PHYS 140

Light & Color

Course Outline

•Ch. 1: Physical Nature of Light •Ch. 2: The Origin of Color•Ch. 3: Colorimetry•Ch. 4: Geometric & Vision Optics•Ch. 5: Wave Optics•Ch. 6: Light & Color in Nature•Ch. 7: Color Vision

Review

•Scientific or “powers of ten” notation.

•Units.

•Algebra & geometry.

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/

http://physics.nist.gov/cuu/Units/units.html

Problem

Speed of light is 3 x 108 m/s. How long does light take to travel from the nearest galaxy, a distance of 2 x 1022 m? Express your answer in years.

Time

DistanceSpeed

Answer: About 2 x 106 years…

That means this galaxy is 2 x 106 light years away!

The Scientific Notation (Continued)

Sometimes, powers of 10 are replaced by prefixes. Examples:

1 1 unit

1,000 103 kilo (k)

1,000,000 106 mega (M)

1,000,000,000 109 giga (G)

0.001 10-3 milli (m)

0.000001 10-6 micro ()

0.000000001 10-9 nano (n)

On The Lighter Side…

•2000 pounds of Chinese soup: Won ton•1 millionth of a mouthwash: 1 microscope•Time between slipping on a peel and smacking the pavement: 1bananosecond•Weight an evangelist carries with God: 1 billigram•Time it takes to sail 220 yards at 1 nautical mile per hour: Knot-furlong•365.25 days of drinking low-calorie beer because it's less filling: 1 lite year•Half of a large intestine: 1 semicolon•1000 aches: 1 kilohurtz•Basic unit of laryngitis: 1 hoarsepower•1 million microphones: 1 megaphone•1 million bicycles: 2 megacycles•10 cards: 1 decacards•1 millionth of a fish: 1 microfiche•1 trillion pins: 1 terrapin•2 monograms: 1 diagram•8 nickels: 2 paradigms

Chapter 1:

The Physical Nature of Light

What is sight?

Early Theories

•550 – 400 BC (Pythagoras):Traveling images

•200 BC – 40 AD (Euclid):Visual Rays

•1000 AD (Alhazen):Light = Physical thingEye = Detector of light

Euclid’s Idea of Vision!

Today

Sight = Light + Eye/Brain

•Direct (Source)•Transmitted or Reflected•Scattered

Detector

Some Properties of Light

•Reflection (light can be reflected)

•Refraction (light can travel in different transparent media)

•Shadows (light travels in straight lines)

•Dispersion (colors)

•Speed…???

Some Early Attempts to Measure the Speed of Light

•Galileo (early 1600’s) Flashing lanterns

•Roemer (1670’s)Indirect asrtonomical observations

Note: Round trip transit time for the light signal would have been 0.0001 second!

Galileo’s Attempt at Measuring the Speed of Light

15 km

Time delay between eclipses as observed from position 1 to position 2 was about 1000 seconds!

Diameter = 300,000,000 km

Some Modern Methods for Measuring the Speed of Light

Date Author Method Result (km/s) Error

1676 Olaus Roemer Jupiter's satellites 214,000

1726 James Bradley Stellar Aberration 301,000

1849 Armand Fizeau Toothed Wheel 315,000

1862 Leon Foucault Rotating Mirror 298,000 +/- 500

1879 Albert Michelson Rotating Mirror 299,910 +/- 50

1907 Rosa, DorsayElectromagnetic constants 299,788 +/- 30

1926 Albert Michelson Rotating Mirror 299,796 +/- 4

1947Essen, Gorden-Smith

Cavity Resonator 299,792 +/- 3

1958 K. D. FroomeRadio Interferometer 299,792.5

+/- 0.1

1973 Evanson et al Lasers 299,792.4574 +/- 0.001

1983 Adopted Value 299,792.458

Newton’s Particle Theory

•Proposed in the mid 1660’s.•Prism experiments:

Spectrumof colors

Prism

WhiteLight

Pinhole

•White light contains all colors of the spectrum.•Colors can be mixed to get white light.•Objects appear colored because they reflect, scatter, or transmit that color while absorbing others.

Newton’s Particle Theory (Contd.)

•Postulates:

•Visible light consists of extremely small particles (corpuscles).•Light particles travel at enormous or infinite speed.•Colors correspond to different particle sizes.

•Newton’s theory could explain all the known (at the time) properties of light.

White Light SpectrumDispersion

Color Mixing

Huygens’ Wave Theory

•Proposed about the same time as Newton’s particle theory.

•Postulates:

•Visible light is a wave phenomenon.•Colors correspond to different wavelengths.

•Drawback: Waves need a medium to travel. How can light travel through vacuum?

www.explorescience.com/activities/Activity_page.cfm?ActivityID=51

Young’s Double Slit Experiment

•Performed in the early 1800’s.

•Demonstrated the “Interference Effect” with visible light. Light must be a wave!

www.Colorado.EDU/physics/2000/schroedinger/two-slit2.html

Waves

•What is a “wave”?

Any periodic disturbance that repeats itself in time and travels through space with definite speed.

Examples: Water waves, waves on a string, sound, visible light, x rays, microwaves…etc.

