Astro 103 - Lecture 3

Lectures Lecture page Astro103 page


PARTICLES, WAVES, BLACKBODIES AND LINES


1. Electro-magnetic radiation

Visible light is but one form of electro-magnetic radiation      

Here all of them:      

gamma-rays

X-rays

ultraviolet light

visible light

infrared light

microwaves

radio waves

What's the difference?

wavelength () or frequency ()      

= c /

(c is the speed of light)

energy 1/

The shorter the wavelength, the more energy the radiation has.


What do we see on Earth from the sun?      



2. Blackbody radiation


A blackbody is defined to be a perfect thermal emitter

Electromagnetic radiation is emitted from a blackbody over a range of frequencies (wavelengths)

The distribution of radiation over wavelength (the ``spectrum'') has a distinct shape

The wavelength where the radiation peaks (is most intense) is proportional to the temperature T

The shape of the spectrum remains the same with T, but shifts to bluer (shorter) wavelengths with increasing temperature, or to redder (longer) wavelengths with decreasing temperature.      


Why the blackbody spectrum has it's shape.


Wien's Law

peak = 2900 / T (peak wavelength)

in units of microns

(micron = 0.001 mm = 0.0001 cm = 0.000001 m )
(micron = 10-3 mm = 10-4 cm = 10-6 m )
(Angstrom (Å) = 10-4 microns)

T in units of degrees Kelvin

degrees Kelvin = degrees Celsius + 273

Stefan's Law

energy T4



3. Particle-Wave duality


Sometimes light (electro-magnetic radiation) behaves like waves

- diffraction of light through a slit

Sometimes light behaves like particles (photons)

- discrete detection (photographs, CCDs)
- discrete amounts of energy in atomic transitions

Light is both a wave and a particle (a photon)!!!



What's Going On?

How can a particle be a wave, and a wave a particle?

What you measure depends on how you measure it!

If you measure the spatial position of the radiation, light behaves like a particle.
If you try and measure the wavelength (or energy), then light behaves like a wave.

This is related to the ``uncertainty principle'' in quantum mechanics:

The position and momentum of a particle/wave cannot both be known with infinite accuracy.

The same idea is true for all fundamental ``particles'' such as electrons, protons, neutrons, etc. Sometimes they behave like waves; other times they behave like particles.



Q3.1 The sun's surface temperature is around 6000 degrees Kelvin. If it were to suddenly become hotter, where would most of it's radiation be emitted?

(a) in the optical, ultraviolet, x-ray or gamma ray region, depending on the temperature change

(b) in the optical

(c) in the optical, infrared, microwave or radio region, depending on the temperature change

(d) uniformly at all wavelengths

(e) in the infrared where it is hottest


Lectures Lecture page Astro103 page
Last updated: Aug 23, 2011 Matthew A. Bershady