Atoms and Spectra October 8th, 2013

Transcription

1 Atoms and Spectra October 8th, 2013

2 Announcements Second writing assignment due two weeks from today (again, on a news item of your choice). Be sure to make plans to visit one of the open observing nights for your fourth writing assignment.

3 News Clouds on planets in other Solar systems Supervolcanos on Mars!

4 Review Objects look the color that they do because they scatter that color of light. Light exhibits signs of being both a particle and a wave. The distance between two subsequent wave peaks is called the wavelength. The number of times per second that a light wave vibrates up and down is called the frequency. For light, the wavelength times the frequency equals the speed of light. Photons with longer wavelengths have smaller energies. Visible light is only a small part of the electromagnetic spectrum, which has gamma rays at the high energy end, and radio waves at the low energy end. A spectrum is a representation of an objects intensity as a function of wavelength. The spectra of stars appears like a blackbody. The wavelength of peak intensity is related to the temperature, where hotter objects appear bluer.

5 Which has the longer peak wavelength? Star A A)Star A B) Star C C) Same Energy Output per second Star C V I B G Y O R Wavelength

6 Which star has the lower surface temperature? Star A A. Star A B. Star C C. Same Energy Output per second Star C V I B G Y O R Wavelength

7 Which of the two stars looks red? Star A A. Star A B. Star C C. Both D. Neither Energy Output per second Star C V I B G Y O R Wavelength

8 The building blocks of matter The term atom comes from the Greek word meaning indivisible. We used to think that nothing was smaller than an atom. We now know that atoms are composed of three types of particles. Protons - Positively charged particles Neutrons - neutral particles, same mass as the proton Protons and neutrons are further divisible into quarks; we don t yet know of anything smaller, so quarks are elementary particles. Electrons, negatively charged particles, with a mass 1/2000 that of the proton. Electrons are also elementary particles, of a category called leptons.

9 Structure of an atom The protons and neutrons stick together in the atom s nucleus, held together by the strong nuclear force. Splitting an atom, or fission, releases this energy (we ll learn about fusion soon). Electrons orbit around the nucleus.

10 Periodic table Elements are atoms with differing numbers of protons. Simplest: Hydrogen (H): one proton and one electron Isotopes are variations on elements with different numbers of neutrons. Deuterium is hydrogen with a neutron alongside the proton in the nucleus. Ions are variations of elements with more or less electrons, making them negatively or positively charge.

11 Atomic number = # of protons (and electrons) Atomic mass ~ # of protons + # of neutrons

12 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

13 Energy levels of H Electrons can only exist in certain energy levels. ground state To move from one to another, they require a photon with an exact amount of energy.

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

15 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

16 Energy levels of hydrogen What happens to the excess energy? energy level energy contained at that level

17 Energy levels of hydrogen Why only certain levels? The energy levels of electrons are quantized, and the study of this is quantum physics. What happens to the excess energy? energy level All atoms have quantized electron energy levels, and even molecules (structures made from multiple atoms) have quantized energy levels. energy contained at that level

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19 What is a spectrum? A spectrum tells us how bright a particular object is at each wavelength. We can see a spectrum by passing an objects light through a prism. It can include continuous light (from a blackbody), emission lines (from atoms moving their electrons to lower levels) and absorption lines (from atoms moving their electrons to higher levels). Spectrum of a distant galaxy; has continuous emission, emission lines, and absorption lines.

20 What is a spectrum? A spectrum tells us how bright a particular object is at each wavelength. We can see a spectrum by passing an objects light through a prism. It can include continuous One-dimensional light (from spectrum a blackbody), emission lines (from atoms moving (plot their of electrons intensity to versus lower levels) and absorption lines (from atoms moving wavelength) their electrons to higher levels). Spectrum of a distant galaxy; has continuous emission, emission lines, and absorption lines. Two-dimensional spectrum

21 What is a spectrum? A spectrum What tells us can how we bright learn a particular from spectra? object is at each wavelength. We can see a spectrum by passing an objects light through a prism. It can include continuous One-dimensional light (from spectrum a blackbody), emission lines (from atoms moving (plot their of electrons intensity to versus lower levels) and absorption lines (from atoms moving wavelength) their electrons to higher levels). Spectrum of a distant galaxy; has continuous emission, emission lines, and absorption lines. Two-dimensional spectrum

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

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

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

25 Spectral Line Demo 22

26 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? 1. dark line absorption spectrum 2. bright line emission spectrum 3. continuous spectrum 4. None of the above

27 Which of these would cause a Violet or high energy absorption line?

28 What physical situation makes this spectrum? A hot dense core surrounded by a low density outer atmosphere

29 The Sun s Spectrum

30 All stars produce dark line absorption spectra

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

32 Tutorial: Analyzing Spectra - p. 71 Work with a group of 2-3 people. Read the instructions and questions carefully Talk to each other and discuss your answers with each another Come to a consensus answer you both agree on 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 as one of us for help. We will discuss as a class when done. Turn in a half sheet of paper with name and EID. Whats your favorite flavor of ice cream?

33 Homework Read Chpt 5.4 and all of Chapter 6 Homework #7 due Thursday Extra credit: Lecture tutorial on Light and Atoms (p. 65) Due Thursday