1. Light Energy
What is light?
How does light travel?
How is light reflected, refracted, and absorbed?
How does light determine how we see the color of an object?

2. What is light?
Light is a form of energy that can be released by an atom. It is made up of many small
particles called light photons.
Atoms release light photons when their electrons become excited. Electrons are
particles that move around an atom's center or nucleus. Electrons move in different
paths around the nucleus. When something happens to an atom to add energy to it,
like it gets heated, the electrons may jump to different paths. Soon they fall back to
their original path. When they return to their original path, they may release energy in
the form of photons of light .

3. How does light travel?
Light photons travel in waves made up of electrical and magnetic energy. But, light
waves can travel through space where there is no material. Light photons travel very
rapidly: 186,000 miles per second.

4. How does light travel?
Electromagnetic waves include more than visible light. They also include cosmic
rays, gamma rays, radio waves, and other forms of electromagnetic radiation. We
only see some of this energy as visible light.

5. How do we see light?
Any light that you see is made up of a collection of one or more photons traveling
through space as electromagnetic waves. In total darkness, our eyes are actually able
to sense single photons, but generally the light we see in our daily lives comes to us in
the form of zillions of photons produced by light sources like the Sun or a flashlight.
If you look around you right now, there is probably a light source in the room producing
photons, and objects in the room that reflect those photons. Your eyes absorb some of
the photons flowing through the room, and that is how you see.

6. How is light reflected?
Reflection is when incoming light hits an object and
bounces off. Very smooth surfaces such as mirrors
reflect almost all incoming light.
What you see in the mirror is reflected light. We now know this can’t happen!

7. How is light refracted?
Refraction is when light waves change direction as
they pass from one material to another. Light travels
at different speeds as it passes through different
materials. Light travels slower in air than in a vacuum,
and even slower in water. The change in speed bends
the light.
Different wavelengths of light are slowed at different
rates, which causes them to bend at different angles.
For example, when the full spectrum of visible light
travels through the glass of a prism or water droplets
in air, the wavelengths are separated into the colors
of the rainbow.

8. How is light absorbed?
Sometimes when light hits an object, the atoms and
molecules in that object start to vibrate as they absorb
the light.
When a dark surface like a black asphalt road
absorbs light, it can get hot enough to cook an egg!
Light energy has turned into heat energy that cooks
the egg.
Darker colored materials absorb more light than
lighter colored materials. The same egg might not
cook on a light grey cement sidewalk!

9. How is light reflected, refracted, and absorbed?

10. Why is the sky blue?
The sky appears blue because of the scattering of light. Light at shorter wavelengths—
blue and violet—is scattered by nitrogen and oxygen as it passes through the
atmosphere. Longer wavelengths of light—red and yellow—transmit through the
atmosphere.
This scattering of light at shorter wavelengths illuminates the skies with light from the
blue and violet end of the visible spectrum. Even though violet is scattered more than
blue, the sky looks blue to us because our eyes are more sensitive to blue light.
The same thing happens in the ocean’s water, so it also appears blue.

11. How does light determine how we see the color of an object?
Color refers to the different wavelengths of light in the visible light spectrum that our
eyes can see.
When we look with our eyes at an object, we are really seeing the wavelengths of light
that are reflected by the object. The object absorbs the other wavelengths.
The physical and chemical composition of matter in the object determines which
wavelength (or color) is reflected.

12. How does light determine how we see the color of an object?
We see objects of a certain color because they reflect part of the light spectrum and
absorb the rest. So, the apple leaf appear green because it absorbs all the colors
except green. The apple skin appears red because it absorbs all the colors except red.
The color we see is the color reflected by the object.

13. How does light determine how we see the color of an object?
Color originates in light. Sunlight, as we perceive it, is colorless. In reality, a rainbow is
testimony to the fact that all the colors of the spectrum are present in white light.
When white light is bent or reflected, as through a prism, it can separate into the
different colors of the spectrum.
A rainbow is white light that is reflected and bent by water droplets in the air, causing
the colors of the light to separate into a range of red, orange, yellow, green, blue, and
indigo/violet.

14. How does light determine how we see the color of an object?
A transparent material,
such as glass, allows light to
pass through it very well.
A translucent material
absorbs some of the light.
An opaque material allows
no light to pass through.

15. How does light cause heat?
When the electromagnetic radiation from the sun
encounters a material, it causes the atoms in the
material to vibrate, generating heat and causing the
object to release thermal energy. Vibrating atoms
hit other atoms and heat is moved through a
material.
In heated liquids and gases, convection causes
the hotter material to move upward and the colder
material to sink.
In heated solids, conduction between the closely
vibrating atoms moves heat through the material
evenly. So, don’t use a metal spoon to stir hot
soup!

16. How is heat
measured?
Heat is measured in
degrees, using the
Celsius, Fahrenheit,
or another type of
scale.
The movement of heat
through a material
depends on the source
of heat and the type of
material. Some
materials, like metals,
move heat well by
conduction. Wood
does not conduct heat
well.

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