Electromagnetic Radiation

Physics 202Professor Lee

CarknerLecture 20

What is Light?

A light wave has no medium

A light particle is called a photon The speed of light in a vacuum is a

constant, called cc = 3 X 108 m/s

As for all waves, f = v = c

The Electromagnetic Spectrum

We often think of light as being visible light

Visible light is just the portion from 400-700 nanometers (nm)

Radio waves, microwaves, gamma rays etc. are all forms of electromagnetic radiation with different wavelengths

We will use the terms “light”, “photons” and “electromagnetic (EM) radiation or waves” interchangeably

EM Spectrum

The EM Spectrum Radio

> 1 meter

Millimeter (microwave) 1 m - 1 mm

Infrared 1 mm - 700 nm

Visible 700-400 nm

Ultraviolet 400 nm - 100 A

X-ray 100 A - 0.01 A

Gamma Ray < 0.01 A hard to produce and

dangerous

Atmospheric Transmission

Gamma+

X-ray

Infrared

O2, N2

AbsorptionH2O, CO2

Absorption

Sensitivity of Your Eye

Intensity of Light

If a light source has a power Ps (in J/s), then the intensity at any point is:

I = Ps / 4r2

This can also be written:F = L / 4d2

Where F is the flux (J/s/m2) and L is the luminosity (J/s)

Light (like sound) falls off with an inverse square law

Inverse Square Law

Radiation Pressure

If someone shines a flashlight on you,

the light is trying to push you away

EM pressure is due to the fact that light has momentum which can be transmitted to an object through absorption or reflection

Comet Hale-Bopp

Comet Tails

Momentum Transfer

p = U/c Where p is the momentum change and

U is the energy change

For reflection the momentum change is twice as much:

p = 2U/c

Light Pressure From Newton’s second law

The amount of energy delivered in time t

is:

where I is the intensity and A is the area Since pressure (pr) is force per unit area the

pressure becomes:pr = I/c (total absorption)

pr = 2I /c (total reflection)

Example: Light Sail

A light sail is a very large, very thin, very reflective piece of fabric to which a spacecraft is attached

Can also use a laser

Do need very large sails

The EM Wave

Lets consider light as a wave What is oscillating?

An EM wave consists of an electric field wave (E) and a magnetic field wave (B) traveling together

An EM wave is transverse (like string waves) The field waves are sinusoidal and in phase

Wave Equations We can generalize the waves as:

E = Em sin (kx -t)

B = Bm sin (kx -t) Nothing is actually moving

A moving E field induces a B field

The two fields continuously create each other The speed of the wave is related to the fields:

Traveling EM Wave

Key Constants Two important constants in E and M are the permittivity

constant 0 and the permeability constant 0

0 = 8.85 X 10-12 F/m

In farads per meter Measure of how electric fields propagate through space

0 = 1.26 X 10-6 H/m

In henrys per meter

The wave speed depends on these constants:c = 1/(0 0)½

Poynting Vector

flux = W/m2 = J/s/m2

Flux for an EM wave can be given by the Poynting vector:

S = (1/0) EB = S = (1/c 0) E

I = (1/c 0) Erms2

Where Erms is the root-mean-square value of the electric field

Next Time

Read: 33.8-33.10