Lecture 9: Light the Messenger

Astronomy 1143 Spring 2014

Key IdeasElectromagnetic spectrum

Energy, wavelength, frequency

Spectrum - amount of light at each wavelength

Nature of light – wave – particle (photon)

Speed of light is a constant for all observers• Surprising! Postulate of Special Relativity• But confirmed by observations• We see distant objects as they were

A light-year is distance light travels in 1 year

Light the MessengerAlmost all information we receive about the Universe comes

in the form of light

Before discussing the size of the Milky Way Galaxy, the distances to nearby galaxies and the expansion of the Universe, we need to get some facts under our belts• How to tell if a star is like the Sun

• Temperature• Absorption lines

• Light has a wavelength, which can be stretched or squeezed

• How knowing brightness and luminosity = distance• Light is special – start of Special Relativity

Electromagnetic Radiation

Light is Electromagnetic Radiation.• Self-propagating Electromagnetic disturbance

that carries energy at the speed of light.

Two ways to describe light:• Electromagnetic Waves

• Photons (particles of light)

Light is a Wave

Wave = a periodic fluctuation traveling through a medium.

Ocean wave = fluctuation in height of

water.

Sound wave = fluctuation in pressure.

Electromagnetic wave = fluctuation in electric

and magnetic fields.

Oscillating Electric & Magnetic Fields

Wavelength ()

Frequency (f) (# waves/second)

Speed (c)

c = f

Wavelength ()

Frequency (f) (# waves/second)

Speed (c)

c = f

Different than water waves

If you run into the waves, the wavelength stays the same, but the frequency will increase – speed increases

Most waves need a medium

But light doesn’t need a medium

The Electromagnetic Spectrum

←medical X rays

←microwave oven

←tanning rays

←cell phones←WOSU FM

←WOSU AM

Waves show diffraction

Example of sea waves & diffraction

Waves vs. Particles

Proof that light is a wave

Radio Waves Diffract!

Photons: Particles of Light

Can also treat light as particles or Photons.

Photon:Massless particle that carries energy at the

speed of light.

Photon Energy:

E = hff = frequency of the lighth = Planck’s Constant

Light is made of particles.

Light shows some properties of particles, such as the photoelectric effect.

Photons kick electrons out of atoms, if they

have enough energy.

Proof that light is a particle

Individual X-ray photons

Wave or particle?

Both

Light has properties of both waves and a stream of particles. It follows the rules of quantum mechanics

Other quantum mechanical particles include the electron, proton and neutron

Electrons are not just particles! They also show diffraction.

Why describe light by both?

Important to think about light as a wave:• Stretching of wavelengths

Important to think about light as a particle:• Knocking electrons off of atoms• Force carrier – part of the Standard

Model of Particle Physics

The Electromagnetic Spectrum

Sequence of photon energies from low to high is called the Electromagnetic Spectrum

low energy = low frequency= long wavelengthExamples:

Radio Waves, Microwaves, Infrared

high energy=high frequency=short wavelengthExamples:

Ultraviolet, X-rays, Gamma Rays

Photons as Dogs

Low energyLong wavelengthLow frequency

High energyShort wavelengthHigh frequency

Most objects emit more than one wavelength of light

Our eyes and brain report one color of light, the dominant color

Primary colors for mixing light – red, green and blue

If all colors of visible light are about equally represented – white light

What is a Spectrum?

A spectrum is the distribution of photon energies emitted by a light source:• Asks: how many photons of each energy

are emitted?

Spectra are observed by passing light through a spectrograph:• Breaks light into its component colors.• Uses either prisms or diffraction gratings.

SpectrumPrism

WhiteLight

Prisms disperse light into its component colors

In Class Demo

Gratings and white light

Lasers

The Beginnings of RelativitySpeed of light is a constant for all light waves

or photons:

c = 299,792.458 km/secIndependent of wavelength or frequency.

It also doesn’t matter how fast the light source is moving or how fast the observer is moving.

Light very different than other waves

Light is special

Common Sense, But Incorrect, Ideas

Time is absolute: The time between two events is independent of the observer. Events are always in the same order.

Space is absolute: The distance between two events is independent of the observer.

Speeds are relative: How fast something is moving depends on how fast you are moving

Test: Michelson-Morley experimentMichelson-Morley experiment showed that

light is different

They attempted to measure the change in the speed of light as the Earth moved around the Sun through “the ether”

They found that the speed of light did not change

Experimental results helped lead to a new theory

Relative Velocities

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -50 mph

50 mph

50 mph

Relative VelocitiesPerson standing on the edge of the road:

Two cars going 50 mph to the left

One car going 50 mph to the right

Cars in the top lanes

One car going 0 mph

One car going 100 mph to the right

One person going 50 mph to the right

Car in bottom lane

Two cars going 100 mph to the left

1 person going 50 mph to the left

Einstein’s Revolution

1905: Proposed his Theory of Special RelativityAccepted the experimental work on the speed of light and the nature of electromagnetic wavesNow the speed of light was absolute and time and space were relative

Consequence: A Real Speed LimitImagine that you start running at the same time your friend turns on a flashlight. You are racing a beam of light. Can you win?

0.99 c 0.99 c

Galactica sees the photons arriving at speed c

- - - - - - - - - - -

Consequence: A Real Speed LimitYou cannot win!

Both you and your friend will measure the speed of light to be the same.

No matter how fast you go, the light will always be traveling at 2.9979x108 m/s relative to you

It will beat you to the end every time

Nothing can travel faster than light

If the Sun stopped shining, when would we know about it?

Lookback TimeThe fact that there is a cosmic speed limit is both good & badcan’t visit galaxiesHave a “time machine” because we see distant things as they were a long time ago.

Looking Back in Time

If we know how many lightyears away something is, we know how far back in

time we are looking