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

For example:

(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.

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

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

4. Charged Particles

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

Electric Force

(attractive or repulsive)  

≈    Gravitational Force

≉                     (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

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

Note also that:

Speed of Transmission

6. EM spectrum

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


7. Thermal radiation

→  distribution  →  properties 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.

→ radiation's peak frequency shifts

→ however shape of the curve remains the same

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

➨ peak in X-ray / Gamma-ray

➨ objects cannot attain high temperatures

➨ i.e non-thermal radiation

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

Thus, black body 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.


9. Spectroscopy

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

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:

Absorption Spectrum

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

Check the screen:

Emission Spectrum

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

Check the screen:


Kirchoff Laws (1859)

10. Atomic Structure

Classical Atom

Modern Atom

Spectral Information