Chapter 2: Light and MatterElectromagnetic Radiation

(How we get most of our information about the cosmos)

Examples of electromagnetic radiation:

LightInfraredUltravioletMicrowavesAM radioFM radioTV signalsCell phone signalsX-rays

a) wavelength

b) frequency

c) period

d) amplitude

e) energy

The distance between successive wave crests defines the ________ of a wave.

Question 2

a) wavelength

b) frequency

c) period

d) amplitude

e) energy

The distance between successive wave crests defines the ________ of a wave.

Question 2

Light can range from short-wavelength

gamma rays to long-wavelength radio

waves.

Properties of a wave

wavelength ()

crest

amplitude (A)

velocity (v)trough

Context: Example from Sound waves

Amplitude => Volume

Frequency () => Pitch, determines which note you hear

Shape of wave => what makes a piano sound different from a trumpet

1. Refraction

Waves bend when they pass through material of different densities.

swimming pool

air

water

prism

airairglass

Things that waves do

2. Diffraction

Waves bend when they go through a narrow gap or around a corner.

3. Interference

Waves can interfere with each other

4. Wave Speed depends on medium – mechanical waves must have a medium to travel in

Example: Speed of sound

In air 340 m/s or 760 MPH at 20 deg C (varies with temperature)

In Water 1497 m/s at

Sound speed depends on elasticity (stiffness) of the material

=> Bell Jar Demonstration

The Doppler Effect

Applies to all kinds of waves, not just radiation.

at restvelocity v1

velocity v1 velocity v3

fewer wavecrests per second => lower frequency!

velocity v1

velocity v2

you encounter more wavecrests per second => higher frequency!

Demo: buzzer on a moving arm

Demo: The Doppler Ball

Doppler Effect

The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

Radiation travels as Electromagnetic waves.

That is, waves of electric and magnetic fields traveling together.

Examples of objects with magnetic fields:

a magnetthe EarthClusters of galaxies

Examples of objects with electric fields:

Protons (+)Electrons (-) } "charged" particles that

make up atoms.

Power lines, electric motors, …

Scottish physicist James Clerk Maxwell showed in 1865 that waves of electric and magnetic fields travel together => traveling “electromagnetic” waves.

The speed of all electromagnetic waves is the speed of light.

c = 3 x 10 8 m / sor c = 3 x 10 10 cm / sor c = 3 x 10 5 km / s

Sun

Earth

light takes 8 minutes

c =

or, bigger means smaller

Demo: White light and a prism

Demo: Spectrum of the Sun

A Spectrum

All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.

Refraction of light

c =

1 nm = 10 -9 m , 1 Angstrom = 10 -10 m

The Electromagnetic Spectrum

Clicker Question:

Compared to ultraviolet radiation, infrared radiation has greater:

A: energy

B: amplitude

C: frequency

D: wavelength

Clicker Question:

The energy of a photon is proportional to its:

A: period

B: amplitude

C: frequency

D: wavelength

We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light).

Brightness

Frequency

also known as the Planck spectrum or Planck curve.

What does the spectrum of an astronomical object's radiation look like?

Many objects (e.g. stars) have roughly a "Black-body" spectrum:

• Asymmetric shape

• Broad range of wavelengthsor frequencies

• Has a peak

cold dust hotter star (Sun)

“cool" star

frequency increases, wavelength decreases

Approximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Law

very hot stars

Laws Associated with the Black-body Spectrum

Stefan's Law:

Energy radiated per cm2 of area on surface every second α T 4

(T = temperature at surface)

Wien's Law:

max energy

α 1T

(wavelength at which most energy is radiated is longer for cooler objects)

1 cm2

Betelgeuse

Rigel

Betelgeuse

The total energy radiated from entire surface every second is called the luminosity. Thus

Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)

For a sphere, area of surface is 4R2, where R is the sphere's radius.

Clicker Question:

A star much colder than the sun would appear:

A: red

B: yellow

C: blue

D: smaller

E: larger

Types of Spectra

1. "Continuous" spectrum - radiation over a broad range of wavelengths(light: bright at every color).

3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.

2. "Emission line" spectrum - bright at specific wavelengths only.

Kirchhoff's Laws

1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum.

2. A transparent hot gas produces an emission line spectrum.

3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.

The pattern of emission (or absorption) lines is a fingerprint of the element in the gas (such as hydrogen, neon, etc.)

For a given element, emission and absorption lines occur at the same wavlengths.

Sodium emission and absorption spectra

Demo - Spectra

Demo - Spectrum of the sun

Spectrum of Helium (He) Gas

Discovered in 1868 by Pierre Jannsen during a solar eclipseSubsequently seen and named by Norman Lockyer

Example: spectra - comet Hyakutake

The Particle Nature of Light

On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons.

cphoton energy is proportional toradiation frequency:

E α (or E α1

example: ultraviolet photons are more harmful than visible photons.

The Nature of Atoms

The Bohr model of the Hydrogen atom:

_

+proton

electron

"ground state"

_

+

an "excited state"

Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.

But, only certain energies (or orbits) are allowed:

__

_

+

The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels.

(photon energy αfrequency)

a few energy levels of H atom

When an atom absorbs a photon, it moves to a higher energy state briefly

When it jumps back to lower energy state, it emits a photon - in a random direction

Other elements

Helium Carbon

neutron proton

Atoms have equal positive and negative charge. Each element has its own allowed energy levels and thus its own spectrum.

So why absorption lines?

.

. .

.

.

.

.

..

.

.

cloud of gas

The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.

Why emission lines?

.

..

...

hot cloud of gas

- Collisions excite atoms: an electron moves into a higher energy level

- Then electron drops back to lower level

- Photons at specific frequencies emitted.

Ionization

+

Hydrogen

_

++

Helium

"Ion"

Two atoms colliding can also lead to ionization.

_

_

Energetic UV Photon

Atom

Energetic UV Photon

Clicker Question:

Astronomers analyze spectra from astrophysical objects to learn about:

A: Composition (what they are made of)

B: Temperature

C: line-of-sight velocity

D: Gas pressures

E: All of the above

Clicker Question:

Ionized Helium consists of two neutrons and:

A: two protons in the nucleus and 1 orbiting electron

B: two protons in the nucleus and 2 orbiting electrons

C: one proton in the nucleus and 1 orbiting electron

D: one proton in the nucleus and 2 orbiting electrons

E: two protons in the nucleus and 3 orbiting electrons