The Electromagnetic Spectrum

! What is Electromagnetic Radiation? ! What are spectra? How could we measure a spectrum?! How do wavelengths correspond to colors for optical

light? Does red or blue have the longest wavelength? ! What is the speed of light? Is it infinite? How was it first

measured? Can anything travel faster than light?! What is the Doppler Shift? A redshift? A blueshift? How

can we use the Doppler Shift to measure the speed at which very distant objects are moving?

! What are the types of Electromagnetic Radiation? Can they all penetrate the Earth’s atmosphere? Can you order them by wavelength? By the energy they carry?

Learning Objectives

Light & Astronomy! Astronomers usually cannot control experiments! We get information from the objects we study

(stars, planets, galaxies) by observing the “light” the objects emit

! We use telescopes to collect the light that comes to the Earth from distant objects

! To be able to extract the maximum information from the light we collect, we must understand light…

Colors of the Rainbow! Newton used a prism to separate white light

into its component colors! Called a spectrum

! He then used a second prism to recombine the spectrum back into white light

! This demonstrated that white light is a mixture of a spectrum of many colors

Particle or Wave?! How does light behave?

! Huygens: light travels in the form of waves of energy

! Newton: light is composed of a large number of particles

Light is a Wave!! In 1803, Thomas Young demonstrated that light

travels as a wave! When light is passed through a double slit, a

pattern of light and dark bands is produced! This can only be explained by wave-like behavior

How Do Colors Arise?! The color of light is determined by its

wavelength! Visible light has extremely small wavelengths

! Usually measured in nanometers (billionths of a meter)

! Wavelengths range from 400 nm (violet light) to 700 nm (red light)

! Colors, from longest wavelength to shortest:red, orange, yellow, green, blue, violet

The Speed of Light! The ancient Greeks thought the speed of light was infinite! In the 1600s Galileo realized “very fast” is not “infinite”

! Galileo was the first to suggest an experiment to measure the speed of light, in which two people with lanterns signal each other

! This isn’t possible over distances of a few miles because of the extreme speed at which light travels

~ 2 miles: 10 microseconds

Speed = 2 × separation / time

The Speed of Light! In 1676, Ole Rømer saw that Io passed behind Jupiter

sooner in time when the Earth was closer to Jupiter! He concluded that light from the event took longer to

reach us when the Earth was farther from Jupiter! He measured that light travelled 8¼ minutes per AU! Today, we know the speed of light to be c = 3×108 m/s

(186,000 miles per second, or 8.3 minutes per AU)

June

December

~16.5mins delay, extra 2AU distance

! The Doppler Effect arises from the relative motion between an observer and a source of light (or sound)

! Waves are squashed in the direction of motion! For light waves, this is called a blueshift

! Waves are stretched in the direction opposite the direction of motion! For light waves,

this is called a redshift

Velocity and the Doppler Effect

Electromagnetic Radiation! In the 1860s, James Clerk Maxwell created new

theories of electricity and magnetism

! These theoriessuggested that light is traveling electric and magnetic waves

! Which is why we refer to light aselectromagnetic radiation

Electromagnetic wave direction

Magnetic FieldElectric Field

“Invisible” Forms of Electromagnetic Radiation

! Visible light has a range of wavelengths from 400-700 nm (nanometers)

! But what about light with longer or shorter wavelengths?

! Maxwell’s theories placed no restrictions on the possible wavelengths of light

! There are types of light outside of “optical” or “visual” wavelengths (the range of wavelengths that we can see with our eyes)

Light is also a Particle!! In 1905, Einstein showed that light also

behaves as a particle! Light particles are called photons! Particles, like little bullets,

have energy! The energy of a photon

decreases as the wavelength of the light increases

! If you half the wavelength of a photon oflight, you double its energy

The energy of E-M radiation that is emitted by a process depends on the atoms and molecules responsible for that process. In general, “violent” processes are energetic and emit short wavelengths (UV, x-rays). “Gentle” processes are less energetic and emit long wavelengths (IR, radio)

Opaque Atmosphere!The atmosphere blocks some wavelengths!This means that IR, UV, X-rays, gamma-rays are

best observed from space or, at least, from high in our atmosphere

Important bad drawing

Next Time

How Hot is a Star?