Electromagnetic Radiation

(How we get information about the cosmos)

Examples of electromagnetic radiation?

LightInfraredUltravioletMicrowavesAM radioFM radioTV signalsCell phone signalsX-rays

Clicker Question:

Which of the following is not an electromagnetic wave:

A: radio waves

B: visible light

C: X-rays

D: sound waves

E: gamma-rays

Radiation travels as waves.Waves carry information and energy.

Properties of a wave

wavelength (l)

crest

amplitude

velocity (v)trough

l is a distance, so its units are m, cm, or mm, etc.

Period (T): time between crest (or trough) passages

Frequency (ν): rate of passage of crests (or troughs), n =

Also, v = λ ν

1T

(units: Hertz or cycles/sec)

Radiation travels as Electromagnetic waves.

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

Examples of objects with magnetic fields:

a magnetthe Earth

Examples of objects with electric fields:

protonselectrons } "charged" particles that

make up atoms.

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 5 km / s

Sun

Earth

light takes 8 minutes

c = λν or, bigger λ means smaller ν

c =

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

The Electromagnetic 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

The "Inverse-Square" Law Applies to Radiation

apparent brightness α 1D2

α means “is proportional to”. D is the distance between source and observer.

Each square gets 1/9 of the light

Each square gets 1/4 of the light

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.

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

Why is the Sun yellow? The Black-Body Spectrum

cold dust average star (Sun)

frequency increases, wavelength decreases

Approximate black-body spectra of stars of different temperature

very hot stars

“cool" stars

infrared visible UV

infrared visible UVB

righ

tnes

s

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:

λmax energy

α 1T

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

1 cm2

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

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

For a sphere, area of surface is 4πR2, where R is the radius.

So Luminosity α R2 x T4

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

Clicker Question:

Compared to blue light, red light travels:

A: faster

B: slower

C: at the same speed

Clicker Question:

A star much colder than the sun would appear:

A: red

B: yellow

C: blue

D: smaller

E: larger

Betelgeuse

Rigel

Applies to all waves – not just radiation. The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

The Doppler Effect

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!

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.

Clicker Question:

If a star is moving rapidly towards Earth then its spectrum will be:

A: the same as if it were at rest

B: shifted to the blue

C: shifted to the red

D: much brighter than if it were at rest

E: much fainter than if it were at rest

Spectroscopy and Atoms

How do we know:

- Physical states of stars, e.g. temperature, density.

- Chemical make-up and ages of stars, galaxies

- Masses and orbits of stars, galaxies, extrasolar planets

- expansion of universe, acceleration of universe.

All rely on taking and understanding spectra: spreading out radiationby wavelength.

Types of Spectra and Kirchhoff's (1859) Laws

1. "Continuous" spectrum - radiation over a broad range of wavelengths (light: bright at every color). Produced by a hot opaque solid, liquid, or dense gas.

2. "Emission line" spectrum - bright at specific wavelengths only. Produced by a transparent hot gas.

3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines. Produced by a transparent cool gas absorbing light from a continuous spectrum source.

The pattern of lines is a fingerprint of the element (e.g. hydrogen, neon) in the gas.

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

Sodium

The Particle Nature of Light

On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. (Einstein 1905).

cphoton energy is proportional toradiation frequency:

E (or E 1

example: ultraviolet photons are more harmful than visible photons.

The Nature of Atoms

The Bohr model of the Hydrogen atom (1913):

_

+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. Number of protons defines element. Isotopes of element have different number of neutrons.

Ionization

+

Hydrogen

_

++

Helium

"Ion"

Two atoms colliding can also lead to ionization. The hotter the gas, the more ionized it gets.

_

_

Energetic UV Photon

Atom

Energetic UV Photon

So why do stars have absorption line spectra?

..

. ..

..

..

.

.

Simple case: let’s say these atoms can only absorb green photons. Get dark absorption line at green part of spectrum.

hot (millions of K), dense interiorhas blackbody spectrum,gas fully ionized

“atmosphere” (thousandsof K) has atoms and ionswith bound electrons

Stellar Spectra

Spectra of stars differ mainly due to atmospheric temperature (composition differences also important).

“hot” star

“cool” star

Why emission lines?

.

..

...

hot cloud of gas

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

- Then electron drops back to lower level

- Photons at specific frequencies emitted.

We've used spectra to find planets around other stars.

Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines.

Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.

Now more than 300 extrasolar planets known. Here are the first few discovered.

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.

Molecules

Two or more atoms joined together.

They occur in atmospheres of cooler stars, cold clouds of gas, planets.

Examples

H2 = H + H

CO = C + OCO

2 = C + O + O

NH3 = N + H + H + H (ammonia)

CH4 = C + H + H + H + H (methane)

They have - electron energy levels (like atoms) - rotational energy levels - vibrational energy levels

Molecule vibration and rotation

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 (1859)

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.