Light the electromagnetic spectrum
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Light & the Electromagnetic Spectrum. Messenger from the Universe. Understand light’s nature Learn its language – what it can tell us about its source. (1) Speed of Light. Light travels damn fast…. Speed first measured by Ole Roemer (1675). Modern value : c = 300,000 km s -1

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Light & the Electromagnetic Spectrum

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Light & the Electromagnetic Spectrum

Messenger from the Universe

  • Understand light’s nature

  • Learn its language – what it can tell us about its source.

(1) Speed of Light

Light travels damn fast….

Speed first measured by Ole Roemer (1675)

Modern value : c = 300,000 km s-1

= 3 x 108 m s-1

(2) Nature of Color

Newton used a prism to find that white light

contains a rainbow of colors (ROYGBV)

(3) Light as waves

a) Huygens argued light is a wave

Waves spread out = diffraction

Incoming waves

Blurring spread

Newton thought light was a particle : “corpuscular”


Wavelength, λ

b) Thomas Young measured its wavelength

using two slit interference pattern


Multiple fringes

c) Light’s wavelength is very small: redblue

400700 nm

4000 7000Å

[roughly cell-sized]

(4) Waves of what?

Maxwell (1860) studied

electric & magnetic fields

Derived four equations:

Predicted E-M waves of

Velocity c=300,000 km s-1

 Light is an EM wave !!

(5) Other wavelengths

Over the next ~100 years,

e-m waves of differentλ discovered


Astronomers now use the entire electromagnetic spectrum

here are some examples of non-visual images 

Radio telescope





Need satellites to observe


(6) Atmospheric Windows

Parts of the em-spectrum are blocked by our atmosphere

(7) Frequency & the wave equation

Frequency, f = number of waves passing any point per second.


c = 300,000 km/s = 3*108 m/s

c = f l

Since f waves each of length λ pass in 1 sec, we have

Em-waves often have very large frequencies :

 The wave equation also reads f = c/λ

 recall: c is huge and λ is tiny  f is gynormous

Example: what is frequency of green light, λ = 500 nm :

f = 3 x 108 / 500 x 10-9 = 6 x 1014 Hz

= 6 hundred thousand billion oscillations per second

Notice: the units of f are Hz (Hertz) = # per sec; (s-1)

Recall: for radio, one often encounters MHz or GHz

Wavelength (m)

Frequency (Hz)

f and l are equivalent measures for an em-wave











(8) Light as particles = photons

Einstein (1905) explained the photoelectric effect:

Photons of light have energy proportional to frequency

E = h f = hc / λ

h = Planck’s constant = 6.6 x 10-34 J s (Joules x seconds)

Photon energies are very small

e.g. red light : f=5 x 1014 Hz

Ered = 6.6 x 10-34 x 5 x 1014 = 3 x 10-19 J

Flashlight of 30 Watt (= 30 J/s) emits :

30/3x10-19 = 1020 photons/s

= 100 billion billion per sec.

(9) Waves and/or particles?

Is light a wave or a particle ?

It has properties of both – one might say it is a “wav-icle”

A wave packet : localized bundle of waves

“Particles” also behave like waves

e.g. protons & electrons will diffract and interfere

In modern (quantum) physics, everything behaves this way.

(more massive things are more particle-like – eg a football)

  • When a chargeaccelerates (changes speed &/or direction)


b) When electrons jump between orbits within atoms


(10) Creation of Light

Two basic ways light is created :

Temperature scales:

Kelvin scale:

0 K = zero motion

“size” same as centigrade

[ K = C + 273 ]




(11) Thermal energy & temperature

What does it mean to say something is “hot”

Molecules/atoms are in constant jostling motion :

qualitatively: hotter faster motion

quantitatively: T proportional to < KEpcle>

(average kinetic energy per particle)

(12) Thermal Radiation

Bouncing molecules  photons

Spread of energies  range of colors

Higher energy bounce  bluer photons

Pure creation of light also means

pure absorption of light.

In this case  pure black

This gives “Black Body Spectrum”

= pure “thermal spectrum”

= unique spectral shape

depends only on temperature.

