Major Concepts of Physics PHY102 – Lecture #  Syracuse University Lecture #9 Everything about electromagnetic (EM) waves Monday, February 22 nd Spring 2016 Prof. Liviu Movileanu hppt://movileanulab.syr.edu/MajorConceptsPhysics2016.html Room 211, Physics Bldg., Major Concepts of Physics PHY 102

Major Concepts of Physics PHY102 – Lecture #  Syracuse University 1. Review of single-slit diffraction: quantitative understanding  A couple of examples pertaining to Homework #3  A couple of examples pertaining to Homework #3 2. What are the electromagnetic (EM) waves?  The electromagnetic spectrum  The electromagnetic spectrum  Example of EM waves and their applications  Example of EM waves and their applications 3. Lecture demonstrations/Movie (EM waves) 4. Speed of EM waves in vacuum and matter 5. Announcements Lecture objectives-Electromagnetic waves

1) Recall: An important result of our study of diffraction: Let any wave go through a narrow slit. Then, the light spreads out after leaving the slit. Its spread can be characterized by the result Sin θ = λ/a. Here, a is the width of the slit-opening, and λ is the wavelength of the wave. Also, θ is the angle for the first minimum for wave-intensity. Hence, objects of angular size smaller than θ cannot be resolved. That is why θ gives the diffraction limit.. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Review – Diffraction

2) Questions to test your understanding: a) A biologist wishes to see in detail a small object in a cell whose size is close to the diffraction limit for light waves. Should red light or blue light be used? Hint: λ(blue) = 400 nm (roughly). λ(red) = 700 nm (roughly). b) A biologist wishes to see a structure in a cell whose size is smaller than the diffraction limit for light waves. She uses an electron microscope (Electrons are waves, as well as particles!) Is the wavelength of the electrons smaller or larger than that of light? Remark: We will understand later, how electrons can behave as waves. This is a subject known as quantum mechanics. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Review - Diffraction

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Minima of diffraction: single-slit experiment

Single-slit diffraction Problem 47 Section 25.7 Diffraction by a single slit Major Concepts of Physics PHY102 – Lecture #  Syracuse University 47. The central bright fringe in a single-slit diffraction pattern from light of wavelength 476 nm is 2.0 cm wide on a screen that is 1.05 m from the slit. (a) How wide is the slit? (b) How wide are the first two bright fringes on either side of the central bright fringe? (Define the width of a bright fringe as the linear distance from minimum to minimum.).

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Single-slit diffraction Solutions Problem 47

Single-slit diffraction Problem 52 Major Concepts of Physics PHY102 – Lecture #  Syracuse University 52. Light of wavelength 490 nm is incident on a narrow slit. The diffraction pattern is viewed on a screen 3.20 m from the slit. The distance on the screen between the central maximum and the third minimum is 2.5 cm. What is the width of the slit?.

We know that light is a wave. But: What is it that is “waving”? Mystery until Then: A breakthrough. James Clerk Maxwell found the answer. He put together all the basic laws known about a “different” subject: Electricity and Magnetism We know that light is a wave. But: What is it that is “waving”? Mystery until Then: A breakthrough. James Clerk Maxwell found the answer. He put together all the basic laws known about a “different” subject: Electricity and Magnetism Major Concepts of Physics PHY102 – Lecture #  Syracuse University Electromagnetic Waves

Whenever a charge accelerates, it sends waves outward in all directions. The waving quantity: An electric field and a magnetic field. He found: The wave-speed v in empty space has one value only. Its value is c=300,000 km/s= 3  10 8 m/s We call this value c. Major Concepts of Physics PHY102 – Lecture #  Syracuse University The results of Maxwell

The entire set of frequencies for EM waves, is called the electromagnetic spectrum. For example, EM waves with frequency f just below red are called infrared waves. EM waves with f just above violet are called ultraviolet. Major Concepts of Physics PHY102 – Lecture #  Syracuse University The Electromagnetic Spectrum

The surprise of Maxwell: The value of c is the same as the known speed of light! The only reasonable conclusion: Light must be an EM wave. He answered the question “What is waving in a light wave?” Before Maxwell: Light was one subject. Electricity and magnetism was another subject. After Maxwell: Both subjects are the same phenomena. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Surprise of Maxwell

Three laws you already studied last semester: 1) Coulomb’s Law: It gives the electric field produced by a static charge. 2) Oersted and Ampere’s experiments: Gives the magnetic field produced by a moving charge. 3) Faraday’s Law: A changing magnetic field produces an electric field. Maxwell combined these three results, and wrote a consistent formulation for electricity and magnetism. Major Concepts of Physics PHY102 – Lecture #  Syracuse UniversityRecall

But: His ideas were later confirmed by the experiments of Hertz (1880) Hertz showed: Accelerating charges produce EM waves. The Frequencies of Light Waves Suppose a charge performs simple harmonic motion. Let f be its frequency. Then: The EM wave it produces must have the same frequency f. Find its value for light. Major Concepts of Physics PHY102 – Lecture #  Syracuse University Maxwell work was theoretical

v = fλ This is valid for any wave. But Maxwell showed: v = c for all EM waves in empty space. Conclude: c = fλ for any EM wave in empty space. Now, we already know the values for λ for light waves. From the double-slit Young experiment or from the diffraction experiments: λ for light is between 400 nm (violet or blue) and 700 nm (red). Use this to solve for the frequency f, using c = fλ. Major Concepts of Physics PHY102 – Lecture #  Syracuse UniversityRecall

f = 430 x Hertz for red light. f = 750 x Hertz for violet light. 1 Hertz = 1 Hz = 1 s -1 = 1 full cycle per second. Important note: This is a very narrow range. It is less than a factor of two. The human eye is “color-blind” to EM waves not in this range. But: Hertz and others showed: Other f than this small range also produce EM waves. The eye, happens to not have sensitivity to anything other than the range from 400 to 750 trillion Hertz. Major Concepts of Physics PHY102 – Lecture #  Syracuse UniversityResults

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Visible electromagnetic waves

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Electromagnetic waves - Summary

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Infrared electromagnetic waves

Major Concepts of Physics PHY102 – Lecture #  Syracuse University Ultraviolet electromagnetic waves a. The large star coral (Montastraea cavernosa) is dull brown when illuminated by white light. b. When illuminated by an UV source, the coral absorbs UV and emits visible light that appears bright yellow.

Major Concepts of Physics PHY102 – Lecture #  Syracuse University 1.Reading 1.Reading: Chapter 22 nd, Sections 22.1 and 22.3 pp and Conceptual example Homework 2. Homework The homework #3 is due on this week’s workshop (Feb 22-26). 3. This week’s lab: Production of light by solids and emission spectroscopy Announcements