Astronomy 101Astronomy 101Section 020Section 020

LectureLecture 5 5

The Nature of LightThe Nature of Light

John T. McGraw, ProfessorJohn T. McGraw, Professor

Laurel Ladwig, Planetarium ManagerLaurel Ladwig, Planetarium Manager

Determining the Speed of LightDetermining the Speed of Light

Galileo tried Galileo tried unsuccessfully to unsuccessfully to determine the speed determine the speed of light using an of light using an assistant with a assistant with a lantern on a distant lantern on a distant hilltophilltop

Researchers later Researchers later (much later!) used (much later!) used rapidly spinning rapidly spinning mirrors to accomplish mirrors to accomplish this difficult task.this difficult task.

Light travels through empty space at a Light travels through empty space at a speed of 300,000 km/sspeed of 300,000 km/s

In 1676, Danish In 1676, Danish astronomer Olaus Rastronomer Olaus Røømer mer discovered that the exact discovered that the exact time of eclipses of Jupiter’s time of eclipses of Jupiter’s moons depended on the moons depended on the distance of Jupiter to Earth distance of Jupiter to Earth

This happens because it This happens because it takes varying times for takes varying times for light to travel the varying light to travel the varying distance between Earth distance between Earth and Jupiterand Jupiter

Light is electromagnetic radiationLight is electromagnetic radiationand is characterized by its wavelength (and is characterized by its wavelength ())

The Nature of LightThe Nature of Light

In the 1860s, the Scottish mathematician, physicist and coffee In the 1860s, the Scottish mathematician, physicist and coffee brewer James Clerk Maxwell succeeded in describing all the brewer James Clerk Maxwell succeeded in describing all the basic properties of electricity and magnetism in four equationsbasic properties of electricity and magnetism in four equations

This mathematical achievement demonstrated that electric and This mathematical achievement demonstrated that electric and magnetic forces are really two aspects of the same magnetic forces are really two aspects of the same phenomenon, which we now call phenomenon, which we now call electromagnetismelectromagnetism

Because of its electric Because of its electric and magnetic and magnetic properties, light is properties, light is also called also called electromagnetic electromagnetic radiationradiation

Visible light falls in Visible light falls in the 400 to 700 nm the 400 to 700 nm rangerange

Stars, galaxies and Stars, galaxies and other objects emit other objects emit light in all light in all wavelengthswavelengths

Three Temperature ScalesThree Temperature Scales

An opaque object emits electromagnetic An opaque object emits electromagnetic radiation according to its temperatureradiation according to its temperature

Visible light from starsVisible light from stars

The wavelength at The wavelength at which a hot object (a which a hot object (a metal rod or a star) metal rod or a star) emits the most emits the most electromagnetic electromagnetic radiation is inversely radiation is inversely proportional to the proportional to the temperature of the temperature of the object.object.

Light has properties of both waves and particlesLight has properties of both waves and particles

Newton thought light was in the form of little packets of energy called Newton thought light was in the form of little packets of energy called photons and subsequent experiments with blackbody radiation photons and subsequent experiments with blackbody radiation indicate it has particle-like propertiesindicate it has particle-like properties

Young’s Double-Slit Experiment indicated light behaved as a waveYoung’s Double-Slit Experiment indicated light behaved as a wave

Light has a dual personality; it behaves as a stream of particle like Light has a dual personality; it behaves as a stream of particle like photons, but each photon has wavelike properties photons, but each photon has wavelike properties

Each chemical element produces its own Each chemical element produces its own unique set of spectral linesunique set of spectral lines

How light is radiatedHow light is radiated

“Fingerprinting” the elements

Iron in our sun

An atom consists of a small, dense nucleusAn atom consists of a small, dense nucleussurrounded by electronssurrounded by electrons

An atom has a small dense nucleus composed of protons An atom has a small dense nucleus composed of protons and neutronsand neutrons

Rutherford’s experiments with alpha particles shot at gold Rutherford’s experiments with alpha particles shot at gold foil helped determine the structurefoil helped determine the structure

A schematic atom

The number of protons in an atom’s nucleus is the The number of protons in an atom’s nucleus is the atomic atomic number number for that particular element for that particular element

The same element may have different numbers of neutrons in The same element may have different numbers of neutrons in its nucleusits nucleus

These three slightly different kinds of elements are called These three slightly different kinds of elements are called isotopesisotopes, (or “baseball players” if you are from Albuquerque), (or “baseball players” if you are from Albuquerque)

Spectral linesSpectral lines are produced when an electron jumps are produced when an electron jumps from one energy level to another within an atomfrom one energy level to another within an atom

The nucleus of an atom is The nucleus of an atom is surrounded by electrons that surrounded by electrons that occupy only certain orbits or occupy only certain orbits or energy level.energy level.

When an electron jumps from one When an electron jumps from one energy level to another, it emits energy level to another, it emits or absorbs a photon of or absorbs a photon of appropriate energy (and hence of appropriate energy (and hence of a specific wavelength).a specific wavelength).

The spectral lines of a particular The spectral lines of a particular element correspond to the element correspond to the various electron transitions various electron transitions between energy levels in atoms between energy levels in atoms of that element.of that element.

Balmer Lines in a Stellar SpectrumBalmer Lines in a Stellar Spectrum

The wavelength of a spectral line is affected by the The wavelength of a spectral line is affected by the relative motion between the source and the observerrelative motion between the source and the observer

Doppler ShiftsDoppler Shifts

Red ShiftRed Shift: The object is moving away from : The object is moving away from the observerthe observer

Blue ShiftBlue Shift: The object is moving towards the : The object is moving towards the observerobserver

//oo = v/ = v/cc

= wavelength shift= wavelength shift

oo = wavelength if source is not moving = wavelength if source is not movingv = velocity of sourcev = velocity of sourcecc = speed of light = speed of light