Properties of Light Chapter 27

Models of Light Socrates and Plato believed "streamers" or antennas were emitted from the eyes. Euclid and Descartes had similar theories. Greeks believed anything that could be seen emitted light waves.

Models of Light Newton introduced the particle theory. He said that light travels in straight lines unlike waves.

Models of Light Huygen disagreed with Newton and said that light is a wave because it spreads out sometimes.

Models of Light Einstein realized that light acts not only as waves but as quantum particles now known as photons. Light has dual properties – Particles and Waves

Black Holes Question: Does light have mass? An event horizon exists inside which the intense gravitational field of the black hole prevents any type of electromagnetic radiation including light from escaping Once was a star at least twice the mass of the sun that exhausted its fuel and collapsed to infinite density

A ravenous black hole This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped disk of dust fueling a massive black hole in the center of galaxy NGC 4261, located 100 million light-years away in the direction of the constellation Virgo. By measuring the speed of gas swirling around the black hole, astronomers calculate that the object at the center of the disk is 1.2 billion times the mass of our sun.

                                                                                                    

Speed of Light A. Roemer was the first person to try to measure the speed of light in observations of Io. His calculation was 300,000 km/s.

In 1907, he won the Nobel prize in physics. Speed of Light In 1880, Albert Michelson measured the speed of light to be 299,920 km/s. Used spinning octagonal mirrors and a mirror far away Adjusted spinning speed to calculate speed of light In 1907, he won the Nobel prize in physics.

Electromagnetic Wave Velocity Light could go around the world 7.5 times each second http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html

Electromagnetic Wave Velocity Light takes 8 minutes to travel from the sun to earth.

Electromagnetic Wave Velocity The next closest star is 4 light years away The diameter of our galaxy is 100,000 light years Some galaxies are 10 billion light years away http://bluepoint.egenet.net/sagan/galaxy.gif

Electromagnetic Spectrum Energy moving at speed of light Vary by f and l only Light is only a small part Below light is infrared, above is ultraviolet

Electromagnetic Spectrum Radio Waves – communication TV and Radio Microwaves –cooking & cell phones Infrared – “heat waves” Visible Light – detected by your eyes Ultraviolet – causes sunburns X-rays – penetrates tissue Gamma Rays – most energetic

The Visible Spectrum A range of light waves extending in wavelength from about 400 to 700 nanometers Less than 1% of the entire spectrum

Questions Is it correct to say that radio wave is a low-frequency light wave? Is a radio wave also a sound wave?

Transparent Materials Transparent - the term applied to materials through which light can pass in straight lines

Visible Light and Glass Visible light maintains the same frequency when it enters glass But the velocity changes: Light travels 0.75x the original speed in water In glass – 0.67x. In diamond – 0.41x When the light comes out it regains its speed

Opaque - the term applied to materials that absorb EM Opaque Materials Opaque - the term applied to materials that absorb EM

Opaque Materials Opaque materials absorb EM ex.) wood, metal, rocks ,etc. The light energy is transformed into random kinetic energy makes the object warmer because it absorbs the electromagnetic energy

http://www.ch.ic.ac.uk/local/projects/steer/chloro.htm

Example Questions Are clouds transparent or opaque to visible light? Answer: opaque Are clouds transparent or opaque to ultraviolet light? Answer: transparent

Example Questions Are windows transparent or opaque to visible light? Answer: transparent Are windows transparent or opaque to ultraviolet light? Answer: opaque Due to Law of Conservation of Energy the EM is given off as heat

Shadows Umbra - the darker part of a shadow where all the light is blocked Penumbra - a partial shadow These terms also apply to Solar Eclipses and Lunar Eclipses.

Solar Eclipse Umbra Penumbra A solar eclipse occurs when the Moon passes in front of the Sun.

A lunar eclipse occurs when the Moon passes into the Earth's shadow.

Questions Which type of eclipse is dangerous to view with the unprotected eye? Why are lunar eclipses more commonly seen than solar eclipses?

Seeing the Light – The Eye Cornea - does most of the focusing Iris - has the eye color and controls light intensity Pupil - the hole in the eye Lens - does remainder of focusing Retina - location of light sensors, has rods and cones Fovea - center of vision, predominantly cones Blind spot - optic nerve exit, no light sensors

Parts of the Eye Detectors on the Fovea Rods light intensity and motion sensitive Cones color sensitive The blind spot for the eye is cause by the optic nerve.

p. 474, Conceptual Physics 2002

Color Vision! Colorblindness - about 10% of the population Red-green is predominant Yellow-blue - a few Total – some Mostly males – X-linked trait

Color Deficiency

Optical Illusions

Optical Illusions

Are the horizontal lines parallel, or do they slope? Optical Illusions Are the horizontal lines parallel, or do they slope?

