Light & Matter II

Attendance Quiz Are you here today? (a) yes (b) no (c) can you please stop with the lame jokes?

Exam #1 again! The first midterm exam is next Monday, 4/18, during the first half of class - please come on time (or better yet, early) It will be multiple choice, and you will take it using a 50-question scantron, so make sure to bring one to class! It will cover the material in Chapters 1, 3, 4, 5 of the optional textbook Most questions will look something like the in-class “Conceptual Questions” or the Lecture Tutorial and Ranking Task questions - your LT book is your textbook to study from Cell phones must be off and put away during the exam (as they should be in every class). If I see or hear a cell phone that is grounds to fail the exam! We will be studying Special Relativity that day as well, so if you have a book, you will want to do the reading as usual

Today’s Topics Types of Spectra Spectra and Composition of Objects Continuous Emission Absorption Spectra and Composition of Objects Thermal (Blackbody) Radiation Laws of Thermal Radiation Stefan-Boltzmann Law Wein Law Doppler Shift

Light & Atoms Quizzes I-V Which figure shows the absorption of yellow light? Which shows the emission of red light? Which shows the ionization of the atom? Which shows the emission of blue light? Which shows the absorption of ultraviolet light? B e- A C D E F

Light & Atoms Quizzes I-V Which figure shows the absorption of yellow light? Which shows the emission of red light? Which shows the ionization of the atom? Which shows the emission of blue light? Which shows the absorption of ultraviolet light? B e- A C D E F

Light & Atoms Quizzes I-V Which figure shows the absorption of yellow light? Which shows the emission of red light? Which shows the ionization of the atom? Which shows the emission of blue light? Which shows the absorption of ultraviolet light? B e- A C D E F

Light & Atoms Quizzes I-V Which figure shows the absorption of yellow light? Which shows the emission of red light? Which shows the ionization of the atom? Which shows the emission of blue light? Which shows the absorption of ultraviolet light? B e- A C D E F

Light & Atoms Quizzes I-V Which figure shows the absorption of yellow light? Which shows the emission of red light? Which shows the ionization of the atom? Which shows the emission of blue light? Which shows the absorption of ultraviolet light? B e- A C D E F

The Three Types of Spectra: Continuous, Emission and Absorption Emission Spectrum

Spectra and Composition of Objects Discharge Tube Demo

The Three Types of Spectra: Continuous, Emission and Absorption Continuous Spectrum

Thermal (Blackbody) Radiation All warm objects emit thermal radiation The complicated interactions between the atoms in a solid or dense gaseous body (such as a star, planet, or hot nail) results in a continuous spectrum “Black” in blackbody refers to an idealized perfectly emitting/absorbing body (the Sun is a good approximation) Showcase Bulb Demo

Continuous Spectra A plot of intensity v. wavelength of a continuous spectrum looks like the curves below The curves below actually shows intensity per square meter The shape of the curve is the same, but position of the curve moves, depending only on the temperature of the object

Laws of Thermal Radiation There are two rules that govern curves of thermal radiation Stefan-Boltzmann Law - each square meter of a hotter object emits more light at all wavelengths than a cooler object (L/m2  T4) Wien’s (“veen’s”) Law - hotter objects emit photons with a higher average energy (shorter wavelength) (max  1/T) Showcase Bulb/Hot Nail Demo

Spectra and Composition of Objects

Stars Emit an Absorption Spectrum Stars (including the Sun) emit an absorption spectrum The hot inner core emits thermal (continuous) radiation The cooler atoms in the outer atmosphere of the star absorb light at the specific wavelengths corresponding to the transitions within those atoms The continuous part of this spectrum can be used to find stellar temperatures The details of the spectrum can be used to determine the composition of stars

Types of Spectra Quiz I Which of the following spectra is emitted by the Sun? continuous spectrum dark line absorption spectrum bright line emission spectrum

The Three Types of Spectra: Continuous, Emission and Absorption Active Figure 5.14

Lecture Tutorial: Types of Spectra, pp. 61-62 Work with one or more partners - not alone! Get right to work - you have 10 minutes 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 all agree on. Write clear explanations for your answers. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.

Types of Spectra Quiz I Which of the following spectra is emitted by the Sun? continuous spectrum dark line absorption spectrum bright line emission spectrum

Doppler Shift When a moving object emits sound waves, the pitch (frequency) of the sound varies (Doppler Demo) For a stationary train, the pitch (frequency) is the same as emitted If the train approaches, the pitch (frequency) is higher If the train moves away, the pitch (frequency) is lower In a similar way, the frequency of light waves shifts if the emitting object is moving towards or away from the observer The spectrum of an approaching object is blueshifted The spectrum of a receding object is redshifted The speed of approach (or recession) can be determined by the size of the shift; distance doesn’t matter! Interactive Figure 5.22

Doppler shift tells us ONLY about the part of an object’s motion toward or away from us: Interactive Figure 5.23

Lecture Tutorial: Doppler Shift, pp. 73-77 Work with one or more partners - not alone! Get right to work - you have 15 minutes 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 all agree on. Write clear explanations for your answers. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don’t understand what the Lecture Tutorial is asking, ask me for help.

Homework For homework Complete the Lecture Tutorial Doppler Shift (if necessary) Complete the ranking tasks, Doppler Shift #2-4, (download from class website) Here at the beginning of the semester, our main goal is simply to make sure that students understand the basic hierarchy of structure from planet to solar system to galaxy to universe (clustering is less important at this stage). You might also wish to note that this slide shows a lot about human history. E.g., we first learned that Earth is a sphere some 2,500 years ago; we learned that Earth is a planet going around the Sun only about 400 years ago; and we learned that the Millky Way is only one of many galaxies with the work of Hubble some 80 years ago…

Doppler Shift Quiz II If a distant galaxy has a substantial redshift (as viewed from our galaxy), then anyone living in that galaxy would see a substantial redshift in a spectrum of the Milky Way Galaxy. Yes, and the redshifts would be the same Yes, but we would measure a higher redshift than they would Yes, but we would measure a lower redshift than they would No, they would not measure a redshift toward us No, they would measure a blueshift