light and color chapters 16 & 19
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Light and Color Chapters 16 & 19. 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). - PowerPoint PPT PresentationTRANSCRIPT
Light and Color Chapters 16 & 19
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”.
Primary and Secondary PigmentsPrimary and Secondary Light
Absorbing and Emitting Light
White paper absorbs and then emits photons of all colors of light.
Black ink absorbs the photons of all colors and emits none.
Relative Speed of Light and the Doppler Effect with LightDoppler Effect with light determined based
on relative speedRelative 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)
Light speed – History of DiscoveryIn 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!
Light…has a dual personality - it can
behave as both a particle (advanced physics) and a wave.
can travel fastest in a vacuum!The speed of light through the other
types of mediums (solid, liquid, and gas) is dependent on the density of that medium. More dense substances slow down light more than less dense. (Opposite of sound!)
Speed of Light
c = f l
wavelength (m)
frequency (Hz)
speed of light3 x 108 m/sec
Speed of Sound vs. Speed of LightLight travels almost a million times
fasterLight travels around the earth 7.5
times per second
The speed of lightSpeed 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.
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 fl
c fl
Answers to practice problemsP. 447 #14. 5.85 x 1014 HzP. 455 #8. 7.43 x 108 Hz
Chapter Review QuestionsAnswer 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)
DUE FRIDAY
Colors of LightWhite 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 and Secondary PigmentsPrimary and Secondary Light
Absorbing and Emitting Light
White paper absorbs and then emits photons of all colors of light.
Black ink absorbs the photons of all colors and emits none.
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. So that shirt must contain cyan pigment.
When mixed, the primary pigments create the secondary pigments (red, blue, and green).
ExamplesWhite light is incident on the three shapes
below. Explain why they appear to be the color they are by using reflected and absorbed colors as well as identifying which pigments must be present in the object.
Yellow color is seen. Red and Green light are reflected, Blue light is absorbed. Blue
pigment must be present.
Blue color is seen. Blue light is reflected, Red and Green light are absorbed. Yellow pigment
must be present.
Magenta color is seen. Red and Blue light are reflected,
Green light is absorbed. Green pigment must be present.
How the human eye sees light
Photoreceptors release chemical signals.
Chemical signals travel to the brain along the optic nerve.
optic nerve
Photoreceptors in the eye
Cones respond to three colors: red, green and blue.
Rods detect intensity of light: black, white, shades of gray.
How we see colors
Which chemical signal gets sent depends on how much energy the light has.
If the brain gets a signal from ONLY green cones, we see green.
Color BlindnessNormal vision
Weak green color vision
No red color vision
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 FilmsColors 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.
Categories of Materials based on absorption or reflection of lightTransparent - 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.
Spectrum Diffraction Patterns on a CDThe 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
DiffractionBending of light around a barrierDiffraction 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 lightNormally, 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
Wavelength and Frequency of Visible Light
Thin slits and diffraction
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 (blue- shifted) or away (red-shifted) from us.