Light, Speed, and Frequency
Light
• Light is a transverse, electromagnetic wave that can be seen
by humans.
• The wave nature of light was first illustrated through
experiments on diffraction and interference.
• Like all electromagnetic waves, light can travel through a
vacuum.
• The transverse nature of light can be demonstrated through
polarization.
• Light is sometimes also known as visible light to contrast it
from "ultraviolet light" and "infrared light".
• Other forms of electromagnetic radiation that are not visible
to humans are sometimes also known informally as "light"
Light is produced by one of two
methods.
• Incandescence is the emission of light
from "hot" matter.
• Luminescence is the emission of light
when bound electrons fall to lower energy
levels.
Speed of Light
The speed of light depends upon the medium through which it
travels.
• The speed of light in a vacuum is a universal constant in all
reference frames.
• All electromagnetic waves propagate at the speed of light in
a vacuum.
• The speed of light in a medium is always slower the speed of
light
in
a
vacuum.
(The difference is usually negligible when the medium is air.)
• The speed of anything with mass is always less than the
speed
of
light
in
a
vacuum.
(The speed of light in a vacuum is the universal speed limit.)
• The speed of light in a vacuum is fixed at 299,792,458 m/s by
the current definition of the meter.
Light cont.
• The amplitude of a light wave is related to its
intensity.
• Intensity is the absolute measure of a light
wave's power density.
• Brightness is the relative intensity as
perceived by the average human eye.
Color and Light
• The frequency of a light wave is related to its color.
• Color is a function of the human visual system, and is not an
intrinsic property. Objects don't "have" color, they give off
light that "appears" to be a color. Spectral power
distributions exist in the physical world, but color exists only
in the mind of the beholder.
• Monochromatic light — that is, light of a single frequency
• The color of the light coming from an object has its origin in
one or more of the following processes …
• Emission: the object itself is a source of light with a color
determined by its spectra
Color and Light cont.
• Reflection: certain frequencies are reflected from the object
while others are not
• Transmission: certain frequencies are transmitted through
the object while others are not
• Interference: certain frequencies are amplified by
constructive interference while others are attenuated by
destructive interference
• Dispersion: the angular separation of a polychromatic light
wave by frequency during refraction
• Scattering: the preferential reradiation of certain frequencies
of light striking small, dispersed particles
Frequency of a light wave
• Monochromatic light is described by only one frequency.
• Laser light is effectively monochromatic.
• There are six simple, named colors in English (and many
other languages) each associated with a band of
monochromatic light. In order of increasing frequency they
are red, orange, yellow, green, blue, and violet.
• Light is sometimes also known as visible light to contrast it
from "ultraviolet light" and "infrared light"
• Other forms of electromagnetic radiation that are not visible
to humans are sometimes also known informally as "light"
• Polychromatic light is described by many different
frequencies.
• Nearly every light source is polychromatic.
• White light is polychromatic.
Wavelength of a light
• The wavelength of a light wave is inversely proportional to
its frequency.
• Light is often described by it's wavelength in a vacuum.
• Light ranges in wavelength from 400 nm on the violet end to
700 nm on the red end of the visible spectrum.
• Phase differences between light waves can produce visible
interference
effects.
(There are several sections in this book on interference
phenomena and light.)
Light Spectrum
• A graph of relative intensity vs. frequency is called a
spectrum
(plural:
spectra).
Although frequently associated with light, the term can be
applied to any wave phenomena.
• A continuous spectrum is one in which every frequency is
present within some range.
• Blackbody radiators emit a continuous spectrum.
• A discrete spectrum is one in which only a well defined set of
isolated
frequencies
are
present.
(A discrete spectrum is a finite collection of monochromatic
light waves.)
• The excited electrons in a gas emit a discrete spectrum.
Visible Light Spectrum
• Visible light region - the very narrow band of wavelengths located
to the right of the infrared region and to the left of the ultraviolet
region. Though electromagnetic waves exist in a vast range of
wavelengths, our eyes are sensitive to only a very narrow band.
Since this narrow band of wavelengths is the means by which
humans see, we refer to it as the visible light spectrum.
