Astro 1 - Lecture 6

Matthew A. Bershady

Fall 1996

Lectures Lecture page Astro1 page

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?

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

_peak = 2900 / T (peak wavelength)

in units of

(micron = 0.001 mm = 0.0001 cm = 0.000001 m )

(micron = 10^-3 mm = 10^-4 cm = 10^-6 m )

(Angstrom (Å) = 10^-4

T in units of degrees Kelvin

degrees Kelvin = degrees Celsius + 273

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 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:

Q6.1 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?