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Let there be ...
Light
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The Nature of Light
What is Light?
By the 17th century,
light had been observed to…
1. travel in straight lines
2. reflect
3. refract
4. transmit energy from
one place to another
Two theories could explain
these phenomena.
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The WAVE THEORY,
advocated by
Christian Huygens
and Robert Hooke, said
that light was a wave.
The PARTICLE THEORY, advocated by Isaac Newton
and later by Pierre Laplace, said that light was made
up of a stream
of
tiny particles.
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Newton’s particle theory could easily explain the
straight line travel of light, reflection, and energy
transmission, but had trouble explaining refraction.
Newton’s explanation of refraction required that
light must travel faster in water than in air.
Huygen’s wave theory could easily explain reflection,
energy transmission, and refraction, but had difficulty
explaining the straight line travel of light.
The wave theory’s explanation of refraction
required that light must travel
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slower in water than in air.
The debate among the two sides continued
through the mid 1800’s.
1801 -interference of light was discovered
1816 - diffraction of light (actually observed
in the 1600’s but not given much significance)
was explained using interference principles
Neither phenomenon could be explained
satisfactorily by the particle theory.
The final blow to the particle
theory came in 1850 when
Jean Foucault discovered that
light traveled faster in air
than in water.
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It was then widely accepted that light was a wave,
but what kind of wave?
In 1865, James Maxwell developed
ideas begun by Michael Faraday
into a series of equations that
proposed the electromagnetic wave
theory. It said that light was a type of
E/M wave: a periodic disturbance involving
electric and magnetic forces.
In 1885, Heinrich Hertz
experimentally confirmed
the E/M theory.
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At the end of the century, many physicists felt that
all the significant laws of physics had been discovered.
Hertz even stated, “The wave theory of light is, from
the point of view of human beings, a certainty.”
That view was soon to change.
Around 1900,
the photoelectric effect was observed.
“the emission of electrons by a substance
when illuminated by E/M radiation”
Careful study of the photoelectric effect
was performed by many scientists.
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PHOTOELECTRIC EFFECT
•The ejection of electrons from certain metals
when exposed to high enough frequencies of light.
•It reinforced the particle theory of light.
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PHOTOELECTRIC EFFECT
• The light has to be at a certain frequency(threshold) where
each individual photon has a certain amount of energy.
• Brightness or Intensity is determined by the number of
photons in the light beam
• If the light is at the correct frequency and is brighter the
metal will emit more electrons vs. a light that is dimmer
which would emit fewer electrons.
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The wave theory could not totally explain the
photoelectric effect, but a variation of the
old particle theory could!
Max Planck and
Albert Einstein
subsequently proposed the
QUANTUM THEORY.
The Quantum Theory
The transfer of energy between
light radiation and matter occurs in
discrete units called quanta, the magnitude
of which depends on the frequency of radiation.
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Although we still commonly characterize
light as a wave, it is actually neither a
wave nor a particle. It seems to have
characteristics of both.
The modern view of the nature of
light recognizes the dual character:
Light is radiant energy transported
in photons that are guided along
their path by a wave field.
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This leads us to the
Duality Principle:
Light is ...
• a wave when it acts like a wave
• a particle when it acts like a particle
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Microwaves
Radio
Larger Waves
Visible
Infrared
X-Ray
Ultraviolet
Gamma
Smaller Waves
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Do you use the radio, television, play video games
or heat food with a microwave?
If you answered YES – then you already use
electromagnetic waves every time you do one of
these activities!
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1.
Produced by the motion of electrically charged
particles (photons) and magnetic field.
2.
Arranged according to their wavelength and
frequency on the Electromagnetic Spectrum.
3.
Do NOT need a medium to transfer energy – often
travel through a vacuum. (no air)
-Example: Light from the stars
-Satellites sending to TV’s radios
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Red-Orange-Yellow-Green-Blue-Indigo-Violet
Wavelength is inversely proportional to frequency and energy
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1.
Radio Waves •
•
•
•
Longest Wavelengths & Lowest Frequency
Have lowest photon energy
Modulation-change radio wave frequency
Used in TV, Cell Phones, MRI, Radar-airports, weather
• Higher frequency radio waves are called
Microwaves and are used in communication.
