The Electromagnetic Spectrum

When a beam of white light passes through a glass prism, the light is separated or refracted into a rainbow-colored band called a spectrum. The numbers on the right side of the spectrum indicate wavelengths in nanometers (1 nm = m).

Electromagnetic Waves

We Can’t Always See Energy

The transparency of a material depends on the wavelength of light. Earth’s atmosphere is relatively transparent to visible light and radio waves, which are referred to as “windows” through which we can view space from a ground based telescope.

WHEN FIRST HEATED THE POKER GLOWS DIMMLY RED AS THE TEMPERATURE RISES, IT BECOMES BRIGHTER ORANGE AT HIGHER TEMPERATURES IT BECOMES BRIGHTER AND YELLOW These stars have roughly the same temperatures as the bars above.

This is a plot of intensity versus wavelength for blackbodies at different temperatures. At higher temperatures the most intense wavelengths are shorter. Since the observed color depends on these emitted wavelengths, blackbodies at different temperatures will appear different colors.

Stellar surfaces emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.

When a chemical is burned, the light produced is made of only specific wavelengths. Different chemical elements have their own series of wavelengths.

When an electron moves from a lower energy to a higher energy level a photon is absorbed. When an electron moves from a higher energy to a lower energy level a photon is emitted. The energy of the photon, and thus its wavelength, are determined by the energy difference between the two energy levels.

The Doppler Effect The radial velocity of a star is its motion along our line of sight either toward or away from us. Using the spectrum we can measure this for nearly every object in space!

The Doppler Effect Recall that a the wavelength of light, and therefore the wavelength of the photons that light contains, is slightly shifted when the source is traveling toward or away from the observer, the Doppler Effect.

C. Doppler Effect We experience the Doppler Effect in sound waves produced by a moving wave source  moving toward you - pitch sounds higher  moving away from you - pitch sounds lower

C. Doppler Effect Stationary sourceMoving sourceSupersonic source same frequency in all directions waves combine to produce a shock wave called a sonic boom higher frequenc y lower frequenc y

The Doppler Effect If the object is moving toward you, the waves are compressed, so their wavelength is shorter. The lines are shifted to shorter (bluer) wavelengths--- this is called a blueshift. If the object is moving away from you, the waves are stretched out, so their wavelength is longer. The lines are shifted to longer (redder) wavelengths--- this is called a redshift.

Doppler Effect Speed The faster the star(object) is moving the greater the spectral shift

1.Wavelength shift: = new - rest. rest is the wavelength measured if the object is at rest and new is the wavelength measured for the moving object. 2.Doppler effect: = rest × V radial /c, where V radial is the object's speed along the line of sight and c is the speed of light. 3.If new > rest, the object is moving away (redshift). 4.If new < rest, the object is approaching (blueshift).