Post on 31-Mar-2015
Physics 1230: Light and ColorIvan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
ivan.smalyukh@colorado.eduPhone: 303-492-7277
Lectures:
Tuesdays & Thursdays, 3:30 PM - 4:45 PM
Office hours:
Mondays & Wednesdays, 3 PM - 4 PM
Grading Assistants:Jian Yao,
Yiheng Lin,
Hao Song
Lectures
Homeworks
Exams
Hands-on experience
Demonstrations
Peer Instruction (with extra credit)
Fun
2
• WebsiteWebsitehttp://www.colorado.edu/physics/phys1230/phys1230_fa09/
• SyllabusSyllabus– Science of light and colorScience of light and color– HW assignments, reading and exam datesHW assignments, reading and exam dates
• Course infoCourse info– GradesGrades– Clicker technologyClicker technology
• Another kind of invisible waveAnother kind of invisible wave
• WebsiteWebsitehttp://www.colorado.edu/physics/phys1230/phys1230_fa09/
• SyllabusSyllabus– Science of light and colorScience of light and color– HW assignments, reading and exam datesHW assignments, reading and exam dates
• Course infoCourse info– GradesGrades– Clicker technologyClicker technology
• Another kind of invisible waveAnother kind of invisible wave
Course Business
Physics 1230: Light and ColorIvan I. Smalyukh
• What is "science"?
• Why is learning science like learning a foreign language?– Words have new and precise
meanings– Wave, image, ray, lens, white, exposure, file, see,
reflection, refraction, dispersion, particle
– What does it mean for the "sun to set?" Is there really global warming? How could complex and magnificent creatures like us have evolved from lower species?
• We all have scientific curiosity:
• Why the sky is blue?
• What is rainbow?
• What is sunset?
• How my laptop display works?
• We all have scientific curiosity:
• Why the sky is blue?
• What is rainbow?
• What is sunset?
• How my laptop display works?
Ornithoptera chimaera chimaeraOrnithoptera chimaera chimaera Ornithoptera goliath supremus, f. titanOrnithoptera goliath supremus, f. titan
Ornithoptera priamus urvillianusOrnithoptera priamus urvillianus Papilio blumei blumeiPapilio blumei blumei
What will be tested in this class? How can I get a good grade?
Ability to: Memorize
new facts
and
definitions
"Understand" new
concepts at a
minimal level
Deeper understanding: Be able to
figure out new things using concepts
Percent of grade: 25% 25% 50% (only 15% uses math)
HW#1 due next Thursday
Survey to be answered by clickers
1. What is your background for light and color? Choose the one which best describes your science background
a) I have had no physics in high school or college
b) I have had physics in high school but not in college
c) I have taken a physics course OR a psychology course at the college level
d) I have taken more than one physics or psychology courses (high school and/or college level)
e) I am a science major
Survey to be answered by clickers
2. Why did you take this course? Give the answer which best describes your reason.
a) Because of requirements by the university
b) Because it was recommended to me
c) Because it looked easy
d) Because it looked interesting
e) I don't know why
Scientific notation and metric system
• Powers of 10 give a shorthand notation for very large numbers.• 103 = 1000
• 102 = 100
• 10 1 = 10
• 100 = 1
• Or very small numbers• 10-1 = 0.1
• 10-2 = 0.01
• 10-3 = 0.001
• Scientists don't use feet or miles to indicate distances
• They use– meters (m)
• 1 meter = 39.4 inches
– kilometers (km)• 1 km = 1000 m = 0.625 mi
– centimeters (cm)• 1 cm = 10-2 m = 0.394 inches
– millimeters (mm)• 1 mm = 10-3 m
– nanometers (nm) • 1 nm = 10-9 m
Clicker question
• What Lottery Prize you prefer?a) $ 103
b) $ 10-8
c) $ 107
d) $ 100
e) $ 10-18
• wavelength = 500 nm = 5 x 102 x10-9 m = 5 x 10-7 m
• Hence 1 m = wavelength/(5 x 10-7) = 107/5 wavelengths =2 million wavelengths
• Since 1 cm is 1/100 of a meter there are 2 x 106/100 = 20,000 wavelengths in a cm
Clicker question
• The wavelength of green light is around 500 nm. How many wavelengths of green light fit into one cm (or a fingertip)?a) 20 thousand
b) 50 thousand
c) Two million
d) Two billion
e) 5 billion
• 500 nm = 5 x 102 x10-9 m = 5 x 10-7 m
• 1 cm = 10-2 m;
• Hence, 10-2 /(5 x 10-7) = 20,000 wavelengths in a cm
Intro and Chapter 1 Continued
• Light belongs to a family of waves called electromagnetic (EM) waves (Physics 2000)
– Other waves: rope waves, water waves, sound waves, etc.
• Sometimes EM waves are called EM radiation
– Radio waves
– Radar and similar waves• microwaves• cell phone waves
– Infrared or heat waves
– Ultra-violet (suntan) waves
– X-rays
– Gamma rays
• EM waves are created and destroyed by emission and absorption– Classical picture (Phys 2000)
• wiggling electrons radiate radio waves or radar waves
• electrons in an atom are resonant with emitted or absorbed light waves or X-rays
– Quantum picture (Phys 2000)• change of state of electrons in
atoms when bundles of wave energy (photons) are emitted or absorbed
• Light sources– Incandescent light bulb– Neon light– Fluorescent light
Rays (a single beam of light, for example)
• Single light ray– Ray from a laser acts like a
single light ray– Illustrate by laser light
through fog– Bounce off mirror– Bounce off white card– Put through water (bending)
• We only see light when a ray enters into our eye– Laser light is visible from
side because it is scattered into our eyes
• Rays from a flashlight• Rays from a light bulb
• What about light coming from everything in this room? Two kinds of objects:– Self luminous objects (lights)
– Objects which are not self-luminous are seen because of light reflected off them
• Turn out the light and we don't see anything in the room
• It's all reflected light with many rays coming from diffuse surfaces
Laser
Flashlight
Lightbulb
Incident ray from a light bulb
MANY reflected rays comefrom all parts of Alex, including
his nose - a diffuse object
Bob sees Alex's nose because a reflected light ray enters Bob's eye!