Optical Illusions

Optical Illusions During the Optical Art (OpArt) Movement of the 1960s, artists would create all sort of puzzling effects with color. This "flashing squares" drawing seems to wobble and flash when you concentrate on one particular area of the image. How many squares can you see in this diagram? Can you feel the "motion" of the image?

Optical Illusions

Can you count the black dots? Optical Illusions Can you count the black dots?

Optical Illusions

Myopia (Near-Sightedness) People with near-sightedness cannot see clearly at distance.

Hyperopia (Farsightedness) People with far-sightedness cannot see clearly up close

You will observe a total eclipse of the sun when... (a) you’re in the penumbra of the moon’s shadow (b) you’re in the umbra of the moon’s shadow (c) sunlight diffracts around the moon (d) sunlight reflects from the moon to the earth (b) you’re in the umbra of the moon’s shadow

The speed of light... (a) has never been measured (b) is about the same as that of sound (c) is infinitely fast (d) is very fast, but not infinite (d) is very fast, but not infinite

In the dark in late evening no color is seen because of lack of stimulation of a. rods. b. cones. c. cornea. d. crystalline lens. b. cones.

Self Test On a Blank Page.... List the Seven Forms of Light Which has the longest wavelength? Which has the highest frequency Which has the highest energy? Which has the fastest speed?

Structure of the Atom Neutron Proton Electron Energy Levels or Orbits

Ground State of Electron Energy level it normally occupies state of lowest energy for that electron

Excited State of Electron Temporarily energy state greater than its ground state e- can become excited if it is given extra energy absorbs a photon, or packet of light or collides with a nearby atom or particle

Emission of Photon Electrons do not stay in excited states for very long they soon return to their ground states emitting a photon with the same energy as the one that was absorbed

                                                                                                                                                

Transitions among the various orbitals are unique for each element because the energy levels are uniquely determined by the protons and neutrons in the nucleus

When the electrons of a certain atom return to lower orbitals from excited states, the photons they emit have energies that are characteristic of that kind of atom This gives each element a unique fingerprint, making it possible to identify the elements present in a container of gas, or even a star

Kirchhoff-Bunsen Experiment These two scientists found that burning chemicals over an open flame resulted in an emission spectrum with bright lines Emission Spectrum They found that each chemical element produced its own characteristic pattern of bright spectral lines

Hydrogen Helium Oxygen Carbon Every element can be “fingerprinted” by it spectra.

Hot, dense solids produce a continuous spectrum. Incandescence Hot, dense solids produce a continuous spectrum. Continuous Spectrum

The color of light emitted by a hot object changes with its temperature hottest glowing object  Bluish White Yellowish Orange-ish coolest glowing object  Reddish

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

Matching Questions Type of Spectrum Appearance 1. Emission Spectra a. All Colors 2. Continuous Spectra b. Dark Lines 3. Absorption Spectra c. Bright Lines

Matching Questions 1. Emission Spectra a. Hot Solids 2. Continuous Spectra b. Hot Stars 3. Absorption Spectra c. Hot Gases

Fluorescence Some materials that are excited by ultraviolet light can emit visible light upon de-excitation The material’s electrons use some of the energy during vibration The emitted light (bluish white) is at a lower frequency (energy) than the UV

Fluorescent Lamps Primary excitation - electron collisions with low pressure mercury vapor, and ultraviolet light is given off Secondary excitation - ultraviolet light is absorbed by phosphors and these emit visible light

                                                                                                                                                

Phosphorescence Phosphorescence - a type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation. Electrons get "stuck" in an excited state and de-excite gradually.

