Chapter 4: Light

“Ever since we crawled out of that primordialslime, that’s been our unifying cry, ’More light.’Sunlight. Torchlight. Candlelight. Neon, incandescent lights that banish the darkness from our caves to illuminate our roads, the insidesof our refrigerators...Little tiny flashlights forthose books we read under the covers whenwe’re suppose to be asleep. Light is more thanwatts ... Light is metaphor. Light is knowledge. Light is life...”

- Diane Frolov and Andrew SchneiderNorthern Exposure,1993

What can we learn by analyzing starlight?

• A star’s chemical composition

• A star’s temperature

• A star’s speed and direction of motion

How fast does light move?

• 186,000 miles per second!

or

• 3 x 105 kilometers per second!

Nothing travels faster than light!

Particle or Wave ?

1905, Einstein verified that light sometimes behaves as a wave and sometimes as particles.

This is called the wave-particle duality of light!

Particle or Wave ?

Wave: Electromagnetic wave

Particles: Photons (packets of energy)

Units: 1 nanometer= 10-9 meters!

Different wavelengths correspond to different colors!

Light as a Wave

Light as a Wave

if wavelength↓, then frequency ↑

if frequency ↑, then energy ↑

Some Important Relationships

if wavelength ↓, then energy ↑

Concept Question

Which color light has more energy blue or red?

Blue coats beat the red coats!

If you pass white light through a prism, it separates into its component colors.

ROY G. BIV !!

long wavelengths short wavelengths

Spectrum

A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors without any specific spectral lines.

A hot, less dense gas, when heated, produces an emission line spectrum - a series of bright spectral lines against a dark background.

A cooler gas in front of a hot dense gas produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.

Each chemical element produces its own unique set of spectral lines when it burns

Spectral Lines

By observing the location of the emission and or the absorption lines, one can identify the element!!!!

Twinkle, twinkle little starI don’t wonder what you are.For by spectroscopic kenI know that you are hydrogen!

- anonymous

The Spectral Type of Stars

• stars are classified by their spectral types:

O B A F G K M

• hottest to coolest

• most massive to least massive

• shortest lifetime to longest lifetime

Stars are classified by their spectra as O, B, A, F,G, K, and M spectral types

Oh Be A Fine Guy (or Girl), Kiss Me

Figure 3.5

Electromagnetic Spectrum

Not all radiation can penetrate Earth’s atmosphere.

What is Color?

• Color of objects due to how much light is reflected, absorbed, or emitted.

Kelvin Temperature Scale

Kelvin Temperature = Celsius Temperature + 273

Peak color (wavelength) shifts to shorter wavelengths as the temperature increases

increasing temperature

Which is HOTTER???

Peak color (wavelength) shifts to shorter wavelengths as the temperature increases

Humans emit infrared

light!

The Doppler Shift

Christian Doppler 1842:

• the observed frequency of an object is affected by its apparent motion

• Doppler shift occurs when the source of the waves is moving with respect to the observer

• occurs for all waves (e.g. water, sound, light)

Sound Waves

low pitchlow frequencylong wavelength

high pitchhigh frequencyshort wavelength

The Doppler Shift

The Doppler ShiftLight Waves

A B

constant wavelength detected by both A and B

A detects longer wavelength redshift

B detects shorter wavelength blueshift

Stationary Star

A B

Star Moving to the Right

The Doppler ShiftIn Astronomy

Spectrum of Stationary Source

Spectrum of Approaching Source

Spectrum of Receding Source

400nm 550nm 700nm

Detection of Extrasolar Planets: Stellar Wobble

• gravity of the planet causes the star to wobble back and forth

• 1990s, used Doppler effect to detect stellar wobbles

Concept QuestionIf a star is moving away from us, which statement best describes what is happeningto the star’s light?

A) the light is blueshifted; we perceive that the wavelength increasesB) the light is redshifted; we perceive that the wavelength increasesC) the light is blueshifted; we perceive that the wavelength decreasesD) the light is redshifted;we perceive that the wavelength decreases

What can we learn by analyzing starlight?

• A star’s chemical composition – by spectrum

• A star’s temperature – by color (peak wavelength)

• A star’s speed and direction of motion – by spectrum and Doppler Shift