Light rays are invisible unless they enter directly into our eye or are scattered bysmoke, fog or some object into your eye!
Rays bounce when they reflect off a mirror or shiny surface
Mirror
• This is called specular reflection.
• How is it different from diffuse reflection?
Rays bounce when they reflect off a mirror or shiny surface
Mirror
• This is called specular reflection.
• How is it different from diffuse reflection?
http://phet.colorado.edu/sims/string-wave/string-wave_en.htmlhttp://phet.colorado.edu/sims/string-wave/string-wave_en.html
Waves
• Rope waves– Created by oscillation of my
hand holding the rope– Finite speed of wave, but rope
segments do NOT move in direction of wave
– Rope segments move up and down, not along wave
– Note the change that occurs when I oscillate my hand faster
• Radio wave transmitter– 3 meters wavelength– (100 Mhz frequency)
• Google search under keyword "physics"– Water waves (circular
pattern)
– Stadium waves
Period and frequency of a wave and relation to wavelength and speed
• The period, T, is the time for the wave to make one complete cycle (say, top-bottom-top) AT ONE FIXED SPOT
Think of my hand moving to make the rope wave. The period is the time for my hand holding the rope to make one complete top-bottom-top (or bottom-top-bottom) motion.
• The frequency, f, of the wave is
equal to one over the period: f = 1/T
f has the units of 1/secs, which we call Hertz (Hz)
• The frequency, f, is related to the speed of the wave, c and its wavelength, (lambda):
f c
What would happen if sound waves could propagate through empty space???
We see color when waves of different wavelengths enter enter our eyes!
Light with wavelength of 650 nmappears red when it enters a viewers eye
Light with wavelength of 520 nmappears green when it enters a viewers eye
Light with wavelength of 470 nmappears blue when it enters a viewers eye
The speed of light in empty space is the same for all wavelengths
Clicker questions
• Which of the light waves has the longest wavelength?
• Which of the light waves is brightest?
• Which of the light waves has the highest speed in empty space?
a) b) c)
d) They all have the same speed
a)
b)
c)
Clicker question
• What does Alex see when the wave at left with wavelength 650 nm goes by him?a) Red
b) Blue
c) Green
d) White
e) Nothing
What happens when two or more waves with different wavelengths reach your eye?
Light with both wavelengths 650 nm and520 nm appears yellow when it enters a viewers eye
Light with only wavelength 580 nmALSO appears yellow when it enters a viewers eye (A DEEPER YELLOWTHAN FOR THE CASE ABOVE)
What is white light?
Light which is a mixture of 650, 520 and 470 nm wavelengths (and possibly more wavelengths) appears WHITE when it reaches your eye
No single wavelength (mono-chromatic) wave appears whitewhen it reaches your eye!
Light at wavelengths which we seeas colors are part of awider familyof electro-magneticwaves
It's a good idea to remember some rough wavelengths associated with colors
• Violet and blue are what we see when shorter wavelength visible rays enter our eyes.– They have
relatively higher frequencies
• Red is how we see longer wavelength visible rays– Red has a relatively
smaller frequency
What happens when those other electromagnetic waves enter our eyes?
Wavelength = 10 million nm• We don't see anything becauseelectromagnetic waves at this frequency cannot excite aresonance in the atoms in our eyes.
• Note that this wavelength is onthe order of a centimeter• These are microwaves just likethose sent out and received by yourcellphones (or used in a microwaveoven).• We cannot see them evenwhen they enter our eyes becausetheir wavelength is too long toexcite a resonance.
Note that the frequency isf = c/
(3 x 108 m/s)/(10-2 m)= 3 x 1010 Hz
How much time it takes for the wave to travel distance of 9000 km?
• Speed of sound 340 m/s
• Speed of electromagnetic waves 300,000 km/s ;
How does an ordinary incandescent light bulb work?
Electrode leading to oneside of the wall plug
Electrode leading to the other side ofthe wall plug
Filament with current of electrons which hitinto atoms causing light to be emitted
Gas
Atom
Electrons
Light emitted at many different resonance frequencies of atoms appears as white light
Atom
Atom
Neon lights have atoms with resonances at special colors inside. They use alternating (AC) household current
(Demo)
Atom
Electrons
Neon light
Atom
Atom
When rays come out in various different directions from an object or objects, the wavefront is defined as
a curve or surface perpendicular to all the rays
• In this case the wavefront is expanding out spherically from the light bulb.
• Wherever it intersects a ray the wavefront is perpendicular to that ray– More technically, the tangent
to the wavefront at the point of intersection is perpendicular to the ray
• The wavefront may be easier to visualize than the rays– You throw a pebble into a
pond. The circularly expanding water waves are the wavefronts
Lightbulb
Rays
Wavefront
How are wave wiggles related to rays?
Amplitude {
Wav
elen
gth
{
Speed of light in empty space is c = 186,000 miles/sec = 3 x 108 meters/sec
Ray
Note, the wave is NOT "red." I have colored it red.We perceive it as red because of its wavelength.
Waveform
They are perpendicularThey are perpendicularThey are perpendicularThey are perpendicular
rayrayrayray
WigglesWigglesWigglesWiggles
Light with a SINGLE wavelength is calledmonochromatic light
Light with a SINGLE wavelength is calledmonochromatic light
HW #1 is due today