Electromagnetic Waves - General Physics - Lecture Slides, Slides of Physics

Download Electromagnetic Waves - General Physics - Lecture Slides and more Slides Physics in PDF only on Docsity! Chapter 25 Electromagnetic Waves 700.0 nm 600.0 nm 500.0 nm 400.0 nm Docsity.com Units of Chapter 25 • The Production of Electromagnetic Waves • The Propagation of Electromagnetic Waves • The Electromagnetic Spectrum • Energy and Momentum in Electromagnetic Waves • Polarization Docsity.com 25-1 The Production of Electromagnetic Waves An electromagnetic wave propagating in the positive x direction, showing the electric and magnetic fields: Docsity.com 25-1 The Production of Electromagnetic Waves The direction of propagation and the directions of the electric and magnetic fields in an electromagnetic wave can be determined using a right-hand rule: Point the fingers of your right hand in the direction of E, curl your fingers toward B, and your thumb will point in the direction of propagation. Docsity.com 25-1 The Production of Electromagnetic Waves Any time an electric charge is accelerated, it will radiate: Accelerated charges radiate electromagnetic waves. Docsity.com 25-2 The Propagation of Electromagnetic Waves The first laboratory measurement of the speed of light was done by Fizeau in the latter part of the 19th century. He used a ray of light passing (or not) through a notched mirror, and was able to derive the speed of light from the rotational speed of the mirror and the distance from the wheel to the mirror. Docsity.com 25-2 The Propagation of Electromagnetic Waves The value of the speed of light is given by electromagnetic theory; it is: This is a very large speed, but on an astronomical scale, it can take light a long time to travel from one star to another. Astronomical distances are often measured in light-years – the distance light travels in a year. Docsity.com 25-2 The Propagation of Electromagnetic Waves Light from the Andromeda Galaxy, left, takes about 2 million years to reach us. From the most distant galaxies in the Hubble Deep Field image, right, it takes 13 billion years. Docsity.com 25-3 The Electromagnetic Spectrum Radio waves are the lowest-frequency electromagnetic waves that we find useful. Radio and television broadcasts are in the range of 106 Hz to 109 Hz. Microwaves are used for cooking and also for telecommunications. Microwave frequencies are from 109 Hz to 1012 Hz, with wavelengths from 1 mm to 30 cm. Docsity.com Infrared waves are felt as heat by humans. Remote controls operate using infrared radiation. The frequencies are from 1012 Hz to 4.3 x 1014 Hz. 25-3 The Electromagnetic Spectrum Docsity.com 25-3 The Electromagnetic Spectrum Visible light has a fairly narrow frequency range, from 4.3 x 1014 Hz (red) to 7.5 x 1014 Hz (violet). Ultraviolet light starts with frequencies just above those of visible light, from 7.5 x 1014 Hz to 1017 Hz. These rays cause tanning, burning, and skin cancer. Some insects can see in the ultraviolet, and some flowers have special markings that are only visible under ultraviolet light. X-rays have higher frequencies still, from 1017 Hz to 1020 Hz. They are used for medical imaging. Docsity.com 25-4 Energy and Momentum in Electromagnetic Waves It can be shown that the energy densities in the electric and magnetic fields are equal: Therefore: Docsity.com 25-4 Energy and Momentum in Electromagnetic Waves The energy a wave delivers to a unit area in a unit time is called the intensity. Docsity.com 25-4 Energy and Momentum in Electromagnetic Waves Substituting for the energy density, An electromagnetic wave also carries momentum: Docsity.com 25-5 Polarization Polarized light has its electric fields all in the same direction. Unpolarized light has its electric fields in random directions. Docsity.com 25-5 Polarization A beam of unpolarized light can be polarized by passing it through a polarizer, which allows only a particular component of the electric field to pass through. Here is a mechanical analog: Docsity.com 25-5 Polarization A polarizer will transmit the component of light in the polarization direction: Docsity.com 25-5 Polarization A polarizer and an analyzer can be combined; the final intensity is: Docsity.com 25-5 Polarization LCDs use liquid crystals, whose direction of polarization can be rotated depending on the voltage across them. Docsity.com 25-5 Polarization Unpolarized light can be partially or completely polarized by scattering from atoms or molecules, which act as small antennas. If the light is already polarized, its transmission will depend on its polarization. Docsity.com Summary of Chapter 25 • Electromagnetic waves are traveling waves of oscillating electric and magnetic fields. • Electric and magnetic fields in an electromagnetic wave are perpendicular to each other and to the direction of propagation, and are in phase. • A right-hand rule gives the directions of the fields and propagation. • Any accelerated charge will emit electromagnetic waves. Docsity.com Summary of Chapter 25 • Electromagnetic waves can travel through a vacuum; their speed in a vacuum is always the same: • Doppler effect: • Electromagnetic waves can have any frequency. Docsity.com Summary of Chapter 25 • The entire range of frequencies is called the electromagnetic spectrum. Named portions of the spectrum, from the lowest frequencies to the highest, are radio waves; microwaves; infrared; visible light; ultraviolet; X-rays; and gamma rays. • Relationship of frequency and wavelength: • Energy density of an electromagnetic wave: Docsity.com