The Origin and Nature of Light

The Origin and Nature of Light Celebration of Knowledge #2 (aka Exam #2) is Thursday March 8th in N210 Tailgate Party (aka exam review) is Wednesday March 7th in N210 from 4-6pm HW #5 – Handed out in class Feb 27th on the topic of Luminosity Area and Temperature, and Due IN-CLASS Tuesday March 6th

The Origin and Nature of Light HW#6 – Masteringastronomy online homework on Properties of Light and Matter. Available March 1st , Due March 8th by 10am.

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

Find the hottest star(s), how do you know ?

Star A Star A Star A Star D Star C Star B Energy Output per second V I B G Y O R Wavelength V I B G Y O R Wavelength V I B G Y O R Wavelength

Which star is larger Star A or Star D? Same Star A Star D Energy Output per second V I B G Y O R Wavelength

Energy Output per second Try to determine EVERYTHING about how these four stars compare!! Temp, Energy output, Color, size (area)….. Object A Wavelength V I B G Y O R visible range Energy Output per second Object C Object B Object D

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

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

Atoms are mostly empty space A nucleus is about 10-15 m in size and the first electron orbits out at 10-10 m from the center of the atom – The size of the electron orbit is 100,000 times greater than the size of the nucleus

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? End 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! Still in Tucson, up in the foothills shopping at La Encantada!!

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

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!!! Hot/Dense Energy Source prism Continuous Spectrum

The type of spectrum given off depends on the objects involved 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.

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

The type of spectrum given off depends on the objects involved 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.

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

Tutorial: Types of Spectra – p.41 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

Tutorial: Light and Atoms – LT Handout Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

Imagine that you observe the Sun using a telescope in an orbit high above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight? dark line absorption spectrum bright line emission spectrum continuous spectrum None of the above

If an electron in an atom moves from an orbit with an energy of 5 to an orbit with an energy of 10, a photon of energy 5 is emitted a photon of energy 15 is emitted. a photon of energy 5 is absorbed. a photon of energy 15 is absorbed. None of the above

Which of these would cause a “Violet” or high energy absorption line? E. None of the above

Which of these shows the atom emitting the greatest amount of light? B C D E

What physical situation makes this spectrum?

The type of spectrum given off depends on the objects involved 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.

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

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

Tutorial: Analyzing Spectra – p. 43 Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group.

Consider the dark line absorption spectra shown below for Star X and Star Z. What can you determine about the color of the two stars? Assume that the left end of each spectrum corresponds to shorter wavelengths (blue light) and that the right end of each spectrum corresponds with longer wavelengths (red light). Star X Star Z Star X would appear blue and Star Z would appear red. Star X would appear red and Star Z would appear blue. Both stars would appear the same color. The color of the stars cannot be determined from this information.