The Origin and Nature of Light

But, what is light? In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. In the 19 th and 20 th Century Maxwell, Young, Einstein and others were able to show that Light behaves both like a particle and a wave depending on how you observe it.

Thomas Young’s interference experiment

Quantum Curiosities Schrödinger's Cat

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

In 1905 Einstein calculated the energy of a particle of light ( photon) and proposed the photoelectric effect. E photon = hc/ e- photon

But, where does light actually come from? Light comes from the acceleration of charged particles (such as electrons and protons)

electron Accelerating charges produce light – electromagnetic radiation! But, where does light actually come from?

Like the flavors of Ice cream – they each provide us with different information.

A B C D E

But what do you get when you put all the flavors (light) together?

Luminosity is the total energy (light) emitted by an object in each second. Stefan-Boltzmann law Luminosity depends on an surface area (A), and its temperature (T 4 ) Luminosity =  67x10   )T 4 Big and Hot objects have greater luminosity than small cool objects

20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is Hot and Dim? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is Cool and Dim? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is Largest? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

Which star is smallest? Temperature (K) 20,000 10,000 5,000 Luminosity (solar units) Temperature (K) , ,000

What can we learn by analyzing starlight? A star’s temperature

If you pass white light through a prism, it separates into its component colors. ROYGBIVROYGBIV spectrum long wavelengths short wavelengths

Photographs of a Star Cluster

Spectra of a Star Cluster

Which object is hotter, an object that is emitting mainly red light or mainly blue light? increasing temperature

Which object is hotter, an object that is emitting mainly red light or mainly blue light? increasing temperature

Filter Detector 81 blue4600 A81

Filter Detector 85 blue4600 A81 green5300 A85

Filter Detector 83 blue4600 A81 green5300 A85 yellow5800 A83

Filter Detector 78 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78

Filter Detector 70 blue4600 A81 green5300 A85 yellow5800 A83 orange6100 A78 red6600 A70 UVIR “Blackbody Curve” - a graph of an object’s energy output versus wavelength. The PEAK of this curve is related to the object’s temperature.

“Blackbody Curve” - a graph of an object’s energy output versus wavelength. The WAVELENGTH that the PEAK of this curve occurs at tells us about the object’s TEMPERATURE and COLOR. UVIR Energy Output Wavelength

Hot objects emit light that PEAKS at short wavelengths (blue). Cool objects emit light that PEAKS at long wavelengths (red) increasing temperature

Wien’s law peak = (2.9 x ) / T kelvin The higher the object’s temperature, the shorter the wavelength of the peak for the light emitted by the object. Relates the temperature of an object to the wavelength of the peak in the black body curve.

What is the wavelength of the PEAK of this “Blackbody” curve

What color is our 5800K Sun? The Sun emits all wavelengths of electromagnetic radiation (light); however, the wavelengths of light it emits most intensely are in the green/yellow part of the spectrum.

Our Sun What if the Sun became hotter?

Our Sun What if the Sun became hotter? What if the Sun became cooler?

Our Sun What if the Sun became hotter? What if the Sun became cooler?

What is this a picture of ? Find the hottest star(s), how do you know ?

The Origin and Nature of Light

An atom consists of a small, dense nucleus (containing protons and neutrons) surrounded by electrons - Model Proposed by Niels Bohr 1913

A nucleus is about m in size and the first electron orbits out at m from the center of the atom – The size of the electron orbit is 100,000 times greater than the size of the nucleus Atoms are mostly empty space

So if a nucleus the size of an orange (10 cm) was located at the center of the football field, where would the electron be? E nd Zone? Grandstands? On Campus? In Tucson?

