Waves, Photons & the

EM Spectrum

Astronomers obtain information about the universe mainly via analysis of electromagnetic(em) radiation:

visible lightradio wavesx-raysinfrared radiationand so on . . .

EM radiation sometimes behaves like waves,sometimes like particles!

Waves

WAVES A wave is a moving disturbance.

Two kinds of waves in a slinky.

The slinky is the wavemedium.

[Wave animations]

crest

trough

Wavelength ()

Amplitude

Words that describe waves . . .

Period (T): time for one wave to pass a pointFrequency (f): # of waves passing a point per second

Compare two waves:

Long wavelength

Short wavelength

Short wavelength short period, high frequencyLong wavelength long period, low frequency

[Animation . . .]

Electromagnetic Waves

Oscillating magnetic and electric fields

Sources: accelerated charge (e.g., vibrating electrons)

Travel through empty space (no

medium)

Travel at speed of light (c) in vacuum:

c = 300,000 km/sec = 186,500 mi/sec

Electromagnetic Wave

Motion

MagneticField

ElectricField

Electromagnetic Spectrum:

Span of all em wavelengths

Visible: part we can see.

p. 101

Visible Spectrum

IR UV“ROY G. BIV”

Units:

Nanometer (nm): 1 nm = 10-9 meter

Ångstrom (Å): 1 nm = 10 Å

Photons

1900 – 1905: Max Planck & Albert Einstein find light sometimes behaves like particles: photons

Photons carry energy (E): E Frequency (E f), orE 1/Wavelength (E 1/)

Long wavelength Low energy

Short wavelength High energy

Interaction of Light & Matter

1. Emission

2. Absorption

3. Transmission

4. Reflection

Continuousemission bya solidBoy

Dog

Infrared

‘Cool’

‘Warm’ ‘Hot’

Continuous emissionby dense gas (Stars)

Warm

Cool

Selective emission by a thin gas

Selective emission by a thin gas

white light

Selective reflection& absorption by solids

selective reflection& absorption by solids & gases

Spectra

I procured a triangular glass prism, to try therewith, thecelebrated phenomena of colors. And for that purpose,having darkened my laboratory, and made a small holein my window shade, to let in a convenient quantity ofthe sun’s light, I placed my prism at the entrance, thatthe light might be thereby refracted to the opposite wall.It was at first a very pleasing diversion to view the vividand intense colors produced thereby.

- Isaac Newton

A spectrum is produced whenever light fromany source is broken-up into its constituent wavelengths (or frequencies):

Prism(Disperses light)

IncomingLight

Spectrum

Three Types of Spectra

1. Emission (Bright) Line

Bright lines on a dark background

2. Absorption Line

Dark lines on a bright background

3. Continuous

Continuous band of colors

p. 105

Emission Line Spectra

H

Na

He

Ne

Hg

Note: unique pattern for each element.

Inte

nsi

ty

Wavelength

Absorption Line Spectra

The Sun’s Spectrum

All three kinds of spectra

Emission/Absorption patterns identical!

En

ergy

Wavelength

Hydrogen

p. 102

Spectrum not equally bright (Intense) at each point . . .

Measure intensity at each wavelength,then plot intensity vs wavelength . . .

Continuous Spectra

. . . You get this:

violet red

Two rules of black bodies

1. As temp (T) increases, more energy is emitted from each unit surface area.

Amt. of energyemitted from each sq meter

T4

2. As temp (T) increases, the peak of the BB curve shiftsto shorter wavelength.

A Couple of Rules for Black Bodies

Compare two stars:

Betelgeuse: T 3,000 K

Rigel: T 12,000 K

Orion

Inte

nsi

ty

7000 K

6000 K

5000 K

As temp drops,location of peakdrifts to longer

wavelength.

Wavelengthp. 104

400 nm 700 nm

“Hot:” Blue

“Cold:” Yellow

. . . So the Color Changes

‘Cool’

‘Warm’ ‘Hot’ p. 103

Spectrum of the Planet Mars (Complicated!)

p. 106

The Doppler Effect

The Doppler Effect: Change in observedwavelength and frequency of waves due toradial motion of source and/or observer.

Observer

Source

Wave crests

No sourcemotion: nochange in

f or λ

Motion towardobserver:

f increases &λ decreases

Motion awayfrom

observer: f decreases & λ increases.

No change inf & λ here!

Doppler animations . . .p. 100

. . . For a star moving toward/away from Earth . . .

Astronomically speaking . . .

. . . We find a shift in the absorption (or emission) lines:

Star moving toward Earth lines shifted toward shorter wavelength: Blueshift

Star moving away from Earth lines shifted toward longer wavelength: Redshift

In either case,

velocity amt of wavelength shift

v

Galaxy spectra –

all redshifted

Larger shift Larger Velocity

1200 km/s

15,000 km/s

39,000 km/s

61,000 km/s