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.