Download Science 8: Unit C – Light and Optical Systems

Science 8: Unit C – Light
and Optical Systems
Topic 1: What is Light?
I. Light is a form of
Radiant Energy
• Light is a form of energy that we can see.
• Light travels in waves. Light, in fact is a
kind of radiation. This means that light rays
can travel across empty space. Other kinds
of waves like sound waves need some kind
of material to travel through. That’s why an
astronaut cannot hear the explosion of his
spaceship, but can see it explode.
• Light travels in straight lines. We call these
straight lines ‘rays’. The more rays there
are in an area, the brighter the light will
look.
I. Light is a form of
Radiant Energy Cont’d
• Since light is a form of energy, different
light sources have more or less energy than
others. We measure light energy by its
intensity. The brighter the light, the more
energy it has.
• Light rays can be reflected, diffracted,
refracted, or absorbed by an object. Black
objects tend to absorb most light that hits
them and white objects tend to reflect most
light rays that hit them.
II. Light Sources
• Light can either come from
natural (eg. sun, stars, animals)
or artificial sources (eg. light
bulbs, TV screens). Here are a
few types of artificial light
sources.
1. Incandescent Light
•
Traditional light bulbs which
use a thin metal wire to
conduct electricity. The
electrical current causes the
wire to glow white hot
producing light. An advantage
of these bulbs is that they’re
cheap, a disadvantage is that
they are very inefficient
losing 90% of its energy to
heat.
2. Fluorescent Light
•
Fluorescent Light bulbs. An electrical
current is run into a tube full of mercury
vapor. The mercury atoms become
excited and produce UV radiation. The
UV radiation reaches phosphor powder
on the tube which also gets
energized/excited and releases light
energy that we see. An advantage of
fluorescent lights is that they are much
more efficient than incandescent lights.
A disadvantage is that they contain
poisonous mercury vapor.
3. Phosphorescent Light
Sources
•
Glow in the dark
toys/stickers/tools. Similar to
fluorescence except that light
energy is stored over time and
then released in the dark
after the light source has
been removed. This process
of gradual release of light is
called phosphorescence.
4. Chemiluminscent
Sources
• Glow Sticks. Chemical reactions
produce energy in the form of
light. So stored chemical energy
produces light energy. In a glow
stick a barrier separates the
two reactants. When the barrier
is broken the reactants mix
producing the light energy.
5. Bioluminescent
Sources
• Deep Sea Creatures.
Biochemical reactions in
certain living things produce
light energy. This is especially
helpful in areas where there is
no light from the sun such as
the ocean floor.
III. The Cost of Lighting
• Light energy in Alberta is provided by
your local power company (Epcor,
Enmax) and is part of your power bill.
The formula to find out how much the
light will cost is:
• Cost (in ¢ or $) = Power of bulb (in kW) * Time being used (in
h) * Price of Power (in ¢ or $)
IV. Transparent,
Translucent, and Opaque
Objects
• Transparent objects allow light rays
to pass through without getting bent.
You see a clear image across a
transparent object.
• Translucent objects allow only some
light rays to pass through and most
of these are bent. You can see light,
but you can’t see a clear image.
• Opaque objects absorb or reflect all
light rays allowing none of them to
pass through. You see no light or
images.