Transcript Light and Color Notes – Chapter 16, 17,18, and 19

Light and Color
Chapters 16, 17, 18, & 19
GPS Standards
Students will analyze the properties and applications of
waves.
a. Explain the processes that result in the production and
energy transfer of electromagnetic waves.
b. Experimentally determine the behavior of waves in
various media in terms of reflection, refraction, and
diffraction of waves.
c. Explain the relationship between the phenomena of
interference and the principle of superposition.
d. Demonstrate the transfer of energy through different
mediums by mechanical waves.
e. Determine the location and nature of images formed by
the reflection or refraction of light.
Journal #
 What is light? Write a paragraph describing your
typical day without light, and what you would
need to do in situations where light was
necessary. Consider how the absence of light
would impact your surroundings as well.
Light…
 is the only thing we can see!
 has a dual personality - it can behave as
both a particle and a wave.
 travels faster than anything else in the
universe.
 can travel in a vacuum, unlike sound!
Light and the Electromagnetic
Spectrum
 Visible Light is an electromagnetic wave that
stimulates the retina of the eye. Its wavelengths
are between 400nm (violet) and 700nm (red).
 The Electromagnetic Spectrum is the full range of
wavelengths and frequencies at which all
electromagnetic radiation exists and the
corresponding names that we give to certain
“zones”.
The Electromagnetic Spectrum
Light speed – History of Discovery
 In a vacuum, light has a speed of 3.00 x 108m/s
(symbol c)
 Before 17th century, people believed light traveled
instantaneously or that speed of light was too fast to be
measured.
 Roemer used the orbit of Jupiter’s moon, Io, and the
orbit of Earth around the sun to first measure the speed
of light in 1674.
 Michelson, using a specially designed experiment
sending light between two mountains, more closely
estimated the speed of light in 1926. His measurement
is almost the exact same that we use today!
Categories of Materials based on absorption or reflection
of light
 Transparent - allow most light to pass through them.
 Translucent - allow some light to pass through, but
some is absorbed and some is reflected.
 Opaque - do not allow any light to pass through, but
instead absorb or reflect all light.
Colors of Light
 White light is a combination of the spectrum of
colors, each having different wavelengths.
 When combined, the three primary light colors of
red, blue, and green will produce white light.
 In other combinations, they will produce other
colors.
 Primary Additive Colors for Light
Red, Green and Blue – combine to make white
light
Primary Pigments
 The three secondary colors of light are also
known as the primary pigments – cyan, magenta,
and yellow – and are used in pigments and dyes
to produce a wide variety of colors.
Why do we see colors?
 Pigments reflect color of light that we see. So… a
shirt that has been dyed to appear red is only red
because it reflects mostly red light and absorbs most
of the other two primary light colors of blue and
green.
 When mixed, the primary pigments create the
secondary pigments (red, blue, and green).
What’s the difference between light and
pigments?
 Unlike light, pigments are referred to as subtractive
colors. For instance, when you mix the primary
pigments of yellow and cyan, the color of green is
what you will see. That is because green pigment
absorbs the light with colors of blue and red and
reflects back only green.
Thin Films
 Colors in soap and oil films are caused by the
interference of specific wavelengths of light reflected
from the front and back surfaces of the thin films.
 The colors actually show up because an antinode
(constructive interference) is formed.
Spectrum Diffraction Patterns on a CD
 The metallic coatings on CDs are less than 100nm thick
 Each coating partially reflects and partially transmits
incident light.
 Light rays reflected from different coating boundaries
interfere with each other to produce the colorful patterns
Diffraction
 Bending of light around a barrier
 Diffraction pattern – a pattern of bright and dark
bands produced by constructive and destructive
interference; white light shows all the colors of
spectrum
 Diffraction gratings (ex: goggles used in 1st lab)
are devices made up of many single slits that
bend light and form diffraction patterns
 See fig. 19-14 p. 528 red light versus white light
Polarized light
 Normally, light (just like all electromagnetic
radiation) vibrates in two dimensions as it travels.
 Polarized light consists of waves vibrating in a
particular plane. Polarization is done by using
VERY small filters (slits) that block vibrations from
other planes.
 Application of Polarization – sunglasses polarized
to reduce glare reflected off water or off the road
Polarized Light
The speed of light
 Speed of light in a vacuum is equal to a
constant c, which equals 3.00 x 108 m/s
 Rearrange the speed of light formula to
find wavelength of a light wave by
dividing c by frequency of that light.
  c/ f
 We can also rearrange the formula to
find frequency, if we know the
wavelength and speed.
 Try p. 447, #14 and #8 p. 455 (Note
that a nanometer is 10-9 m)
c  f
Answers to practice problems
 P. 447 #14. 5.85 x 1014 Hz
 P. 455 #8. 7.43 x 1014 Hz
Relative Speed of Light and the
Doppler Effect with Light
 Doppler Effect with light determined based on
relative speed
 Relative speed – magnitude of the difference
between the velocities of the light source and the
observer of the light
Blue-Shifted or Red-Shifted?
 Blue-shifted - When light
source approaches
observer, there is an
increase in measured
frequency, so wavelength
of light shortens. Called
blue-shifted because the
increase is towards the
higher frequency (or blue)
end of the color spectrum.
(Ex: side of a star turning
towards us as it spins)

Red, green and blue channels represent the
red-shifted and blue-shifted motions of the
ionised material in the halo. The positions of
the two galaxies C11 and C15 are marked.
Red-Shifted?
 Red-shifted - When a light source recedes (goes
away) from observer, there is a decrease in
measured frequency, so wavelength lengthens.
Called red-shifted because the decrease is
towards the lower (or red) end of the color
spectrum (Ex: the side of a star spinning away
from us; distant galaxies show a red shift as they
move away)
How do scientists analyze light?
 A Spectroscope
(containing a diffraction
grating) is used to
measure light from
glowing elements and
separate that light into its
various frequencies. It is
able to measure Doppler
shift of the wavelengths of
the light from elements.
What does a spectroscope do?
 Helps chemists determine the elemental
composition of heated gases or materials using
the spectra they emit.
 Astronomers can use spectra seen to determine
what elements make up the stars and galaxies
and to see if they are moving towards (blueshifted) or away (red-shifted) from us.
Chapter Review Questions
 Answer the following 19 questions:
 p. 438 #7 and 12 (hint: v = d/t - rearrange)
 P. 447 : #18 – 22 (4 questions)
 13 chapter review questions on page 452-454:
#25, 33, 35, 36, 37, 38, 39, 43, 48, 51, 54 (hint:
remember v = d/t ), 62, and 68 (just explain in
general terms, no need to solve)