• This visible light region consists of a spectrum of wavelengths
which range from approximately 700 nanometers (abbreviated
nm) to approximately 400 nm. Expressed in more familiar units,
the range of wavelengths extends from 7 x 10-7 meter to 4 x 10-7
meter. This narrow band of visible light is affectionately known as
ROYGBIV.
• Each individual wavelength within the spectrum of visible light
wavelengths is representative of a particular color
Visible Light Spectrum cont.
• Each color is characteristic of a distinct wavelength; and
different wavelengths of light waves will bend varying
amounts upon passage through a prism.
• For these reasons, visible light is dispersed upon passage
through a prism.
• Dispersion of visible light produces the colors red (R), orange
(O), yellow (Y), green (G), blue (B), and violet (V). It is
because of this that visible light is sometimes referred to as
ROY G. BIV. (Incidentally, the indigo is not actually observed
in the spectrum but is traditionally added to the list so that
there is a vowel in Roy's last name.) The red wavelengths of
light are the longer wavelengths and the violet wavelengths
of light are the shorter wavelengths. Between red and violet,
there is a continuous range or spectrum of wavelengths.
Test
Fill in the blank.
Monochromatic light
Color
Intensity
Incandescence
•
•
•
•
•
•
Visible Light
Luminescence
Spectrum
Frequency
Light is also know as_________.
The amplitude of a light wave is related to its __________.
A graph of relative intensity vs. frequency is called a ________(plural: spectra).
The frequency of a light wave is related to its_________.
_____________ that is, light of a single frequency.
Light is produced in these two ways___________ and ______________.
Answer Key
6.
1.
Visible Light
2.
Intensity
3.
Spectrum
4.
Color
5.
Monochromatic light
Incandescence Luminescence
Bibliography
• http://www.physicsclassroom.com/Class/light
/u12l2a.cfm
• http://galileoandeinstein.physics.virginia.edu/l
ectures/spedlite.html
• http://www.desy.de/user/projects/Physics/Rel
ativity/SpeedOfLight/measure_c.html
Light, Speed, And Frequency
Dylan Rooney
Kristaps Skrodelis
What is Light?
• Light has a dual nature: It is made up of
photons which are packets(quanta) of energy
that have both particle and wave properties
• Light can be used as energy: Solar Power,
Photosynthesis, fireplace, etc…
Light
•
•
•
•
Light travels in straight lines.
Properties of light: color, reflection, refraction.
When light is blocked a shadow is formed.
What we call light is a small portion of the
electromagnetic spectrum
• All the different colors are electromagnetic
waves with different wave lengths
What happens when light
hits these objects?
•
•
•
•
•
•
•
•
•
Glass of water
School bus window
Notebook paper
Waxed paper
Plastic wrap
Tissue paper
Cardboard
Textbook
Hand lens…
Waves
• The waves have high points called crests.
• Waves also have low points called troughs.
• The distance from one crest to the next crest
is called a wavelength.
• The number of waves passing a given point in
one second is called the Frequency
• Wave speed = frequency x wavelengthwavelength
Frequency
• Frequency is the number of wave crests which
pass a point per second.
Color
• Lowest frequencies of visible light are red
• Highest frequencies are violet
• The order is red, orange, yellow, green, blue,
violet
Speed
Yxf=c
Y = wavelength , f = frequency
c = 3.00 x 108 m/s = speed of light
wavelength x frequency = speed of light = constant
Speed
• The speed of sound is 340 m/s (about 1/5
mile/sec)
• The speed of light is 3x108 m/s
• You hear the thunder five
seconds after seeing the
lightning.
• How far away is the lightening?
Figure it out?
• Takes 5 seconds for you to hear it
• Sound travels about 1/5 mile per second
• 5 times 1/5 = 1 mile
TEST
The difference between one wave crest to the next is called the
A- wave B-wavelength C-crest D-troughs
Frequency x wavelength =
A-acceleration B-crest C-wave speed D- none of the above
The number of waves that pass a giving point in one second is called the
A-frequency B-trough C-wave speed D-acceleration
Light travels in ________ lines
Light is made up of ________which are packets of energy that have both
particle and wave properties
High point of wave is called the _______
Low point of the wave is called the _______