Light and Color

Chapter 27 and 28

Light – Wave or particle?• For many years scientists argued over the

nature of light, "Is light a wave or a stream of particles?"

• In some experiments light exhibits wave like properties, the Doppler effect, interference, refraction, diffraction

• and in other experiments, like the photo electric effect, it exhibits particle like properties

• The fact is that light exhibits behaviors which are characteristic of both waves and particles.

Models of Light - Waves• Electromagnetic waves (light) originate from vibrating or

accelerating electric charges• Electromagnetic waves are made up of an electric field and a

magnetic field oscillating at right angles relative to one another

Unlike other waves, light waves can travel through a vacuum

• An electromagnetic wave (light) is a transverse wave

Models of Light - Particles• Particle of light are called photons

• Photons have zero rest mass and travel at the speed of light through a vacuum.

Light and Energy• For waves, the amplitude or intensity is usually

related to the energy of the wave• For light, this is not true. The energy of light waves

was found to be directly related it is frequency.• An experiment demonstrating the photoelectric

effect demonstrated the particle nature of light and that E = hf, where E is energy, h is Planks constant, and f is frequency.

http://phet.colorado.edu/new/simulations/sims.php?sim=Photoelectric_Effect

R O Y G B I V

The Electromagnetic SpectrumThe electromagnetic spectrum is the range of electromagnetic waves extending from radio waves to gamma rays

Increasing frequency

The Visible Spectrum• We can only see a small part of the

electromagnetic spectrum• The visible spectrum is a range of light waves

extending in wavelength from about 400 to 700 nanometers.

Increasing frequencyIncreasing energy

Increasing wavelength

Speed of Light - c• In the early 17th century, many scientists believed that

there was no such thing as the "speed of light"; they thought light could travel any distance in no time at all.

• In the 1670's Roemer was able to calculate a value for the speed of light by carefully studying the orbit of one of Jupiter’s moons, Io. He noticed that the time between the eclipses of the moons of Jupiter was less as the distance away from Earth is decreasing than when it is increasing.

• In 1926 scientist Albert Michelson used the reflection from a rotating mirror on a distant mountain and measured the speed of light at 299,796 km/second

• The current accepted value is 300,000,000 meters per second (3 x 108 m/s) or 186,000 miles per second. Light waves

obey the wave equation, c = f

Transparent vs. Opaque• Transparent - the term applied to materials through which light can

pass in straight lines. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object.

• Opaque - the term applied to materials that absorb light. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material; rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave.

Light Questions1) Rank the following in terms of energy, frequency and wavelength:

microwaves, visible light, IR, UV, x-rays, gamma rays, and radio waves.2) Rank the following in terms of energy, frequency and wavelength: Red,

orange, yellow, blue, and green.3) Why is it possible to get a sunburn with the window down in the car, but

not when the window is up?4) Find the frequency of 80 m radio waves.5) What did the photoelectric effect show about light?6) Is light a wave or a particle?7) What is the name of a particle of light?8) What is the origin of light?9) Why is light also called electromagnetic radiation?10) Why is it possible to see a cell phone ringing in a bell jar (vacuum jar) but

not possible to hear it ringing?11) Do radio waves travel at the speed of light or the speed of sound?

Color

• The color of an opaque object is the result of the color of the light which strikes the object and the color of the light that it reflects.

• The color of a transparent object is the result of the color of the light which strikes the object and the color of the light that it transmits.

• Red filters transmit red light and absorb the other colors, etc.• Red objects reflect red light and absorb the rest, etc.

green

green

red

redwhite ( )

Visible Colors• Different wavelengths of light are perceived as

different colors. • Pure Colors: ROYGBIV• White light contains equal amounts of these colors.

(ROYGBIV)• At an interface, light can be...

