Physics 55Friday, October 14, 2005

1. What light can tell us about astronomical objects (a lot!).2. Doppler shift with applications.

Light and Its Properties

What Can We Learn From Light?

1. Atomic composition.

2. Surface temperature T of a remote object.

3. Whether object has an atmosphere, composition of the atmosphere.

4. Speed of object toward or away from Earth

5. Whether an object is rotating, speed of rotation.

6. Whether an object is expanding or contracting.

7. Presence and strength of magnetic fields.

8. Presence and strength of electric fields.

Example of Astronomical Spectrum:What Are We Looking At?

Make sure you understand intuitively what is meant by “intensity”:amount of energy emitted or absorbed per unit time (watts) per unit area.

But What Is Light?

Modern answer is precise, complete, but rather confusing, involves quantum properties. Light is not like any macroscopic thing with which you are familiar on a day-to-day basis.

Sometimes light acts like waves rippling on a pond. (But unlike sound, there is no medium that is rippled by the passage of light.)

Sometimes light acts like particles that can knock other particles like atoms and molecules about.

Different experiments show wave or particle behavior. Behavior is intrinsically random!

Some Properties of Light

1. reflection2. transmissio

n3. refraction4. dispersion5. absorption

Light Has a Wavelength and Frequency f

cf

Wavelengths of visible light rather small, 400-700 nm.Frequencies of visible light rather high: =500 nm implies f~1015 Hz

Names of Different Regimes of Light

cf

Warning: Wavelength Determines ColorBut Color Does Not Determine Wavelength!

Color is a complex perception of the world constructed by your brain.

Quantum (Crazy) Properties of Light:Two-Slit Experiment for Electrons/Photons

Akira TonomuraAdvanced Research Laboratory, Hitachi, Japan

http://www.pnas.org/content/vol0/issue2005/images/data/0504720102/DC1/04720Movie1.mpg

The experiment. What should be seen for sane waves.

Photons Have Energy

photon energy /E hf hc

The constant h ~ 7 x 10-34 J-s is called “Planck’s constant” and shows up in any calculation involving quantum properties.

The formula E=hf was first proposed by Einstein in 1905, he won a Nobel prize in 1921 for the photon idea behind the formula.

Quantum and particle properties of light important roughly when:1. low intensity, i.e., few photons present 2. light interacts with individual atoms and molecules.

Photons Have Energy: Example

34 8

9

19 -19

(6.6 10 J-s) (3 10 m/s)

550 nm 10 m/nm

3.6 10 J (1 eV / 1.6 10 J)

2.3 eV

hcE

What is energy of photons corresponding to green light in middle of spectrum, with wavelength =550 nm?

Photons Also Have Momentum

photon momentum / / /p h hf c E c

Example: A motionless hydrogen atom of mass m ~ 2 x 10-27 kg is in its first excited state and changes to its ground state by emitting a photon of wavelength ~ 122 nm (see Fig. 6.8 on page 159 of text). What is the approximate recoil speed of the atom?

H

34

-27 -9

0 = total momentum before

= total momentum after

= m v /

so

7 10 J-s mv = ~ ~ 3

(2 10 kg)(100 10 m) sH

h

h

m

PRS Question: Photon Collision

A photon corresponding to green light with =500 nm

bounces off a hydrogen atom at rest. If the hydrogen atom

moves off with a certain speed, then the photon that

bounces back will have:

1. The same wavelength.

2. A longer wavelength.

3. A shorter wavelength.

Light Has Momentum, Can Apply A Force

The Big Picture of What Produces Light Spectra

Quantum Levels Versus Emission orAbsorption of Light

Quantum energy levels for a hydrogen atom H.

Interaction of Light With Matter:Emission and Absorption Spectra

Emission spectrum of hot H gas.

Absorption spectrum of white lightpassing through cold H gas

Read Section 4.4 for somereview.

Each Element, Compound, and Their Ions Has Unique Emission and Absorption Spectra

Can estimate temperature by observing which atoms are partially ionized!

You give it a try:

Use hand-held spectroscope based on diffraction grating to examine spectra of light from gas-discharge tubes at front of lecture hall. Look also at fluorescent lights overhead (mercury vapor) and at tungsten filament.

Solar Spectrum Is Complex!

Maxwell’s Light Wave