Chapter 5: Light

Measuring the speed of light Early 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 and the best timing instruments available at the time. His conclusion: “The speed of light is at least 10 times faster than the speed of sound.”

1850…Fizeau & Foucault measure speed to be about 300,000 km/s By the 1800’s two Frenchmen were able to measure the speed of light with some accuracy 1850…Fizeau & Foucault measure speed to be about 300,000 km/s

The Speed of Light in vacuum is a fundamental constant of the universe The Speed of light in vacuum is the same for all observers everywhere in the universe regardless of their motion. We define the speed of light to be c = 299792458 m/s exactly. For most purposes, though, we use 3.00 x108 m/s or 300,000 km/s

When light travels through anything other than vacuum it moves slower We 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

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 Waves Wavelength = l in meters Frequency = f in cycles per second or Hertz (Hz) Speed = v in meters per second

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

Different wavelengths of light are created by things of similar size

Even though light is an electromagnetic wave, it sometimes behaves like a particle c = speed of light n = frequency l = wavelength

When things get very small, Quantum Mechanics rules

The Rutherford Atom Early model of the atom. Like a mini-solar system, the electrons orbit around a tiny but massive nucleus The Nucleus: Protons, Neutrons and 99.98% of the mass

The Bohr Atom Neils Bohr Can’t tell where the electron is, only the probability of where it might be

The energy associated with the electron is quantized States above the ground state are excited states

Atoms emit light when an electron goes from a high energy state to a low one The energy of the emitted photon is exactly equal to the difference in energy between the two states

A hot gas will emit specific wavelengths

Atoms absorb photons when an electron is “bumped up” to a higher energy state

If we pass white light through a “cool” gas we can see absorption

Most atoms have many emission lines due to many different electron energy levels

The Doppler Effect The light is redshifted (longer wavelength) if the source is moving away from the observer, blueshifted (shorter wavelength) if it is moving towards

The Doppler effect can change a stars spectrum in two ways If the star is rotating the absorption lines are broadened If the star is moving away or towards Earth the entire spectrum is shifted

The Solar Spectrum has lots of absorption lines We know absorption comes from electron transitions but where does the continuous rainbow of color come from?

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 As the temperature increases, the peak of the blackbody curve shifts to shorter (bluer) wavelengths. The total intensity also increases dramatically as the temperature increases.

It isn’t a perfect match, but it’s close The Sun is a 5800° Blackbody It isn’t a perfect match, but it’s close It also has lots of absorption lines due to the gasses in its’ atmosphere

How bright something appears depends on how far away it is Brightness versus intensity is another inverse square law

What can be learned from the light of a star? Surface Temperature…Blackbody Spectrum Elemental Composition…Emission/Absorption Radial Motion…Doppler Effect Rotation…Doppler Line Broadening Surface Pressure/Density…Pressure Broadening