03 Law of Radiation

1. Light & Radiation

How do astronomers know anything about objects far from Earth
without traveling to them?

How do we obtain detailed information about any planet, star, galaxy
too distant to travel or to perform controlled experiment?

Andromeda Galaxy. What do you see in the picture?

Interpret the electromagnetic radiation emitted by these objects

  • Radiation: transmitting energy between two points with no physical contact between the points

  • Electromagnetic: Energy is carried in the form of electric and magnetic fields.

For example:

  • Visible Light: part of EM spectrum that human eye is sensitive.

  • Invisible Light: part of EM spectrum undetected by human eye.

(Radio, Infrared, Ultraviolet, X-rays, Gamma-rays).

EM spectrum. It contains both visible (to human eye) region and invisible regions.

Therefore, the following terms refer almost to the same thing:


2. Wave motion

All EM radiation travel through space in the form of waves.

  • Wave is a way in which energy is transferred from one place to another without the physical movement of material from one location to another.

  • Wave motion: The energy is carried by a disturbance of some sort.

  • A wave is not a physical object:

Pattern of up and down motion.

If a wave moves at HIGH (SLOW) speed

the number of crests/through passing any given point per unit time is


Waves of Radiation ≇ Waves of Water/Sound

(doesn't need medium) (does need medium)

3. Light

  • Color = frequency/wavelength of light

(e.g. Red: 700 nm, Violet: 400 nm)

  • Measuring Wavelength:

4. Charged Particles

  • a charged particle : MASS + CHARGE

(note that mass and charge are properties of the matter)

  • electron (e) or proton (p):

    • building blocks of atoms

    • they carry the basic unit of charge

    • this is true for every charged particle and for every other charged particle in the universe

Electric Force

(attractive or repulsive)

≈ Gravitational Force ∝ 1 / r2

≉ (always attractive)

5. Transmission of electric force

Electric Field

Electric Field and Charged Particles. Electric field lines extends from the charged particles. These lines are a measure of its force exerted to other charged particles

Transferring Information Through Waves. Particles vibrates (heat, collision etc.):

→ position changes

→ electric field changes

→ the force exerted on other particles changes

By measuring variations (i.e. on waves) of Electric Field on the distant charges one can gather information from actual vibrating particles.

Magnetic Field

  • Magnetic Field must accompany every changing Electric Field.

  • Magnetic Field exerts forces on moving electric charges (i.e. electric currents).

  • Conversely, moving charges create magnetic fields.

Thus, Electric and Magnetic Fields are linked to one another. A change in either one necessarily creates the other.

Note also that:

  • moving charge create disturbance (consists of both E and B).

  • E and B are perpendicular to one another.

  • They don't exist as independent entities.

  • They are different aspects of the same phenomenon: ELECTROMAGNETISM.

Speed of Transmission

  • How quickly does one charge feel the change in the EM field when another begins to move?

    • EM wave moves at c (speed of light): c = 299 792.458 km/s ~ 300 000 km/s

  • It is large but finite !

    • We can never observe the universe as it is, only as it was.

6. EM spectrum

Electromagnetic Spectrum. Comparing all at once: colors, bands, frequency, wavelength, size, atmosphere and opacity.


  • The more opaque an object is, the less radiation gets through it (opposite of transparency).

  • Opacity varies; why?

    • Certain atmospheric gases absorb radiation at some wavelengths.

    • H2 and O2 → absorbs ... Radio waves ( ƛ < 1 cm)

    • H2O and CO2 → absorbs ... Infrared waves

    • Ozone layer → absorbs ... UV, X-ray, Gamma rays

    • Ionosphere → reflects ... ƛ > 10 m. (the layer is at 100 km above sea level)

7. Thermal radiation

  • All macroscopic objects emit radiation at all times regardless of their size, shape or chemical composition.

  • Temperature: Temperature of an object is a direct measure of the microscopic motion within it.

  • Intensity: The amount/strength of radiation at any point in space.

  • Energy is generally spread out over a range of frequencies.

    • not just one frequency

distributionproperties of the object can be reached.


An object that absorbs all radiation falling on it and it must re-emit the same amount of energy it absorbs.

  • No real object absorbs and radiates as a perfect black body. However, the blackbody curve is a good approximation to the reality.

  • As the object's temperature increases

→ radiation's peak frequency shifts

→ however shape of the curve remains the same

  • Well known experience:

as the temperature of the object increases

the color of the object changes:

→ normal color

→ beginning to glow in red

→ red hot

→ white hot

Wien's Law

So, the relation between wavelength and absolute temperature is:

(where ƛ is wavelength of the peak emission)

Reality and Applications in Astronomy

  • No natural object reach temperatures high enough to emit radiation at very-high-frequencies

  • e.g. thermonuclear explosions

➨ peak in X-ray / Gamma-ray

  • e.g. high-frequency radiation from the most human invented devices

➨ objects cannot attain high temperatures

➨ i.e non-thermal radiation

  • Many extraterrestrial objects

➨ radiation in UV, X-ray even Gamma-ray

Thus, blackbody curves are used as thermometers to determine the temperature of distant objects

8. Doppler Effect

When either the observer or the object is in motion the EM waves received by the observer shifts according to the direction of the motion.


  • observing stars which rotate with very high velocities

  • listening sound waves near the traffic.

9. Spectroscopy

Spectrum: a splitting of the incoming radiation into its component wavelengths.

  • All spectra deviate from its idealized form (i.e. Blackbody Curve).

  • However; this deviation contains the detailed information about physical conditions in the source of the radiation.

Spectroscope: the instrument to analyze the radiation.

Type of Spectrums

Continuous Spectrum

Radiation in all wavelengths with an intensity distribution that is well described by the blackbody curve

Check the screen:

  • observe rainbow of colors without interruptions

Absorption Spectrum

Wavelengths of light that have been removed (absorbed) by the gas between the source and the detector.

Check the screen:

  • Absorption lines associated with a gas occur at precisely the same wavelengths as the emission lines produced when the gas is heated.

Emission Spectrum

The particular pattern of light emitted by a gas of a give chemical composition.

Check the screen:

  • a few narrow well defined lines

  • with a black background (i.e not emitted by Hydrogen)


  • Intensity can be altered but not the wavelength.

Kirchoff Laws (1859)

  1. A luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum

  2. A low density, hot gas emits light whose spectrum consists of a series of bright emission lines that are characteristic of the chemical composition of the gas.

  3. A cool, thin gas absorbs certain wavelength from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum.

10. Atomic Structure

Classical Atom

Modern Atom

  • How: ground state ➨ excited state by

    • atom absorbs EM radiation

    • matter collides with another matter

  • How long: ~ 10-8 s then drops to ground state

  • How much: Energy frequency ➨ E = h f (where h=6.63 × 10-24 J)

Spectral Information

  • Observe Peak Frequency (for continuous spectrum only) ➨ Derive Temperature (using Wien's Law)

  • Observe Lines or Line Intensities ➨ Derive Composition Temperature

  • Observe Line Width ➨ Derive Temperature and/or Rotation Speed and/or Density

  • Observe Doppler Shift ➨ Derive Line of Sight Velocity