Science Focus 8
Light and Optical Systems
Topic 1 Topic 1 – What is Light?
* Have you ever seen a fire burning? Ever wonder why you can see it? Turn to page 176
of your textbook and read the introductory paragraph to find out more.
Light ​
is a source of energy that you can see.
Natural Light Sources ​
would be things that come from nature like the Sun or fire.
Energy that ​
radiates out, or spreads out from a source in all directions and does not
require a conductor or something to travel through is known as ​
radiation or ​
radiant
energy​
.
Man made sources of light (such as light bulbs) are known as ​
artificial light sources​
.
The First Basic Principle of Light – “Light is a Form of Energy”
When light reaches a surface, it can be absorbed and transformed into other types of energy.
● When absorbed by a surface, ​
light can turn into ​
heat​
or ​
thermal energy​
.
● When absorbed by a ​
solar cell​
, light is turned into ​
electricity​
.
● When absorbed by a​
tree​
​
, light is turned into ​
chemical energ​
y.
* Turn to page 177 and 178 of your textbook to read more about light intensity.
Sources of Light
* Turn to page 179 of your textbook and read “Sources of Light”.
Living today, we are lucky to have so many different sources of light. Can you imagine
studying next to a candle?! Unfortunately, all this light that’s available to us can also be
a burden. ​
Light pollution occurs when sources of light interfere with our view of the sky
and stars at night. That’s why sometimes you’re not able to see the stars at night even
when it’s clear. Because of this, most observatories are out in the country and usually at
high altitudes (see Figure 3.3 on page 179).
Incandescent Sources
An object can be heated to such a high temperature that it emits visible light. Such an
object is called an ​
incandescent source ​
of light.
EXAMPLE​
: the thin filament wire in a light bulb gets white hot and gives off energy in
the form of mostly heat (about 95%), but also light
Electrical Energy
Thermal Energy
* Turn to page 180 and read “Incandescent Sources”.
Visible Light Energy
Science Focus 8
Light and Optical Systems
Topic 1 Fluorescent​
Sources
​
High energy invisible ultraviolet light is absorbed by particles which then emit this
energy as light.
Ultraviolet Light Energy
Energy Absorbed by Particles
Visible Light Energy
In a fluorescent light bulb, electrical energy causes mercury vapor inside the light bulb
to give off ultraviolet energy which is absorbed by the phosphor particles which coat the
light bulb. The phosphor particles then glow, which gives off the visible light. This glow
is known as ​
fluorescence​
.
Florescent tubes use less energy, last longer and waste less energy to heat than
incandescent light bulbs, but they are more expensive to manufacture and the phosphor
coating and mercury vapor are both toxic, which make disposal more difficult.
* Turn to page 181 and read “Fluorescent Sources”.
Phosphorescent Sources
A phosphorescent source of light is similar to a fluorescent source. Light energy is
absorbed by certain particles that can store this energy for a while. The stored energy is
later released as visible light. ​
The main difference between fluorescent sources and
phosphorescent sources of light is that fluorescent light is released immediately while
phosphorescent light takes longer to emit.
* Turn to page 182 and read “Phosphorescent Sources”.
Chemiluminescent Sources
Light can also result from the energy released in chemical reactions. The chemical
reaction produces energetic particles that give off visible light energy.
Chemical Energy
Visible Light Energy
* Turn to page 183 and read “Chemiluminescent Sources”.
Bioluminescent Sources
Light produced by living organisms through chemical reactions is known as
bioluminescence​
. Examples may include some very deep ocean creatures, fungi and
fireflies.
* Turn to page 183 and read “Bioluminescent Sources”.
All objects that emit light are called ​
luminous​
, while the ones that do not emit light are
called ​
non-luminous​
.
Science Focus 8
Light and Optical Systems
Topic 1 The Cost of Lighting
Electricity is charge for in kilowatt hours. A kilowatt hour is one thousand watts of
electrical power operating for one hour. ​
The symbol for watt is W and the symbol for
kilowatt hour kW•h.
In other words if you left a 100 watt light bulb on for 10 hours, it would use 1000 watts
of energy or 1 kW•h worth of energy.
QUESTION 1​
:
If a 60 watt light bulb was left on for twenty hours, how much energy would it use? If
electricity sells for 9¢/kW•h how much would this cost?
1. Convert 60 watts into kilowatts by dividing it by 1000 =
60W 1000W = 0.06kW
2. Calculate the number of kilowatt hours by multiplying the power (in kW) by the
number of hours =
0.06kW x 24 h = 1.44 kW•h
3. Calculate the cost of leaving the light on for 24 hours by multiplying the number
of kilowatt hours by the cost per kilowatt hour:
1.44kW•h X 9¢/kW•h = 12.96 cents
Cost (in cents) = Amount of energy (in kW•h) × Unit price (in ¢/kW•h)
The Ray Model of Light – “Light Travels in Straight Lines”
A ​
ray is a straight line which has a definite starting point, but no ending point. It
represents the path of a beam of light. The ray model ​
can be used to predict where
shadows will form, and how large they will be.
* Turn to page 184 and 185 of your textbook and read “The Ray Model of Light”.
Science Focus 8
Light and Optical Systems
Topic 1 The ​
ray model can help us understand what happens when a beam of light hits different
surfaces.
When a beam of light passes straight through a substance, we call that substance
transparent​
.
T​
ransparent = TRANSMITTED
QUESTION 2​
:
Provide TWO examples of transparent substances. ​
window glass, water, air, lenses
When a substance allows light to pass through, but not actual images, we call this
substance ​
translucent​
. The translucent substance bends the light rays and scatters
them.
Translucent = SCATTERED
​
QUESTION 3​
:
Provide ONE example of a translucent substance. ​
paper, wax paper
When a substance or materials completely blocks the light from passing through, the
substance is called ​
opaque​
.
Opaque = Blocked/Reflected
​
QUESTION 4​
:
Provide THREE examples of opaque materials. ​
wood, brick, desk, wall
QUESTION 5​
:
Turn to page 187 of your textbook and complete the following questions from Topic 1
Review: ​
p. 187 # 1-6