Unit E: Light and Geometric Optics
page 455
Start with: “What do You Already Know?”
Questions 1 – 7, page 457
Chapter 11: The Production and Reflection of Light
11.1: The Nature of Light
Page 459
page 463
Wavelength – the distance from one crest to a wave to the next
crest.
Light is the only source of energy that can pass through an empty
space and through some materials in the form of waves.
This type of energy is called electromagnetic wave.
Electromagnetic wave is a wave that has both electric and
magnetic parts, that does not require a medium, and travels at the
speed of light.
These waves are similar to water waves, and the movement of
energy from one point to another.
Electromagnetic waves are invisible and can travel through a
vacuum (empty space). The speed of light is 3.00 x 108 m/s.
Electromagnetic waves are more complicated then water waves.
They are made up of electric and magnetic fields.
The Electromagnetic Spectrum
The electromagnetic spectrum is a diagram that illustrates the
range, or spectrum, of electromagnetic waves in order of
wavelength or frequency.
The colours of light are just different wavelengths of light.
Red has the longest wavelength of visible light, and measure 700
nm, (10-9 m). Violet has the shortest wavelength of visible light,
measured at 400 nm.
Figure 5: The electromagnetic spectrum. Note the different categories as the
energy of electromagnetic waves increase or decrease.
Medium is any physical substance through which energy can be
transferred.
Radiation is a method of energy transfer that does not require a
medium; the energy travels at the speed of light. (3.00 x 108 m/s)
Visible Light is the electromagnetic waves that the human eye can
detect.
The Colours Associated with Visible Light
White visible light is composed of a continuous sequence of
colours. The seven distinct colours that have been identified are
red, orange, yellow, green, blue, indigo, and violet.
11.2: How is Light Produced
page 470
Algae can emit light at night, only after storing energy from the
Sun. Algae emit the light naturally
Zebra fish will emit light only when they are exposed to ultraviolet
light. Zebra fish are genetically engineered by scientist. The
scientists inserted a gene from jellyfish into the zebra fish making
them glow, this is not natural.
Types of Light Emissions
There are many sources of light. Some sources are natural while
others are artificial. The Sun is natural light, and candles and light
bulbs are examples of artificial light.
The Sun is luminous, which means that it produces its own light.
Candle burning, light bulbs and a flashlight are also luminous
sources.
A tree, the Moon, or a rock, do not produce their own light,
therefore they are non-luminous sources. These items can only be
seen by reflecting light.
Each of these light sources is related to heat. Light from hot
objects, is made up of many colours mixed together, also known as
White Light.
Sources of light that is emitted from a source that does not produce
heat, will only produce one colour of light.
For light sources to emit light, they must absorb energy. Atoms
that absorb energy become excited. When these atoms return to
ground state, they loose energy in the form of light.
Light from the Sun
The Sun is undergoing a fusion reaction (nuclear reaction).
Hydrogen is being combined with helium. A tremendous amount
of energy is released. This energy is transmitted to the outer gas
layer, where it is eventually released as light.
Light from Incandescence
The source of light found in an average home is the incandescent
light bulb. An incandescent light bulb creates light through heat.
The filament in the light bulb is made from tungsten wire. As
Electric current passes through it; it will heat up and begin to glow.
The incandescent light bulb is a very inefficient source of light.
Only 5% of the energy actual produces light the rest (95%) is lost
as heat energy.
Light from Electric Discharge
Light produce by this method, uses electricity to heat a gas in a
vacuum tube. The gas is sodium - mercury vapour which will blow
yellowish colour when excited by an electric discharge.
Light from Fluorescence
Tubular fluorescent bulbs are an excellent replacement for the
inefficient incandescent bulb.
How a Fluorescent Bulb Works
The fluorescent bulb is an electric discharge tube. The tube
contains mercury and argon (inert gas). The inner lining of the tube
is coated with phosphor. As the electrons travel through the tube
they exciting the mercury atoms. These atoms release energy in the
form of ultra violet light. The ultra violet light is absorbed by the
phosphor coating creating light, or fluorescence.
Efficiency of Fluorescent Lighting
This type of lighting is more efficient, 20% of the energy is
converted into light, while 80% is released as heat, but last longer.
The problem with fluorescent lights is the hazardous waste. The
mercury in the lights are very dangerous, therefore these light need
to be disposed at hazardous waste centres.
Types of Luminescence
Luminescence is the light produced without heating an object.
Phosphorescence, chemiluminesence, and bioluminescence are
three types of luminescence.
Phosphorescence – is the excitement of a phosphorescent material.
They retain energy for long periods of time. Ultraviolet light is the
source of energy that caused the phosphorous material to glow.
Chemiluminescence – light is generated through a chemical
reaction. Glow sticks that emit light, occurs because of a chemical
reaction.
Chemiluminescence is the direct production of light as the result of
a chemical reaction with little or no heat produced.
Bioluminescence – is light produced by living organisms.