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Question:
Why do you think they make passengers in airplanes
and visitors in hospitals turn off their cell phones?
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2.
Infrared Radiation • Has a wavelength () slightly longer than Visible
Light on the Electromagnetic Spectrum.
• Felt as Heat
• Examples:
 Security Monitoring Systems, night vision googles
 Some Restaurants to keep food warm
 Auto paint shops to dry car finishes
 Thermal imaging
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3.
Visible Light • Only part on the Electromagnetic Spectrum
that you can see.
• Causes CHEMICAL reactions, like:
 Photosynthesis – plants to make own food
 In your eyes – allows you to see
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Colors of Visible Light
From the LONGEST wavelength and the Lowest frequency to
the SHORTEST wavelength to the HIGHEST frequency the
colors of visible light are:
RED-ORANGE-YELLOW-GREEN-BLUE-INDIGO-VIOLET
You can remember this by remembering…
ROY G. BIV
…the colors of the rainbow.
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4.
Ultraviolet Radiation • Exposure to UV rays (the sun or artificial
sunlight) enables the skin cells to produce
Vitamin D – that is needed for healthy bones
and teeth.
• Overexposure to UV rays kills healthy cells
and may cause skin cancer.
• Used in hospitals to kill bacteria and sterilize
surgical instruments, tanning beds, SUN
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Question:
Why do you think that it is important to wear sunscreen
if going outside – even on a cloudy day?
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5.
X-Rays • Carry high energy and have greater penetrating
power that travels through skin and muscles.
• When x-rays hit dense material – like a bone –
then the wave is absorbed and the bones appear
brighter.
• Doctors, dentists, airport scanners, hospitals
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Questions:
Have you ever gotten x-rays taken at the dentist or
doctor’s office?
Ever wonder why they put that jacket on you and then
leave of the room?
Are x-rays safe?
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6.
Gamma Rays • Shortest Wavelengths & Highest Frequency
• Emitted from the nuclei of radioactive atoms
• Concentrated rays are destructive and are
used to kill cancer cells.
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• Albert Einstein – in 1905 hypothesized that
light is composed of tiny energy particles
• He named these particles – PHOTONS.
These are different from atoms in matter.
• The highest energy photons emit the highest
frequency and can damage matter & living
cells.
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Light Qualities
•
•
•
•
Carries heat and warmth
Has color
Can be bright or dim
Travels almost unimaginably fast and far, 300
million meters per second
• Travels in a straight line, but can be bent by
lenses or reflected by mirrors
• 20% of U.S. electricity use is in lighting.
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1. Incandescent Light
• Tungsten (W)
• a thin wire filament is
heated to white hot and
emits light.
• Uses: toaster, light
bulbs, incubator
• 80% more heat than
light.
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2. Fluorescent Light
• Phosphorus (P)
• Bulb filled with UV gas
absorbs UV rays and
then gives off light
• Under low pressure
• School and office lighting
• 4X as much light as
incandescent
• Less heat given off
• More efficient
• Last longer
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3. Neon Light
• Neon (Ne)
• Electricity passes through
a tube, excites Neon and
causes it to emit light.
• Advertisement, business
signs.
• The color of the light
depends upon the gas and
the coating of the tube.
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The type of matter
determines the amount of
light it absorbs or reflects.
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What matter does with light?
1. Absorb:
if the
light falling on the
material is of
“natural
frequency”
2. Re-emit: after
absorption it can
then release light
(glow-in-the-dark)
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What matter does with light?Continued
3. Reflect:
If the
light is not of
“natural
frequency”
4. Transmit: Allows
the light to pass
through the material
`
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•
Opaque –
• does NOT transmit light
• only reflects or absorbs
light
• completely blocks out light
• example: wall and wood
•
Translucent –
• transmits some light
• mostly reflect or absorbs light
• examples: frosted or stained glass
• Transparent –
• mostly transmits light
• reflects very little light
• example: windows
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The color we see objects depends on:
1. The kind of light emitted from the object.
2. The kind of light transmitted by an object.
3. The kind of light reflected by the object.
4. The kind of light shown on an object.
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The 3 primary colors of light:
• Red, Green, and Blue
• Red, green and blue
combine to make white
light
• All other colors of light are
made by combining these
in different ratios.