Stars approximate thermal spectra

(13) Two Characteristics

(a) Hotter objects  curve moves to blue

Wein’s Law : λpeak(nm) = 3x106 / T(K)

(b) Hotter objects  more photons made

Stefan-Boltzmann Law : E = σ T4

E in Watt per square meter

T in Kelvin

σ is Stefan’s constant = 5.7 x 10-8



10 µm



  • Atoms are tiny ~ 10-10 m

(b) Nucleus much smaller ~ 10-5 atom

contains 99.95% of the mass; +ve charge

(d) # neutrons defines isotope :

e.g. C12 : 6p6n ; C13 : 6p7n ; C14 : 6p8n

(14) Atoms & Particles

We need to understand these for two reasons :

(i) electrons jumping  colors/light  diagnostics

(ii) nuclei combing  powers stars  makes elements

(c) # protons defines element :

1:H, 2:He, 3:Li, …. 6:C, 7:N,…. 26:Fe, …. 92:U

Cosmic abundances for all

isotopes of all elements

(relative to Si = 106)

α elements

Iron peak

“rare” earths




(f) Orbits

since e- attracts  nucleus+

& e’s less massive than nucleus

(analogous to earth orbiting sun)

However, not all orbits are possible:

must have whole number of waves.

1,2,3 waves  shells 1,2,3

e’s fit in nested shells,

fill inner (lowest energy) ones first.

} e’s orbits nucleus

(e) Electrons : 1/2000 mass of p,n ; –ve charge

if #e = #p  neutral atoms

if #e ≠ #p  charged ions (usually fewer e)

Three types of spectra:

consider also graphs of

spectra – not just bars






Electron ejected,


(15) Light  Atom interactions

Photons interact quite strongly with electrons in atoms

  • electron jumps down photon emitted

  • electron jumps up  photon absorbed


Energy of photon = energy difference

between orbits

Ephoton= hf = hc/λ = E4– E3




Single colors emitted/absorbed

Color set depends on orbit set

Different for each atom.

c) electron ejected(ionization) energetic photon absorbed

Emission line spectra from specific atoms

The specific set of lines is unique to each atom.

It is like a “bar-code” identifier, or a fingerprint.

(16) Hydrogen Atom & its Spectrum

Atomic orbits are often redrawn as energy level diagrams

Bohr orbit figure

Energy level diagram







Hydrogen spectra are particularly simple

Several line series: Lyman (UV); Balmer (optical); Paschen (IR)……

Hydrogen Spectrum








1st three lines of

the Balmer series

Hα , Hβ , Hγ ,

Nebulae often appear pink

due to strong Balmer Hα

emission at 656nm from the

hydrogen gas.

Continuous spectrum :

hot solid or dense gas

Emission spectrum : hot rarefied gas

Absorption spectrum :

Continuous source seen

through hot thin gas

(17) Kirchoff’s Laws

Define conditions for the formation of spectra

Hot dense gas below a cooler

thin gas (atmosphere)

The Sun’s Spectrum has many absorption lines

Two representations of an

absorption line spectrum

showing three Balmer lines

of hydrogen.

Star spectra have absorption lines

Balmer lines are weak in v. hot stars

 all hydrogen is ionized

Balmer lines are weak in v. cool stars

 need electrons in n=2 level

Sequence of line strengths

ionized He

neutral He

hyd. Balmer

ionized “metals”

neutral “metals”


v. hot ~ 20,000K








“cool” ~ 3000K

(18) Absorption line strengths

The strength of an absorption line depends mainly on temperature

(19) Spectral sequence

Oh Be A Fine Girl/Guy Kiss Me

Detailed Sunlight spectrum – with many absorption lines

(20) Abundance analysis

Absorption line strength depends on temperature and

abundance– how much element is present

Possible to measure/calculate relative abundances :

~all stars have ~74% Hydrogen

~24% Helium

~2% all others

(Star surfaces, not interiors).

(21) Doppler Shifts: velocity

Moving light source changes apparent wavelength:

Towards/away  blue/red shift

Only radial velocity component, Vr , affects wavelength

Vr/c = (λ – λ0)/ λ0

Binary star in orbit

(22) Line widths: gas pressure

Some stars have wider lines than others.

Higher gas pressure blurs the atom energy levels

 gives broader line.

Dwarf stars  compressed atmosphere  broad lines

Giant stars  fluffy low pressure atmosphere  narrow lines



High pressure

Broad line

Low pressure

Narrow line

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