Chemical reactions occurring in the cells produce his light.
Bioluminescence is the production of light in living organisms as
the result of a chemical reaction with little or no heat produced.
Light from a Light-Emitting Diode (LED)
Light-emitting diode (LED) light produced as a result of an electric
current flowing in semiconductors.
Semiconductor is a material (silicon) that allows an electric current
to flow in only one direction.
LED differs in many ways from an incandescent bulb: it does not
require a filament, it does not produce heat, and it is more efficient.
LED’s are used in electronic devices, Christmas lights, and
illuminated signs.
Check Your Learning, questions 1 – 11, page 476
11.3: The Laser – A Special type of Light
page 477
Previous sources of light emitted electromagnetic radiation, of
many different energy levels and in all directions. A laser is a
different light source. It emits only one type of electromagnetic
radiation at exactly the same energy level. This produces a light
beam of pure colour.
A prism is an optical device which can separate white light into its
individual light colours (light spectrum) that make up white light.
If a laser is pointed at a prism, the light ray will remain unchanged.
Laser emits a very intense beam of light. The light is a pure colour,
very intense, and concentrated in one narrow beam. A high energy
laser can be used to burn a hole through steel; it also can be used to
measure distances. The Moon has been measure to be 385 000 km
away from Earth.
Figure 1: a Laser emits electromagnetic waves that are all exactly the same.
Check Your Learning, questions 1 – 6, page 478
11.4: The Ray Model of Light
page 479
All light behaves the same way, no matter the source.
Reflection is the change in direction of a wave when it reaches a
surface and bounces off that surface.
All light originates from the sun and reflects off of objects and then
into your eyes.
Light travels in a straight line that can not bend around things,
when traveling in the same medium.
Medium is the term for the substance through which light is
traveling.
Light Ray is a straight line with an arrowhead that shows the
direction in which light waves are traveling.
Geometric Optics applies when light rays are used to determine
the path of light when it strikes and object.
Transparent object is an object that transmits all or almost all the
incident light. Objects are clear and see through.
Translucent object is an object that transmits some of the incident
light, but absorbs or reflects the rest. These objects you can not see
clearly through.
Opaque objects are objects that do not allow any incident light to
be transmitted. The light is either absorbed or reflected. These
objects you can not see through.
Ray Tracing
Ray tracing uses rays to predict the location, size, and shape of
objects and their shadows.
Light travels in straight lines from a source of light, in every
direction.
Fermat’s Principle
Fermat’s principle predicts the path that light will travel after
reflecting from a surface or passing through more then one
medium. Light will travel the path that will take the least amount
of time. This leads to the Law of Reflection.
The Terminology of Reflection
Plane – flat.
Incident ray – a ray of light tat travels from a light source toward a
surface.
Angle of Incidence – the angle between the incident ray and the
normal in a ray diagram.
Normal – is a line that is perpendicular to a surface where a ray of
light meets the surface.
Perpendicular - is a line that runs at a right angle.
Reflected ray – a ray that begins at the point where the incident ray
and the normal meet.
Angle of Reflection – the angle between the reflected ray and the
normal in a ray diagram.
Check Your Learning, questions 1 – 6, page 481
11.6: The Laws of Reflection
page 484
When you know the angle of incidence, you are able to determine
the angle of reflection because they are equal. The reflection ray is
found in the same plane as the normal and incident ray. This makes
up the Law of Reflection.
1. The incident ray, the reflected ray, and the normal always lie
on the same plane.
2. The angle of reflection, ‫ ے‬r, is equal to the angle of incidence,
‫ ے‬i.
‫ے‬r =‫ ے‬i
Figure 1: Diagrams illustrating the two laws of reflection.
Reflecting Light off Surfaces
Specular reflection is the reflection of light off a smooth, shiny
surface, such as; mirrors, still water, and shiny steel.
Diffuse reflection results from the reflection of light off an
irregular or dull surface. The reflected light is not parallel when
leaving the surface, unlike when it arrived.
Check Your Learning, questions 1 – 6, page 486
11.7: Images in Plane Mirrors
- Light Rays
- Laws of Reflection
*are used to find images
in a plane mirror
You need to be concerned with only 1 light ray at a time, as
opposed to all light rays.
Light rays that hit the mirror and reflect back to your eyes are used.
These rays follow the Law of Reflection.
ۧ i = ۧ r
Your brain uses this information and determines the light source
comes from behind the mirror.
You see the image behind the mirror.
This image is called a VIRTUAL IMAGE - not real.
The light does not come from behind the mirror.
Using Light Rays to Locate an Image
Virtual Images an image formed by light coming from an apparent
light source; light is not arriving at or coming from the actual
image location.
Using Equal Perpendicular Lines to Locate an Image
Plane mirrors produce virtual images.
The use of ray diagrams to determine location of the virtual image.
