirror
“Normal line”
is perpendicular
(90) to the surface
The Law of Reflection
Angle of incidence (θ i ) = Angle of reflection (θ r )
In other words, light gets reflected from a surface at
____ _____ angle it hits it.
The same !!!
Greek letter, theta (θ).
Common symbol for
angle. Angle (θ) is
measured FROM
the NORMAL to the ray
Clear vs. Diffuse Reflection
• Smooth, shiny
surfaces have a clear
or regular reflection.
Rough, uneven surfaces
have a diffuse reflection.
Diffuse reflection is when
light is scattered in
different directions
Ray Diagrams
 Allows us to trace the path
that light travels
• Why? So we can predict the
location of the reflected
image of an object
• Note: The rays (incident ray
and the reflected ray) are
drawn as straight arrows
How Do We See Reflected
Images?
Tim
• Using ray diagrams…
• We will show how TIM will be able to see the
reflected image of our object, the GREEN ARROW.
• 4 Steps
Step 1: Draw the Image of the Object
• Pick ONE DISTINCT feature on
the object
• Carefully measure the distance
from this point to the mirror
• Distance:_____
• When drawing the reflected
image (in the virtual plane),
ensure that it is the same
distance from the mirror
• Repeat this process for ALL
distinct features on the object
until the object and the image
appear identical
Image
5 cm
Virtual
Plane
5 cm
Object
*Note
The object’s distance from the mirror
is EQUAL to the image’s distance
from the mirror for ALL DISTINCT
points.
Step 2: Drawing Reflected Rays
• Pick one DISTINCT feature
on the image and draw the
reflected ray (a straight line)
from here to TIM’s eye.
Image
* Note: Draw a SOLID LINE for
the reflected ray and a
DASHED LINE in the virtual
plane.
• The reflected ray should
have an arrow pointing
towards TIM’s eye to indicate
the direction that the light is
travelling
Virtual
Plane
 Reflected Ray
Object
Step 3: Drawing Incident Rays
*Remember, Law of reflection:
ANGLE OF INCIDENCE = ANGLE OF REFLECTION.
• Draw the incident ray from
the object’s DISTINCT feature
to the point of incidence on the
mirror’s surface.
Point of Incidence
Reflected Ray
Incident ray
• Point the arrow to indicate
the direction of light
− Toward the mirror!
Step 4: Finishing it off!
• Repeat Step 2 and 3
until all incident rays for
each DISTINCT feature
have been drawn.
Points of Incidence
Reflected Rays
• Now you have shown
how light travels from
all DISTINCT the
features on the object,
towards TIM’s eye
Incident rays
Now YOU try!
Ray Diagrams Continued….
•
What if the mirror is curved?
1) Concave Mirrors
- Converging rays
- Produce larger images
2) Convex Mirrors
- Diverging rays
- Produce smaller images
Which is Concave? Convex?
Terminology for Curved Mirrors
We will first use a Concave Mirror to label these terms..
• Vertex – midpoint of the curved mirror
• Principal Axis – a straight line that passes through the
vertex (symmetrical & perpendicular)
• Center of Curvature (C) – think of it as the center of a
circle; it lies on the principal axis
• Radius of Curvature (R) – distance from the vertex to
the center of curvature
Terminology for Curved
Mirrors
 From the focal point
to the vertex has a
length of… f .
 What is the length
from C to the vertex?
• Focus or Focal Point (F ) – the half way point from
the center of curvature (C ) to the vertex; this is
where reflected rays* pass through and converge
*Reflected rays pass the focal point when incident
rays are parallel to the principal axis
• Focal length (f ) – the distance from the vertex to
the focus or focal point (F )
Diagram for Curved Mirrors
• We applied the new terminology for a Concave Mirror.
• Now, YOU try labelling the terms for a Convex Mirror!
Which way is convex?!
• Remember :
* Principal Axis (PA)
* Vertex
* Center of Curvature (C)
* Radius of Curvature (R)
* Focus or Focal Point (F)
* Focal Length (f )
Ray Diagrams for Concave Mirrors
Depending on where the object is located on the
principal axis, ray diagrams are used to determine…
S A L T
• S = the SIZE of the Reflected Image (smaller, larger,
or the same as the object)
• A = the ATTITUDE of the RI (right-side up or upside
down
• L = the LOCATION of the reflected image (RI)
• T = the TYPE (real or virtual)
We will look at 5 cases…
Drawing Ray Diagrams
for Concaved Mirrors
•
1st incident ray is drawn parallel to the principal axis, starting
from the top of the object to the mirror (blue).
•
Can you predict and draw the reflected ray (red)?
•
2nd incident ray is drawn from the top of the object through
the focal point to the mirror (blue)
•
Predict and draw the reflected ray (red).
•
The image is formed where the rays intersect. This
intersection point on the image is the same point that was on
the object.