If the electron’s orbit is 100,000 times bigger than the nucleus then the electron would be 10,000 m or 6.21 miles away from the center of the Football Field! Red Bank or Eatontown

The electron should be thought of as a distribution or cloud of probability around the nucleus that on-average behave like a point particle on a fixed circular path

Nucleus

Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower energy level Nucleus

Photons (light-waves) can also be absorbed by an atom when an electron moves from a lower energy level to a higher energy level Nucleus

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

We will study three types of spectra!!! Continuous Spectrum Hot/Dense Energy Source prism Emission Line Spectrum prism Hot low density cloud of Gas Absorption Line Spectrum Cooler low density cloud of Gas Hot/Dense Energy Source prism

The type of spectrum given off depends on the objects involved Law #1 – The excited atoms within a hot dense object give off light of all colors (wavelengths) and produce a continuous spectrum -- a complete rainbow of colors (range of wavelengths) without any spectral lines.

We will study three types of spectra!!! Continuous Spectrum Hot/Dense Energy Source prism

Law #2 – The excited atoms within a hot, cloud of gas give off only particular colors (wavelengths) of light and produce an emission line spectrum - a series of bright spectral lines against a dark background. The type of spectrum given off depends on the objects involved

We will study three types of spectra!!! Emission Line Spectrum prism Hot low density cloud of Gas

Law #3 – When the light from a hot dense object passes through a cool cloud of gas, the atoms within the cloud can absorb particular colors (wavelengths) of light and produce a absorption line spectrum - a series of dark spectral lines among the colors of the rainbow. The type of spectrum given off depends on the objects involved

We will study three types of spectra!!! Absorption Line Spectrum Cooler low density cloud of Gas Hot/Dense Energy Source prism

What physical situation makes this spectrum?

Law #3 – When the light from a hot dense object passes through a cool cloud of gas, the atoms within the cloud can absorb particular colors (wavelengths) of light and produce a absorption line spectrum - a series of dark spectral lines among the colors of the rainbow. The type of spectrum given off depends on the objects involved

Absorption Line Spectrum Cooler low density cloud of Gas Hot/Dense Energy Source prism

What physical situation does a star like the sun present? A hot dense core surrounded by a low density outer atmosphere

The Sun’s Spectrum

All stars produce dark line absorption spectra

What can we learn by analyzing starlight? A star’s temperature A star’s chemical composition

What can we learn by analyzing starlight? A star’s temperature A star’s chemical composition - peak wavelength of the spectral curve - dips in the spectral curve or the lines in the absorption spectrum A star’s motion

The Doppler Effect Definition: “The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”

Astronomers use the Doppler Effect to learn about the radial (along the line of sight) motions of stars, and other astronomical objects.

Real Life Examples of Doppler Effect Doppler Radar (for weather) Airplane radar system Submarine radar system –Ok, anything with radar Radar gun, used by Law Enforcement Officers…

The Doppler Effect Definition: “The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”

Doppler Effect When something which is giving off light moves towards or away from you, the wavelength of the emitted light is changed or shifted V=0

Doppler Effect When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase. We call this a “redshift”.

Doppler Effect When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease. We call this a “blueshift”.

The Doppler Effect Definition: “The change in wavelength of radiation due to relative motion between the source and the observer along the line of sight.”

Doppler Effect “Along the line of sight” means the Doppler Effect happens only if the object which is emitting light is moving towards you or away from you. –An object moving “side to side” or perpendicular, relative to your line of sight, will not experience a Doppler Effect.

Astronomy Application V=0

Doppler Shifts Redshift (to longer wavelengths): The source is moving away from the observer Blueshift (to shorter wavelengths): The source is moving towards the observer  = wavelength shift o = wavelength if source is not moving v = velocity of source c = speed of light

What can we learn by analyzing starlight? A star’s temperature A star’s chemical composition - peak wavelength of the spectral curve - dips in the spectral curve or the lines in the absorption spectrum A star’s motion - Doppler shift

The Doppler Effect causes light from a source moving away to: 1. be shifted to shorter wavelengths. 2. be shifted to longer wavelengths. 3. changes in velocity. 4. Both a and c above 5. Both b and c above