Light and Color
Light as a Wave
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Light is also known as electromagnetic radiation.
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Gamma rays, X-rays, Ultraviolet, Infrared,
Microwaves, Radio and TV waves
All of these have different wavelengths.
A form of radiant energy.
Light and Energy
Not the same as radioactive.
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All the light we see (and the light we can’t see) travels
as waves.
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What else travels as waves?
The energy of light is proportional to its frequency.
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The higher the frequency the higher the energy.
Frequency and wavelength are inversely proportional.
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Therefore the higher the wavelength the lower the
energy.
Properties of Waves
All waves have:
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Atoms and Color
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wavelength (, “lambda”).
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The distance between 2 crests of a wave.
frequency (, “nu”).
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The # of waves that oscillate per second.
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This is true in a vacuum (space).
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c = 186,000 miles per second.
c = 300 million meters per second.
• 3 x 108 m/s
It slows down a little when passing through
material like air or water.
The equation that relates the three is:
c=•
When one goes up the other goes down.
Wavelengths: ~ 400 to 800 nanometers.
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Hot coals and metal glow.
Electricity:
The filament of a light bulb glows.
The gas inside a neon light glows.
Atomic Emission Spectra
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Each element emits (gives off) its own, individual set
of colors when energized.
This is called its atomic emission spectrum.
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The colors show up as a series of lines when
viewed through a prism.
Like a fingerprint for that element.
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Used in astronomy to determine the composition of
stars.
• Around 60 elements have been identified in our
sun.
Absorption of Light
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There are 109 nanometers in a meter.
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Each wavelength corresponds to a different color of
light.
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Red light: ~ 620 - 770 nm
Violet light: ~ 390 – 450 nm
White light is a mixture of all the colors of the rainbow.
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A prism can split white light into its colors.
Electromagnetic Radiation (EMR) Spectrum
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Heat:
This is called an inverse relationship.
Visible Light: The Light our Eyes Can Detect
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Light is given off by atoms subjected to various forms
of energy.
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So if c is a constant in the equation above, when
wavelength is high, frequency is low and vice-versa.
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The speed of light is a constant
Wavelength, Frequency & Speed
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speed (c, for light only).
The Speed of Light
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Which light is higher in energy: blue or red?
Visible light is only one type of EMR.
Other types include:
Atoms can also absorb light shined on them.
Any light not absorbed by the atoms in an object is
scattered (reflected) back.
If only some colors are absorbed, the rest are scattered
back.
We see the colors scattered back.
* White objects don’t absorb any light, all colors
reflected.
* Black objects absorb most colors, little light
reflected.
* Transparent objects let most light pass through
without being absorbed or scattered.
Example
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Let’s say you are wearing blue jeans.
The dye in the jeans is absorbing most of the visible
light, but not blue light.
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White light is composed of all the colors.
When white light hits the jeans, the blue light is
reflected back.
The blue part of the white light hits our eyes and we see
blue.
Photons
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Light is comprised of particles called photons.
Like “little packets” of light.
Photons have no mass.
A photons energy is proportional to the frequency of
the light.
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Remember: each color of light has a different
energy.
They interact with the subatomic particles of atoms.
Review: What’s Inside the Atom?
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Atoms are not indivisible!
There are three types of “subatomic” particles inside
each atom:
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Neutron
Electron
Protons and neutrons are much heavier than electrons
(1800x heavier).
Electrical charge:
* Protons “positive” +
* Electrons “negative” –
* Neutrons “neutral”
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Electrons “fly around” outside the nucleus in a cloudlike area.
The electrons of an atom have different energies.
Not all energies exist, only certain allowed energy
levels.
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They are termed discrete energy levels.
These levels are numbered: 1, 2, 3, 4....
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The energy levels are further divided into sublevels
and orbitals.
Electrons with more energy are able to get farther away
from the nucleus and its + charges.
Therefore, electrons in higher energy levels spend more
time farther away from the nucleus.
The higher energy levels are larger so they can hold
more electrons.
What’s Going on Inside the Atom when it Interacts with
Light?
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Remember: Electrons can be found in discrete energy
levels.
Atoms emitting or absorbing light have electrons
undergoing transitions between these levels.
If an atom takes in photons: absorption.
If an atom gives off photons: emission.
Absorption
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A photon of light hits an atom.
If the energy of the photon is “right”, it is absorbed
by the atom.
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An electron in the atom is “promoted” to a higher
energy level.
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The energy of the photon must be exactly equal
to the difference of the electron energy levels.
Photons that aren’t absorbed are scattered (or
transmitted).
Emission
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Protons and neutrons are located in the center of the
atom in a densely packed “nucleus”.
Electron Energy Levels
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Proton
Protons, Neutrons and Electrons
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Location of Electrons
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An electron in an atom is excited to a higher energy
level by an outside energy.
* Heat, electricity, etc.
The electron returns to a lower energy level.
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A photon of light is emitted.
The energy of the photon is equal to the
difference between the energy levels of the
electron.
Since each element has a unique set of electron
energy levels, each element emits a unique emission
“signature” set of lines.