Image
Characteristics
1. Object
is Located
S-
Center of Curvature
Beyond the
Any Incident
Rays Rays
Any Incident
• 1st incident
ray parallel
is drawn
parallel
travelling
to
thetheto
passing
through
the principal
axis,
starting
from
principal
axis
will
reflect
FOCAL
POINT
will the
•A- top ofand
the pass
object
to the
through
the (blue).
reflect
andmirror
travel
focal
point
parallel
thethe
principal
• Can you
predict
and to
draw
reflected rayaxis
(red)?
•
2nd incident ray is drawn from the
•L- top of the object through the focal
point to the mirror (blue)
• Predict and draw the reflected ray
(red).
General Conclusion
••T- The image is formed where the rays
An object located beyond the Center
intersect. This intersection point on
of Curvature will reflect an image that is:
the image is the same point that
was on the object.
• located between C and F
• Is real, smaller and upside down
2. Object is Located at the
Center
of Curvature
Image Characteristics
SSame as previous slide…
• 1st incident ray travels parallel to
Aprincipal axis from the top of object
towards the mirror (blue)
• Predict & Draw reflected ray (red)
• 2nd incident ray travels from the top
L-of the object through the focal point General Conclusion
An object located at the Center of
to the mirror (blue)
Curvature
will reflect an image that is:
• Predict & Draw reflected ray
(red).
• The imageWhat
is formed
where the rays
do you
T-intersect.
• Also located at C
notice about
the image? • Is real, same size and upside down
3. Object
is Located
Image
Characteristics
Between
the Center of Curvature and
Focal Point
S-
Same as previous slide…
• 1st incident ray travels parallel to
A- principal axis from the top of object
towards the mirror (blue)
• Predict & Draw reflected ray (red)
• 2nd incident ray travels from the top of
to
L- the object through the focal point General
Conclusion
the mirror (blue)
Anray
object
• Predict & Draw reflected
(red). located between the Center
of Curvature
• The image is formed where
the rays and the Focal Point will
reflect an image that is:
intersect.
T-
• located beyond C
Is real, larger size and upside down
Image
Characteristics
4. Object
is Located
S-
Between
the Focal Point and Mirror
Same as previous slides…
• The 1st ray is drawn parallel to
A- the principal axis from the top of
the object to the mirror (red)
• Can you predict and draw the
reflected ray?
• The 2nd ray is drawn from the top
of the object through the focal
L- point to the mirror (blue)
General Conclusion
• Predict and draw the reflected
ray.
An object that is located between the
• The image is formed where the
Focal
point and the mirror will reflect
rays intersect. This point
on the
Timage is the same point that was an image that is:
on the object
• located behind the mirror
• Is right side up, larger and is virtual
5. Image Is Located at the
Focal Point
• 1st incident ray travels
parallel to principal axis from
the top of object towards the
mirror (blue)
• Predict & Draw reflected ray
(red)
• 2nd incident ray travels from
the top of the object through
the focal point to the mirror
(blue)
• Predict & Draw reflected ray
(red).
• What do you notice…?
No SALT
Ray Diagrams for Convex
Mirrors
What does a Convex Mirror look like again ?
Depending on where the object is located on the principal
Why do
axis, ray diagrams are used to determine an images’…
we use
Ray Diagrams
Size (smaller, larger, the same)
Again?!
Attitude (upright, upside down)
•
•
• Location
• Type (real or virtual)
We will look at 2 cases…
Using mirrors
• Two examples:
2) A car headlight
1) A periscope
Light Refraction
•
•
•
1.
2.
3.
What happens when light
is NOT reflected?
In a vacuum, light travels
in a straight line
But, if light encounters
matter, such as water
then….
Light is absorbed by
water
Straight-line path of light
CHANGES
Light bends as it passes
through water
Index of Refraction
• Light bends in different ways
depending on the material it is
passing through
• The amount of bending is the
angle of refraction
• The more medium slows
down the light, the greater
the angle of the refraction
• For example, diamonds
slow down (bend) light
more than water, thus
diamonds have a greater
index of refraction
• Certain materials bend
light in similar ways…thus
they have similar indexes
of refraction
The Physics of Sparkle
1. The index of
refraction of a
diamond is 2.4
• One of the highest
for a clear
substance
• Explains why
diamonds are so
sparkly
2. The angle of
incidence of
diamonds should be
24.5…making it
glitter
Optical Illusions:
Both Reflection and/or
Refraction
1.
Desert Mirages
• Light is reflected as it
passes through the hot air
lying near the ground
• There appear to be a lake
in the distance, but this is
actually the image of the
sky reflected by the hot
water…
2.
End of the Rainbow
• Location of rainbows are
relative to YOU, the SUN,
and the RAIN
Application of Optics
1. Telescopes
Amateur astronomers use
reflecting and
refracting telescopes
• A reflecting telescope
uses mirrors to focus
light from a distant
object…how?
• A refracting telescope
uses a lens…how?
• Check out refraction with the following web applet:
•
http://www.coolschool.ca/content/showcase/content/physics/refraction.html
• Other resources can be found at
•
http://www.classconnect.ca/sph2d0_optics.htm