»absorbed»reflected»transmitted

Primary Colors of LightWhite

Red

Green

Blue

Yellow

Cyan

MagentaPr

imar

y Co

lors

Combining Colors of Light• When two of primary colors of light are combined they form

the primary colors of pigment; yellow, magenta and cyan • Combining colors of light is an additive process, when all

three primary colors are combined, you get white light• Color Addition Example:

– Tiny dots called pixels on Color TV's and Computer Monitors are colored only red, green, or blue.

Red + Green = Yellow

Red + Blue = MagentaGreen + Blue = Cyan

Complementary Colors

• Complementary Colors - any two colors that add together to produce white

Complementary Colors of LightRed and Cyan Green and MagentaBlue and Yellow

Primary Colors of Pigment (Subtractive Colors)• Subtractive Primary Colors:

• Yellow• Magenta• Cyan

• One can produce any color by varying the amount of yellow, magenta and cyan pigments

• Combining pigments is a subtractive process because each color absorbs a color or colors of light. Combining all three primary colors produces black (all colors are absorbed)

• Color Subtraction Example: – Newspapers and zip-lock sandwich bags (yellow and cyan makes green) use

color subtraction.

Color Blindness• Colorblindness - about 10% of population

• Red-green is predominant• Yellow-blue - a few• Total - some

Optical Illusions

Color Questions1) List the primary colors of light. What happens when combine the three primary

colors of light?2) List the primary colors of pigment. What happens when combine the three

primary colors of pigment?3) _______ + green = yellow Blue + _______ = cyan Yellow + _______ = white

Cyan + _______ = white4) You have a blue opaque object (an object that appears blue under white light).

What color will it look under the following colors of light?– Blue– Red– Green– Cyan– Yellow

5) What color does a red shirt appear when the room lights are turned off and the room is entirely dark? ____________ What about a blue shirt? ____________ ... a green shirt? ____________

White blue

More Color Questions6) Fill in the blanks:

7) True or False/Explain: White and black are colors of light.8) You have a cyan opaque object (an object that appears cyan under white

light). What color will it look under the following colors of light?– Blue– Red– Green– Cyan– Yellow

White cyan

Things that can separate white light

• Prism• Raindrops• CD’s• Diffraction Grating

Things the produce electromagnetic waves

• Radio waves – electrons moving up and down an antenna

• Visible Light – electrons changing energy states in an atom

The Structure of the Atom and Emission• An atom is composed of electrons,

protons and neutrons.• When an electron is raised to a

higher energy level, the atom is said to be excited.

• When the electron returns to a lower energy level, energy is released in the form of light.

• Different transitions from high levels to low levels result in different colors of light.

The Kirchhoff-Bunsen Experiment

• These two scientists found that burning chemicals over an open flame resulted in a spectrum with bright lines.

• They found that each chemical element produced its own characteristic pattern of bright spectral lines.

Emission Spectra of Hydrogen

Photographic Film

Slit

Low DensityGlowing

Hydrogen Gas

Discrete Emission Spectrum

Film

Prism

Hot gas produces a bright line emission spectrum.

Every element can be “fingerprinted” by it spectra.

Helium

Hydrogen

Oxygen

Carbon

Incandescence• Hot, dense solids produce a continuous

spectrum.

• The brightness and color of light emitted by a hot object changes with its temperature.

• Glowing object colors:• Reddish coolest glowing object• Orange-ish• Yellowish• White• Bluish hottest glowing object

Continuous Spectrum

Absorption Spectra

• Cool gas in front of a continuous source of light produces an absorption line spectrum.

• Fraunhofer lines in our Sun's spectrum showed that cool helium gas surrounds the Sun

Absorption Spectrum

Absorption Spectra of Hydrogen

Discrete Absorption Spectrum

Prism

Photographic Film

Film

Slit

White Light Source

Discrete Emission Spectrum

Hydrogen Gas

Sources

• Conceptual Physics by Paul Hewitt• www.physicsclassroom.com• http://observe.phy.sfasu.edu/courses/

phy101/lectures101/