1. The distance from the object to the mirror (do) is equal to
the distance from the mirror to the image (di).
do = di
2. The object – image line is perpendicular to the plane
mirror.
Another Way …
Applying the Law of Reflection
1. From the top of the object, draw two incident rays toward
the mirror
2. Draw normal and reflected rays. Make sure that each angle
of reflection equals its corresponding angle of incidence.
3. Use similar procedure for the bottom of the object.
4. To draw the image, you must make the reflected rays meet.
You must therefore extend them (with broken lines)
behind the mirror. Where the reflected rays appear to
come from is the location of the image
The Acronym ‘SALT’
When describing properties of an image, you need to use the
following four characteristics;
1. Size of the Image: in comparison to the object the image can
be either, the same size, larger, or smaller then the object.
2. Attitude of the Image: is how the image is orientated in
comparison to the object, upright or inverted.
3. Location of Image: either in front or behind the mirror.
4. Type of Image: the image is either real or virtual.
The image found in a plane mirror will always be the same size as
the object, upright, behind the mirror, and virtual.
SALT – Size, Attitude, Location, and Type.
** For plane mirrors will always produce:
Same Size
Lateral Version (Upright and Backwards)
Behind - Same Distance (do = di)
Virtual
Check Your Learning, questions 1 – 12, page 493.
11.9: Images in Curved Mirrors
page 496
There are two Types of curved Mirrors;
1. Concave (Converging) Mirror – the inner surface of a sphere
is the reflecting surface. The centre of the mirror bulges away
from the object.
2. Convex (Diverging) Mirror – the outer surface of a sphere is
the reflecting surface. The centre of the mirror bulges toward
the object.
The Terminology of concave Mirrors
a) Centre of Curvature – centre of the sphere of a reflecting
surface, and is labeled as ‘C’.
b) Principal Axis – is the line that runs from the centre of
curvature to the centre of the mirror, (also know as the radius
of a circle).
c) Vertex – is the point where the principal axis intersects the
mirror. It intersects the reflecting surface at 900. It is labeled
as ‘V’.
d) Converge – the point where the parallel rays come together
or intersect.
e) Focus – is the point where converging lines come together, or
intersect. It is labeled as ‘F’.
Parallel lines to the Parallel Axis reflecting off a concave mirror,
will reflect off the mirror and converge at the focus. (Mid – point
between the C and V.
Figure 3: The focus is the point where all incident rays that are parallel to the principal
axis converge when they are reflected off the mirror surface.
How to Locate the Image in a Converging (Concave) Mirror
To find an image in a concave mirror, you will require at least 2
incident rays from the top of the object.
These rays will reflect off the mirror, and eventually intersect. At
this point, you will find the top of the image.
Rules for Finding the Image:
1. A Light ray running parallel to the Parallel Axis will reflect
through the Focus.
2. A Light ray running through the Centre of Curvature will
reflect back onto itself.
3. A Light ray running through the Focus will reflect back,
parallel to the Principal Axis.
4. A Light ray running through the Vertex will follow the Laws
of Reflection.
Figure 5: Imaging rules for a concave mirror.
Applying the Rules for finding an Image in a Concave Mirror.
There are 5 situations to be familiar with;
a) When the Object is located beyond C.
SALT: Smaller, Inverted, Between C & F, Real
b) When the Object is located on C.
SALT: Same Size, Inverted, At C, Real
c) When the Object is located between C and F.
SALT: Larger, Inverted, Outside C, Real
d) When the Object is located on F.
SALT: No Image is Formed, Lines do not
intersect!
e) When the Object is located in front of F.
SALT: Various Sizes, Upright, Behind the Mirror,
Virtual
Figure 6: A converging (concave) mirror produces a real image in the first three object
locations. The characteristics of each image are shown.
Figure 9: No image is formed when the object is at the F because the reflected rays are
parallel.
Figure 10: A virtual image behind a concave mirror is formed when an object is between
F and the mirror.
Images in a Converging Mirror
Real Image is an image that can be seen on the screen as a result
of light rays actually arriving at the image location.
How to locate the Image in a Diverging (Convex) Mirror
The rules applied in the Concave mirror are also used in the
Convex mirror.
The difference is that F (a virtual Focus) and C are located behind
the mirror. Therefore the light rays appear to come from a light
source behind the mirror.
1. A ray running parallel to the Principal Axis is reflected as if
it had come through the Focus (F).
2. A ray aimed at the Centre of Curvature (C) is reflected back
upon itself.
3. A ray aimed at the Focus (F) is reflected parallel to the
Principal Axis.
Figure 12: Imaging rules for a convex mirror.
Images in a Diverging Mirror
The ray s reflected off a Convex Mirror always diverges in front of
the mirror (no real image formed). Therefore to have intersecting
rays, you must extend them behind the mirror, producing an image
behind the mirror, forming a Virtual Image.
Check Your Learning, questions 1 – 10, page 501
Chapter 11 Review, questions 1 – 21, page 506