Light and the Electro-

magnetic Spectrum

Light and the Electromagnetic Spectrum

• Almost all of our information on the heavens is derived from the light we see

• We have returned samples from the Moon and a comet – Also obtained meteor samples

• We have landed (with unmanned probes) on only a handful of planets and moons

EM Spectrum in Astronomy

• If we could only observe in visible light, our knowledge of the universe would be greatly limited

• By looking at objects at different wavelengths, we get a different view and lots more information

• Some objects are only visible at certain wavelengths

Objects in space that emit

different frequencies on

the EM Spectrum

The Electromagnetic Spectrum• Human eyes are only able

to process information from the visible part of the spectrum

• Toward longer wavelengths, the spectrum includes infrared light, microwaves, and radio

• Toward shorter wavelengths, the spectrum includes ultraviolet light, X-rays, and gamma rays

• All of these are forms of electromagnetic radiation

Wavelength• The distance from one wave crest to the

next

• Radio waves have longest wavelength and Gamma rays have shortest!

What do we mean by wavelength?Light as a Wave

• One way to think about light is as a traveling wave

• A wave is just a disturbance in some medium (water, air, space)

• A wave travels through a medium but does not transport material

• A wave can carry both energy and information

Wave Terminology

• Wavelength - distance between two like points on the wave• Amplitude - the height of the wave compared to undisturbed state

• Period - the amount of time required for one wavelength to pass• Frequency - the number of waves passing in a given amount of time

Wave Relationships

• Note that light is always traveling at the same speed (c ~ 3 x 108 m/s, or 300,000 m/s, or 186,000 miles/sec, or 7 times around the earth in 1 second!)

– Remember: velocity = wavelength x frequency• If frequency increases, wavelength decreases• If frequency decreases, wavelength increases

Wavelength Units

• Meters: More commonly in nanometers (1 nm = 10-9 meters)

• Angstroms still used– Named for Swedish Astronomer who first

named these wavelengths– 1 nanometer = 10 Ao

Wavelengths of Light - Visible

• What we see as white light is actually made up of a continuum of components

• Traditionally, we break white light into red, orange, yellow, green, blue, indigo, and violet (ROY G BIV)

• There is actually a continuous transition of color, each with its own wavelength and frequency

Bright Line Spectra

Doppler Effect

• The motion of an object can be measured through a change in the frequency of the waves emitted by the object

• The increase in pitch of an approaching police car is caused by the compression of the sound wave– The pitch decreases as the police car moves away

Doppler Shift

• In astronomy, the same effect happens to light waves

• A source that is moving away will appear redder (redshift)

• A source that is moving toward us will appear bluer (blueshift)

• Note: Only objects moving toward or away from us (radial motion) will show this effect

Which spectrum (spectra) indicates an object is moving towards earth?

Which spectrum (spectra) indicates an object is moving away from earth?