Transcript what is light? - Fort Thomas Independent Schools

LIGHT
WHAT IS LIGHT?
 All
frequencies or wavelengths of
electromagnetic radiation
 Seven bands of electromagnetic waves,
based upon differences in frequency and
wavelength.
 A continuous spectrum
 A form of energy.
WHAT CAUSES ELECTROMAGNETIC
WAVES (OR LIGHT)?
Vibrating electrically charged particles
 A changing electric field sets up a changing
magnetic field, which sets up a changing electric
field, and so on.
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Electromagnetic wave (a
combination of electric
and magnetic fields
ELECTROMAGNETIC WAVE
http://www.nrc-cnrc.gc.ca/obj/inmsienm/images/research_images/optical_comb/COMBFIG1.gif
Electromagnetic Waves
Light is a portion of the family of electromagnetic waves that
includes radio waves, microwaves, and X-rays.
The range of electromagnetic waves is called the electromagnetic
spectrum.
The Electromagnetic Spectrum
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Non-mechanical waves are produced by vibrating electric charges (changing
electric field produces a changing magnetic field and vice-versa)
Spectrum means a continuous series of wavelengths and frequencies
Divided into 7 characteristic portions-differ by wavelength
As frequency increases, wavelength decreases
Light shows a dual nature
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Modeled as Wave (reflects, refracts, diffracts, interferes)
Modeled as Particles: a stream of photons
As the frequency increases, so does the energy of the photon
All objects emit electromagnetic radiation (higher the temperature, the higher the
frequency of the electromagnetic spectrum)
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http://phet.colorado.edu/en/simulation/photoelectr
ic
Visible Light
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Smallest portion of electromagnetic spectrum
Different frequencies represent different colors
White light contains all frequencies of visible light (all colors)
ROYGBV—from longest to shortest wavelength or lowest to highest frequency, red light has
lower frequency and is less energetic than violet light
White light is the presence of all color (reflection); black light is the absence of all color
(absorption).
The color we see represents only those frequencies of visible light that are reflected by the
object
Primary colors of light (Red + Blue + Green = White)
Red + Blue (magenta) Blue + Green (cyan) Red + Green (yellow)
Photoelectric effect: visible light photons cause electrons to move in solar cells, producing
electricity.
The cone cells in the retina are light-sensitive and respond to the different frequencies of
color
The pupil will change size to adjust for intensity (brightness) of light
Graphical Representation of Wavelength
and Light
HOW DO WE MODEL LIGHT?
TWO
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MODELS
Wave Model
Particle Model
 Physicists
recognize the dual nature of
light (light travels like a wave, but hits
like a particle).
 Photon: a massless particle of light
(carries energy—greater the frequency,
the greater the photon energy)
LIGHT AS A WAVE AND A PARTICLE
Light behaves like a wave because light reflects,
refracts, diffracts and interferes.
 Light also behaves like a particle, traveling in a
stream photons.
 Photons are discrete packets of light energy.
 Evidence for the particle model is the
photoelectric effect.
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ENERGY OF A PHOTON
Photon energy is proportional to the frequency of
light. The higher the frequency, the higher
the photon energy.
 For example, radio wave photons carry less
energy than do ultraviolet light photons.
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Light and Transparent Materials
A light wave incident upon a pane of glass sets up vibrations in
the atoms. Because of the time delay between absorptions and
reemissions, the average speed of light in glass is less than c.
INTENSITY OF LIGHT
The amount of light illuminating a surface or
passing through space is called the intensity
(brightness).
 Intensity relates to the number of photons, not
the energy of a photon.
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INTENSITY OF LIGHT
An increase in the number of waves or photons
(or the amount of light)
 Brightness of light means intensity.
 Not related to frequency or the energy per
photon!
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Same frequencies
of light, just
different
intensities!
100 Watts
40 Watts
SPEED OF LIGHT
Nothing in the Universe travels faster than the
speed of light.
 Speed
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3 x 108 m/s
 300,000 km/s
 186,000 mi/s
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WHAT AFFECTS THE SPEED OF
LIGHT?
Speed of light is affected by the type of medium it
travels through.
 In general, light travels fastest through a
vacuum, then gases, liquids and solids.
 This is opposite of mechanical waves. Sound
waves travel fastest through solids, not gases.
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PHOTOELECTRIC EFFECT: THE EJECTION OF
ELECTRONS FROM CERTAIN METALS WHEN EXPOSED
TO CERTAIN FREQUENCIES OF LIGHT.
Electromagnetic radiation
(visible light range)
Electrons emitted
Photovolatic cell
(solar cell)
Produces electron flow (electricity)
ENERGY TRANSFORMATIONS OBSERVED WITH
PHOTOVOLTAIC CELL, MOTOR AND PROPELLER
Visible light absorbed by photvoltaic cell (light
energy to electrical energy)
 Electric motor causes propeller to spin (electrical
energy to mechanical energy)
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WHAT IS LIGHT?
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All forms of electromagnetic radiation
http://www.colorado.edu/physics/2000/waves_particles/
http://didaktik.physik.uniwuerzburg.de/~pkrahmer/ntnujava/emWave/emWave.html
http://imagers.gsfc.nasa.gov/ems/waves3.html
http://ihome.cuhk.edu.hk/~s016969/physproj/index.htm
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectr
um.html