EM Spectrum

Physics 102Professor Lee

CarknerLecture 27

PAL #26 EM Waves Changing C of LC oscillator

f and C have an inverse square root relationship

Increase the frequency

Decrease the frequency

Double the frequency

Tune in WVIK (90.3 MHz) with L = 1X10-8 H (LC)½ = 1/2f C = [1/(2)()(90.3X106)]2(1/1X10-8) = 3.1X10-10 F

Energy in EM Wave

Each has an energy density: uE = uB =

The total energy density is the sum of each term

This energy density is not very useful

Called the intensity, I

Intensity of Flashlight

Intensity

Use Erms or Brms to get average The energy density of each field is the

same

The intensity depends on how much energy the wave delivers, which depends on the energy density and the speed:

I = I = c0Erms

2

Units: joules per second per square meter (W/m2)

Intensity of Light

We normally don’t know much about the E field of a light wave, but rather we know something about the source of the wave

As the waves travel out the power is spread out over a sphere of radius r (r is the distance away):

I = Ps / 4r2

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

Momentum Transfer The change in momentum due to light is

given by:

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

pr = I/c (total absorption)pr = 2I /c (total reflection)

Solar Sail

Example: Light Sail Radiation pressure can be used to power a

spacecraft

The sail can gather light from a star to propel the spacecraft

Light sail powered craft need no engines or fuel

Today’s PAL Consider a light sail craft near the Earth with a mass

of 5 tons (5000 kg) and a sail area of 2.25 x 108 m2 (about 15 by 15 km). The sail is made from a lightweight, thin, and highly reflective fabric and is powered by sunlight (distance to sun = 1.5 X 1011 meters, power of sun 3.9 X 1026 W). 

a)       What is the intensity of sunlight for the spacecraft?

b)       How much light pressure does the spacecraft feel?

c)       What is the acceleration of the spacecraft (remember F = ma, p = F/A)?

The Electromagnetic Spectrum

Electromagnetic radiation can have any wavelength

This is where our Sun produces the most light, so our eyes have evolved to detect it

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

EM Waves and Energy

The energy of an EM wave is proportional to the frequency

where h = Planck’s constant = 6.63 X 10-

34 J s

Radio waves and X-rays produce different effects

The Electromagnetic Spectrum

The EM Spectrum Radio

Low energy but good penetration

Millimeter (microwave) Also good for communication

Infrared Emitted strongly by living things

Visible What your eyes see

The EM spectrum

Ultraviolet High energy

X-ray Strongly penetrating

Gamma Ray Hard to produce Get from H bombs, etc.

Atmospheric Transmission

Gamma+

X-ray blocked

Infrared blocked

O2, N2

AbsorptionH2O, CO2

Absorption

Sensitivity of Your Eye

Light from the Sun

The sun (like all stars) is actually producing EM waves with a variety of wavelengths via many different processes

The sun’s visible light is thermal (blackbody)

radiation

The sun produces more yellow and red light than blue, so the sun looks orange

Solar Emission Spectrum

Monday’s Sun in Visible Light

Radio Waves from the Sun

A Coronal Loop

Solar Spectrum Sun emits most strongly at visible

wavelengths

Produces stronger IR at cooler regions

Produces high energy radiation in outer layers

Also produces low energy radiation in magnetic loops

Next Time

Read 24.10 Homework, Ch 22, P 21, Ch 24, P

53, 57 Final exam:

Section 1: Tuesday 9 am Section 2: Thursday 12 noon