2.Light Energy and Its Sources

Light Energy and Its
Sources
308-8 identify and describe properties of visible
light
What is light?
Light is a form of energy that can be
detected by the human eye. Light cannot
be touched or tasted, but it can warm
things up so it gives heat energy to
objects.
Energy
Energy is the ability to do work. Energy can
be neither created nor destroyed.
Energy can only be transformed from one form to
another.
Where does light energy
come from?
Light energy comes form two different sources, natural and
artificial.
What is the most important
source for natural light?
The sun is the most important source for natural
light.
• Objects that emit (give off) their own
light are called luminous.
• Objects that do not emit light, but
reflect it from other sources are
called nonluminous.
What are some examples
of luminous objects?
Luminous objects are objects that give off their own
light.
What are some examples
of nonluminous objects?
Nonluminous objects are objects that do not give off light.
Why is the Moon
nonluminous?
The Moon is nonluminous because it reflects light from the Sun
What are some common
forms of luminous energy?
Chemical energy
Electrical energy
Nuclear energy
Thermal energy
Sources of Light
1. Light from Incandescence –
Incandescent source means the
process of giving off light because of
the high temperature. Light
produced by a flame, candle,
campfire, or light bulb.
2. Light from Phosphorescence –
Some materials, called phosphors,
give off light for a short time after
you shine a light on them. They
store the energy and then release it
as light energy.
3. Light from Electric Discharge – An
electric discharge is the process by
which a gas releases light because of
electricity passing through it.
4. Light from Fluorescence –
The release of light from a
substance caused by
exposure to radiation from
an external source.
Where are fluorescent lights
used?
Schools
Offices
Homes
What happens inside a
fluorescent light?
In fluorescent lighting, the tube glows when ultraviolet
light energy strikes the phosphors lining the tube.
The tube stops emitting light immediately after the electric
current is turned off.
Phosphorescent light sources continue to emit light for much
longer period after receiving light energy.
What is a chemical
reaction?
When two chemicals react and a new compound is formed.
When substances are mixed and new substances are formed, a
chemical reaction has occurred.
Change that takes place when two or more substances
(reactants) interact to form new substances (products).
5. Light from Chemiluminescence –
Chemiluminescene is the process of
changing chemical energy into light
energy with little or no change in
temperature.
6. Light from Bioluminescence –
some living things, such as fish, can
make themselves luminous using a
chemical reaction like
chemiluminescence.
Give examples of
bioluminescence.
Firefly
Zebra Fish
Hawaiian bobtail squid
Glow
worm
Bacteria
Fungi (mushrooms)
Daytime
Nighttime
Properties of Light
• Light travels in a straight line (rectilinear
propagation)
• Light travels at a speed of 300 000 km/s
in air
• Reflection
• Refraction and dispersion
• Light travels in a vacuum and in some
other types of media
* We will explore these properties in more
detail throughout this unit.
Understanding Concepts
• Questions
• Page 271
• 1, 2, 3, 4
Which of the following are
luminous?
A. Campfire
B. The Moon
C. A hot toaster filament
The campfire and hot toaster are
luminous.
WHY?
For, luminous objects are objects that
give off their own light.
Describe how a flashlight can be luminous.
A flashlight becomes luminous when it is switched
on, allowing the chemical energy in the battery to
become electrical energy, which in turn becomes
light energy (heat) in the bulb.
Describe how it can also be
nonluminous.
The flashlight can be nonluminous when it is turned off.
Investigation
• On your own, you are going to investigate
“fuzzy” and “sharp” shadows.
• What are they? Show some examples
(draw or print pictures 2 pictures for each)
• What causes one to be sharp and the
other to be fuzzy?
• I am just looking for a little write-up about
each to be passed in for evaluation.
Light Box
• You are going to make a light box to
see how light travels in a straight
line.
• See instruction sheet.