Chapter 5:Light

Light!Measuring the speed of lightEarly attempts to measure the speed of

light were done in 1638 by an apprentice of Galileo

Hilltop to hilltop around Padua Italy using hand lanterns

“The speed of light is at least 10 timesfaster than the speed of sound.”

Rømer estimates the speed of light1676…Olaus RømerEclipses of Io by Jupiter. Found speed to be finite but very large (didn’t know diameter of Earth’s orbit)

By the 1800’s two Frenchmen were able to measure the speed

of light with some accuracy1850…Fizeau & Foucault measure speed to be

about300,000 km/s

The Speed of Light in vacuum is a fundamental constant of

the universeThe speed of light in vacuum is the same for all observers everywhere in the universe regardless of their circumstances. We define the speed of light to be

c = 299792458 m/s exactly.

For most calculations, though, we can use 3.00 x108 m/s

We can use the speed of light to define distances

seconds2997924581intravelslightdistancemeter1

The meter is defined as

The lightyear is defined as yearoneintravelslightdistance1ly

The year used in the definition is the sidereal year: 365 days 6 hours 9 minutes and 9.5 seconds

When light travels through anything other than vacuum

it moves slowerWe define the index of refraction of a material to be the ratio of the speed of light in vacuum to the speed of light in the material

v

cn

What is light?

Just as a cork bobbing in

water creates waves in the

water, charges “bobbing” in space create electric and magnetic

waves

“Light” is an Electromagnetic Wave

Basic Properties of WavesWavelength = l in metersFrequency = f in cycles per second or Hertz (Hz)Speed = v in meters per second

fc

Each “color” is characterized by its

wavelength

Using c = lf we can see that the frequency of visible light is in the 1014 Hz range

Visible light is only a very small part of the

Electromagnetic Spectrum

Watch ClassAction Light & Spectra module EM Spectrum Module animation

Photon Torpedoes Away!

hhc

E sJh 3410625.6

Light also behaves like a particle: a photon

c = speed of lightn = frequencyl = wavelength

But didn’t we say that light was a wave? What’s going on? How can it be both?

To understand how light can be both a wave and a particle, we

need new physicsQuantum Mechanics deals with the very small like atoms, molecules, subatomic particles and light particles.

The Bohr AtomAn early model of the atom. Like a mini-solar system, the electrons orbit around tiny nucleus

The Nucleus: Protons, Neutrons and 99.98% of the mass

The problem is, it doesn’t work! According to electromagnetic theory, the electron will radiate away all its energy and collapse into the nucleus in a fraction of a second.

The Quantum Mechanical Atom

The Heisenberg Uncertainty Principle of quantum mechanics says you can’t tell where the electron is, only the probability of where it might be. The electron is “delocalized”.

The energy associated with the electron is quantized

Light comes from Electron Transitions

The energy of the emitted photon is exactly equal to the difference in energy between the two states

photon12 Ehc

EE

A hot gas will emit specific wavelengths

An atom can also absorb a photon if it has the right

energy

Most atoms have many emission lines

Absorption and emission lines have the same

wavelengths The same states are involved in emission and absorption. The only difference between emission and absorption is the way the electron is changing. For emission the electron loses energy and thus goes from a high energy state to a low energy state. For absorption, the electron gains energy and so goes from a low energy state to a high energy state.

Three Views Spectrum Demonstrator in ClassAction Light & Spectra module

The Solar Spectrum has lots of absorption lines

All the naturally occurring elements are present in the Sun

The Doppler Effect

01 1

c

vs

+ If the source is moving away from the observer

- If the source is moving towards the observer

The light is redshifted if the source is moving away from the observer, blueshifted if it is moving towards

What do we mean when we say something is hot?

On a microscopic scale, temperature is a measure of how fast things are moving

In astronomy, we use the absolute

temperature scale

Absolute zero is the temperature at which all motion stops. Quantum mechanics says that isn’t possible so you can never reach absolute zero

All objects emit light

according to their

temperature:

Blackbody Radiation

Hotter objects glow

brighter and

become bluer

The Blackbody Spectrum

The hotter the object is, the shorter the wavelength of maximum intensity. The hotter the object is, the greater the total intensity of the light it emits is.

Objects reach

equilibrium between energy

absorbed and energy

emitted

There is more to the temperature of a planet

than just sunlight absorbed and blackbody

emission

What can be learned from the light of a planet or

star?• Surface Temperature…Blackbody Spectrum

• Elemental Composition…Emission/Absorption

• Radial Motion…Doppler Effect

• Rotation…Doppler Line Broadening

• Surface Pressure/Density…Pressure Broadening