Lecture 1 Notes 53760 - Lecture 1 Notes B. A. Rowland 53760...

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Lecture 1 Notes B. A. Rowland 53760 I will outline a few highlights from the first lecture in these notes. These are written in conjunction with the slides presented in class. Light For years light was thought to be a wave. There are some terms you should be familiar with concerning waves: Wavelength : The distance between two successive crests (or troughs) in a wave. Units of distance (m, mm, cm, nm, etc). Frequency: The number of cycles per second that pass through a given point in space. Units of 1/s (also known as the Hertz—Hz). Speed : The velocity of the traveling wave. In a vacuum, for EMR this value is c = 2.998 x 10 8 m/s. Wavelength and frequency are inversely proportional. This means that as the wavelength increases, the frequency decreases and vice versa. Important equation: c λ υ = , where λ is the wavelength and ν is the frequency. Make sure to watch your units! For any wave the wavelength times the frequency should equal the speed of light ( c ). The energy of light is directly proportional to the frequency, while it is inversely proportional to the wavelength. Practice Problem: Green light has a wavelength of 530 nm (nanometers). What is the frequency of green light? Red light is less energetic than violet light. This means that red light will have a _________ (higher, lower) wavelength and ____________ (higher, lower) frequency than violet light. EMR Light is a form of Electromagnetic Radiation and represents only a very small fraction of the Electromagnetic Spectrum. Other examples of EMR and their effects on matter include: Radio waves—excites nuclei. Microwaves—makes molecules rotate. Infrared—makes molecules vibrate.
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Visible light—causes electronic transitions in molecules which produce pretty colors. Ultraviolet—also causes electronic transitions. X-ray—removes electrons from molecules. Gamma rays—likely would obliterate molecules. Each of these types of radiation does something a little different to a molecule, and we can use this information to make predictions as to the structure of molecules (field of spectroscopy). The concepts of wavelength and frequency can be applied to EMR, and all the principles discussed above apply as well. The energy ranking of EMR would go like: Radio < Microwave < Infrared < Visible < Ultraviolet < X-rays < Gamma Rays Practice problems: Rank the EMR spectrum in terms of i) increasing wavelength; ii) increasing frequency. An FM radio station broadcasts at 99.5 MHz (megahertz). Calculate the wavelength of these radio waves. Problem 12-24 in the text. Interference and Diffraction Basically, waves can interfere with each other in two ways. The first is called constructive interference. This happens when a wave has its crest aligned with the crest of another wave. This results when a wave that has a greater amplitude than either of the component waves (will create a bright spot in a diffraction pattern). Destructive interference occurs when the crest of one wave is aligned with the trough of another. This results in their being no wave, as the trough will cancel out the crest and leave
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This note was uploaded on 01/07/2009 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas at Austin.

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Lecture 1 Notes 53760 - Lecture 1 Notes B. A. Rowland 53760...

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