Note: Most of the information comes to us in the form of waves!

Waves transmit energy!

http://surendranath.tripod.com/Applets.html

Wave Characteristics

•Waves consist of periodic motion in time. This repetition rate is measured in terms of time period or frequency.

•Waves transport energy. “Strength” or “loudness” of the wave is measured by its amplitude.

•Waves travel through space. The distance traveled during 1 time period is called the wavelength.

Tf

1

Wave Speed

•Wavelength is the distance traveled during one time period. By definition,

T

λ v fv

Note: Above relations are true for any wave.

For a wave with constant speed, wavelength is inversely proportional to its frequency.

or

Review Problems

1. If the frequency of a wave is 10 Hz, what is its time period?

2. A given wave has a wavelength of 10 cm and a frequency of 200 Hz. What is the speed of the wave?

3. The speed of sound on planet Mongo is 800 m/s. A normal person can hear sound in the frequency range of 20 Hz to 20 kHz. What wavelength range is this?

4. A typical AM radio frequency is 1000 kHz. What wavelength does this correspond to?

0.1 s

20 m/s

40 m – 0.04 m

300 m

Properties of Waves

•Superposition Principle:

Two or more waves can overlap in space or time and produce a single net disturbance.

This principle gives rise to many interesting effects!

http://surendranath.tripod.com/Applets.html

Interference Effect

Constructive Interference

•Amplitudes are added.

•Result: Single wave with large amplitude.

Overlapping waves are “in phase”

=

Interference Effect (Contd.)

Destructive Interference

•Amplitudes are subtracted

•Result: Single wave with small or zero amplitude.

Overlapping waves are “out of phase”

www.colorado.edu/physics/2000/index.pl www.physics.nwu.edu/ugrad/vpl/waves/superposition2.html

Particle-Wave Controversy (1800’s)

Light

Particles?Particles?

Could explain:Could explain:•ReflectionReflection•RefractionRefraction•PolarizationPolarization

Could not explain:Could not explain:•InterferenceInterference

Waves?Waves?

Could explain:Could explain:•ReflectionReflection•RefractionRefraction•InterferenceInterference

Could not explain:Could not explain:•PolarizationPolarization•Travel without mediumTravel without medium

The Electromagnetic Wave Model

•Proposed by Maxwell in early 1870’s.•An electromagnetic field exists around a moving charge. (Field can exist in a vacuum!)

+

•Maxwell’s calculations showed that if this charge is oscillated, the disturbance in the field travels at the speed of light! Is light an electromagnetic wave?

Shake me!

http://webphysics.davidson.edu/Applets/Retard/Retard.html

Electromagnetic Wave Theory

•Postulates:

•Light is an EM wave produced by accelerated charges within atoms.

•It consists of oscillating electric and magnetic fields that are mutually perpendicular.

•All EM waves travel at the speed of light, but they have different wavelengths (colors) and frequencies.fc

The Electromagnetic Wave

http://employees.oneonta.edu/labroos/animations/phe/emwave.htm

The Electromagnetic Spectrum

3 x 108 m/s = Wavelength x Frequencywww.Colorado.EDU/physics/2000/waves_particles/index.html

Electromagnetic Wave Model (Contd.)

•This model is adequate in explaining the “wavelike” characteristics of light.

•Drawbacks of the EM theory:

•Blackbody radiation.

•Photoelectric effect.

•How does an EM wave carry energy?

Blackbody Radiation

•Shape of the blackbody radiation curve could not be explained by the classical theory.

ww2.unime.it/dipart/i_fismed/wbt/ita/physlet/blackbody/corponero.htm

Photoelectric Effect

•Presence of a cut-off frequency could not be explained by the classical theory.

http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm

Electrons being ejected from Electrons being ejected from a metal surface by absorption a metal surface by absorption

of lightof light

The Quantum Idea

•Initially proposed by Planck in 1890’s to explain “blackbody radiation”, and later adopted by Einstein in 1905 to explain the “photoelectric effect”.

•Mechanical wave: Energy is related to its amplitude, so it can carry any continuous amount!

•EM wave: Planck proposed that energy is related to its frequency, so it can only carry discreet amounts.

Gravitational Analogy of the Quantum Idea

Gravitational potential energy is proportional to the height!

Ramp:Energy = Any amount

Steps:Energy = 1, 2, ..Quanta

The Quantum Model

•Postulates:

•EM waves carry energy in “bundles” or “packages” called PHOTONS.

•Photon energy is given by,

•Note: Planck’s constant, h = 4.1 x 10-15 eV-sec. Where 1 eV = 1.6 x 10-19 Joules of energy.

hc

hfE

Review

Calculate the photon energy in eV of the following waves:

• 100 MHz radio wave

• 500 nm visible light

• 0.1 nm x-ray

4.1 x 10-7 eV

2.46 eV

12,300 eV

Wave-Particle Duality

Light

Waves? Particles (Photons)?

•Wave-like characteristics:•Shows interference.

•Particle-like characteristics:•Has momentum.•Exerts pressure (comet’s tail always points away from the sun!).

•Note: Particles like electrons and neutrons show wave-like characteristics!