• Additive colors
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The 3 primary pigments are:
• Magenta, Yellow, and Cyan
• Magenta, Yellow and Cyan
combine to make BLACK
pigment
• Used in paints and crayons
• Subtractive colors
Magenta
yellow
cyan
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• The color of an object depends on both
the pigment present and the light shown
on it.
• Example: White shirt with red light shown
on it will appear to be red, because there
is only red light to reflect.
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How do we see color?
• Retina in the eye collects light, chemical reaction
changes to nerve impulse and transmitted to brain
• Photoreceptors
– Rod cells- respond to brightness of light, white,
shades of gray or black
– Cone cells- respond to color, red, green and blue
– Our brain sees combinations of red, green and blue
to make all the shades of colors we see
• Colorblindness- most common defected red cones
and sometimes green.
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Photoluminescence
Materials that absorb light energy, then gives light off later
– Phosphorus atoms embedded in plastic absorb light
energy and slowly release the light in a glow-in-the-dark
process.
– The phosphorus atoms absorb energy as its electrons
move from a low energy level to a high energy level.
– When the electrons fall back to the low energy level the
glow is given off.
eP&N
absorbs light and gains energy
to move out an energy level
releases energy and falls back
in and gives off light
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Polarization
Vertical Polarization
(allows only vertical light through)
Horizontal Polarization
(allows only horizontal light through)
Vertical and Horizontal Polarization
(cuts out most light)
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• LASER
– Light Amplification by
Stimulated Emission of
Radiation
– a single frequency of light
• Fiber Optics
– Light pipes that use total
internal reflection to carry
light and signals from one
point to another
– Example-telephone calls,
television signals, computer
data
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• Optical Illusionerroneous perception
of reality
• Diffraction Gratingsmall slits in a material
that when looked
through breaks white
light into the
spectrum-ROYGBIV
• Filter-a transparent
material that transmits
one or more colors of
light but absorbs all
others
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Two color models
• RGB model– The primary colors used to construct an image on a color
TV monitor
– Tiny RBG dots separated by black lines
– All devices that make their own light use RGB model
• CMYK model– Color-making process based upon subtraction of color
– Reflection of color rather than its transmission
– Printing inks, fabric dyes, even color of your skin
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• Optics-the study of how light behaves
– Optic devices-mirror, lens, prism, fiber optics
• Object-many real physical things that give
off or reflect light rays
• Image
– collection and use of light
– a picture of objects that are formed in places
where light rays from the object meet
• Focus-(focal point) the place where all light
rays from the object meet to form the image
• Lens-type of optical device used to bend
light
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Angle of incidence = Angle of reflection
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1. Plane mirror
• flat surface
• virtual image
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2. Concave mirror
• curves inward to form a focal point
• Light rays CONVERGE – come together
• Examples: telescopes, searchlights,
flashlights
>
>
>
•Focal point -place where rays of light
cross after they have been bent
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3. Convex mirror
• curves outward and forms
no focal point
• Light rays DIVERGE –
Spread out
• Examples: rear-view
mirrors, store security
mirrors
>
>
>
>
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What is refraction?
Refraction is the bending of light.
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Refraction
• Caused by a change in the waves speed
when it moves from one material to another
• Glass, plastic, water
• Amount of refraction depends on the
wavelength of the wave
• Short wavelengths bend more
• Long wavelengths bend less
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Refraction concluded
• Index of refraction-ratio of
speed of light reduced when it
passes through a material
• Higher index-more bending
• Prism-polished triangle shape of
glass that bends different
frequencies of light into
separate colors
• Rainbow are produced by
raindrops refracting sunlight.
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1. Plane lens
• flat surface
• Rays go straight
through
• Examples: Regular
window and
microscope slide
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2. Concave Lens
• Thinner in center than
at the edges
• Light rays DIVERGE –
spread apart
• Used to correct
nearsightedness
• Examples: glasses
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3. Convex Lens
• Thicker in the center than
at the edges
• Light rays CONVERGE –
come together
• Used to correct
farsightedness
• Examples: magnifying
glass and microscope
lens
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