Lasers Light Amplification by Stimulated Emission of Radiation Lasers produce coherent light. all the light waves have the frequency, phase and direction. Demo - Laser and chalk dust

Rhodamine 6G dye (tunable) Laser Type Wavelength (nm) Argon fluoride (UV) 193 Krypton fluoride (UV) 248 Nitrogen (UV) 337 Argon (blue) 488 Argon (green) 514 Helium neon (green) 543 Helium neon (red) 633 Rhodamine 6G dye (tunable) 570-650 Ruby (CrAlO3) (red) 694 Nd:Yag (NIR) 1064 Carbon dioxide (FIR) 10600

Matching Questions Incandescence a. mercury vapor light tubes 2. Fluorescence b. glow-in-the-dark paints and plastics 3. Phosphorescence c. light bulbs with filaments

Matching Questions 1. Fluorescent Lamp a. color changes with temperature 2. Incandescent Bulb b. only one color of light 3. Laser c. converts ultraviolet light to visible light

Color Chapter 27

Different wavelengths of light are perceived as different colors Pure Colors: ROY G. BIV White light contains equal amounts of these colors (ROYGBIV)

Selective Reflection and Transmission At an interface, light can be... absorbed reflected transmitted

Fill in the Blanks absorb Black objects _______ all colors of light. White objects _______ all colors of light. Transparent objects _______ all colors of light absorb reflect transmit

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

In red light, what color do the red petals and green leaves of a rose appear? Answer: Petal appear red Leaves appear to be black.

Mixing Colored Light Color Addition Additive Primary Colors of Light: Red Green Blue One can produce any color by varying amplitude and mixture or red, green, and blue light.

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

                                                                                                                                                     

Mixing Colored Pigments - Color Subtraction Subtractive Primary Colors: Yellow Magenta Cyan One can produce any color by varying the amount of yellow, magenta and cyan pigments.

Green Pigment Red Pigment Absorbed =

Colors of Light

Yellow Pigment Cyan Pigment + + Absorbed =

Yellow + Cyan = Green Pigment Absorbed =

In-Class Exercise Make a list of the seven forms of light in order of decreasing wavelength. Draw a Color Addition Diagram using overlapping colored spotlights. Label all 7 colors. Draw a Color Subtraction Diagram using overlapping paints. Label all 7 colors.

Colors of Light red magenta blue yellow white cyan green

yellow green cyan red black blue magenta

scatter high frequencies of light Why is the Sky Blue? Nitrogen and Oxygen in our atmosphere scatter high frequencies of light

Why is the Ocean Greenish Blue? How have fish adapted? Red light is absorbed by the molecules in the water

Why are Sunsets Red? Red light is scattered the least by our atmosphere The greatest path of sunlight through the atmosphere is at sunset or sunrise

Why are Clouds White? Clouds are composed of water droplets Different-sized droplets create different types of scattered frequencies Small droplets scatter blue Bigger scatter higher frequencies (like green) Even bigger droplets scatter red This creates a cloud that appears white

e.g. magenta + green = white Complementary Light Colors - any two colors that add together to produce white e.g. magenta + green = white

After Images - Conal Fatigue The human eye will see complimentary colors after staring at a color picture. Demos: Colored Shapes Texas Flag American Flag Rose Lincoln

If you look at the yellow Sun just before sunset for a few seconds and then look at a white cloud you see _______ afterimages of the Sun. a) red b) green c) blue d) cyan 

A mixture of cyan and yellow paints gives __________ paint. a) green b) red c) black d) white e) blue 

Adding red and green light gives us what color? a) yellow b) cyan c) blue d) magenta 

Suppose that two flashlight beams are shone on a white screen, one through a pane of blue glass and the other through a pane of green glass. What color appears on the screen where the two beams overlap? a) yellow b) green c) cyan d) magenta e) red 

If a car headlight emitted only yellow light, the normally green grass appears to be a) green. b) red. c) black. d) white. e) yellow. 

a) yellow, violet, and green. b) cyan, yellow, and magenta. To see an after image of a red, white and blue Texas flag one would first stare at a flag with the complimentary colors a) yellow, violet, and green. b) cyan, yellow, and magenta. c) cyan, black, and yellow. d) yellow, cyan and green. 

The worst thing that you can do for the health of a green-leafed plant is to illuminate it with only a) red light. b) green light. c) blue light. d) all are equally bad. e) none of these. 

Most of the light that we see has undergone (a) selective interference (b) selective transmission (c) selective reflection (d) selective refraction (c) selective reflection

A mixture of magenta and green lights give white light A mixture of magenta and green lights give white light. These two colors are (a) additive primaries (b) secondary colors (c) complementary colors (d) fluorescent colors (e) interference colors (c) complementary colors

Mixing yellow paint and magenta paint gives what color?  (a) red (b) green (c) blue (d) cyan

What color would red cloth appear if it were illuminated by cyan light? (a) cyan (b) red (c) yellow (d) green (e) black 