Chapter E16 Electromagnetic Waves

An Electric Field in Empty Space Empty space is a medium for electric waves Suppose an electric field suddenly appears at x=0 –The blackboard became positively charged This would cause a planar disturbance that would move in the x direction (toward the back of the room) at the speed of light.

The Electromagnetic wave If the electric field in the plane x=0 varies sinusoidally, an electric wave is formed that propagates in the x direction with a velocity c. –This can easily be done with a vibrating electrical charge According to the equations on the last slide, this causes a magnetic wave. –It is impossible to have one without the other –A changing electric field causes a magnetic field. The magnetic field is perpendicular to the electric fields and the two fields are in phase. –When the electric field is max the magnetic field is also maximum.

A planar electromagnetic wave traveling in the direction v.

If the line above were a radio antenna, what would be the direction the electric field would be waving at your seats? What is the plane of vibration of the magnetic field?

Types of Electromagnetic radiation from longest to shortest Radio waves –No known effect on humans Microwaves –Destroy tissue and can cause genetic defects Infra-red –Feels like heat, not dangerous Visible –Electromagnetic waves we can see –Long wavelength = red –Short wavelength = violet

Types of Electromagnetic radiation from longest to shortest (2) Ultra-violet radiation –Can cause chemical reaction –Sunburn –Genetic defects X-rays and gamma rays –Can kill cells –Can cause genetic defects

This shows the entire electromagnetic spectrum, frequency (ν) and wavelength (λ)

This shows what electromagnetic wavelengths can penetrate the atmosphere and reach the surface of the Earth.

Why the sky is blue Light from the sun strikes the electrons in the atoms of the gases in the atmosphere causing them to vibrate. These electrons radiate energy proportional to ω 4, so blue (shorter) is scattered (deflected) much easier than red. This explains why the sky is blue at noon and red at sun rise and sun set.