Since we will be studying electromagnetic waves, let s review some general features of waves:

Transcription

1 The Nature of Waves Since we will be studying electromagnetic waves, let s review some general features of waves: 1. A wave is a traveling disturbance. 2. A wave carries energy from place to place.

2 The Nature of Waves Longitudinal Wave - the disturbance caused by the wave moves along the direction that the wave propagates, e.g., sound waves, compressed slinky waves

3 The Nature of Waves Transverse Wave - the disturbance caused by the wave moves perpendicular to the direction that the wave propagates, e.g., water waves, shaken slinky waves, electromagnetic waves.

4 Periodic Waves Periodic waves consist of cycles or patterns that are produced over and over again by the source. In the figures, every segment of the slinky vibrates in a simple harmonic motion, provided the end of the slinky is moved in a simple harmonic motion.

5 Periodic Waves In the drawing, one cycle is shaded in color. The amplitude A is the maximum excursion of a particle of the medium from the particles undisturbed position. The wavelength is the horizontal length of one cycle of the wave. The period is the time required for one complete cycle. The frequency is the number of cycles per time. It is related to the period and has units of Hz, or s -1. f 1 T

6 Periodic Waves The propagation velocity of a periodic wave is related to its frequency and wavelength. Consider the motion of a long train as a periodic wave which repeats itself with the passing of each identical car: Since velocity is distance/time è λ v T fλ

7 Chapter 22 Electromagnetic Waves

8 The Nature of Electromagnetic Waves How to produce an electromagnetic wave Two straight wires connected to the terminals of an AC generator can create an electromagnetic wave. Electric part of wave: In each part of the drawing, the red arrow represents E produced at point P by the oscillating charges on the antenna at the indicated time. The black arrows represent E created at earlier times. For simplicity, only the electric wave traveling to the right is shown here.

9 The Nature of Electromagnetic Waves Magnetic part of wave: The current used to generate the electric wave creates a magnetic field. Using RHR to find the direction of B at point P for this current direction shows that B is perpendicular to E since E is parallel to I.

10 The Nature of Electromagnetic Waves We just showed how the electromagnetic (E&M) wave is initially generated by the ac voltage source near the antenna (near field). As the wave moves farther away, it propagates itself by the changing E-field producing a B-field and the changing B-field producing an E-field (radiation field). è E&M wave is transverse and can travel through a vacuum radiation field wave far from the antenna. The speed of an electromagnetic wave in a vacuum is: c m s

11 The Nature of Electromagnetic Waves A radio wave can be detected with a receiving antenna wire that is parallel to the electric field: è E generates an oscillating current along the antenna wire.

12 The Nature of Electromagnetic Waves With a receiving antenna in the form of a loop, the magnetic field of a radio wave can be detected, è From Faraday s law, the changing magnetic flux in the loop will create an oscillating current in it.

13 The Electromagnetic Spectrum Like all waves, electromagnetic waves have a wavelength and frequency, related by: c fλ

14 The Electromagnetic Spectrum Example: The Wavelength of Visible Light Find the range in wavelengths for visible light in the frequency range between 4.0 x Hz and 7.9 x Hz. λ c f m s Hz m 750 nm red λ c f m s Hz m 380 nm violet

15 The Electromagnetic Spectrum Example: The Wavelength of Radio Waves A station broadcasts AM radio waves whose frequency is 1230 x 10 3 Hz and FM radio waves whose frequency is 91.9 x 10 6 Hz. Find the wavelength of each type of wave. AM c v λ f m s Hz 244 m ~ three football fields FM c v λ f m s Hz 3.26 m ~ 10 ft

16 The Electromagnetic Spectrum Conceptual Example: The Diffraction of AM and FM Radio Waves Diffraction is the ability of a wave to bend around an obstacle or the edges of an opening. Would you expect AM or FM radio waves to bend more readily around an obstacle such as a building? AM waves are much longer than FM waves (as seen in our example), and waves tend to bend easier around objects (i.e. diffract) when the object s size is on the order of or less than the size of the wavelength. It is found that AM waves bend easier around buildings and hills than FM waves, which are essentially line-of-sight.

17 The Speed of Light The speed of light in a vacuum c m s Michelson device to measure the speed of light (c. 1926). If the angular speed of the rotating mirror is adjusted just right, the observer can see the light source after it has reflected from the path shown. From this angular speed and knowing the distance to the fixed mirror, the speed of light can be calculated.

18 f L Calculate the minimum frequency the rotating mirror must turn to measure the speed of light. Period of turning t path 2L c For observer to see light T 8 t path 1 8 f 2L c f c 16L ( ) 540 Hz

19 The Speed of Light Conceptual Example: Looking Back in Time A supernova is a violent explosion that occurs at the death of certain stars. The figure shows a photograph of the sky before and after a supernova. Why do astronomers say that viewing an event like this is like looking back in time?

20 The Speed of Light Maxwell s prediction of the speed of light Assuming that E&M waves are produced by oscillatory electric and magnetic fields, in 1865 Maxwell predicted the speed of light using the values known at the time for the permittivity of free space, ε 0, and the permeability of free space, µ 0, as, c ε µ o o ( C ( N m ))( 4π 10 T m A) m s This is in excellent agreement with the experimental value.