EM Spectrum Physics 102 Professor Lee Carkner Lecture 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 = 1/2  f   C = [1/(2)(  )(90.3X10 6 )] 2 (1/1X10 -8 ) = 3.1X F

Energy in EM Wave   Each has an energy density:  u E =  u B =  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 E rms or B rms 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 = c  0 E rms 2  Units: joules per second per square meter (W/m 2 )

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 = P s / 4  r 2   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 = 2  U/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  p r = I/c (total absorption) p r = 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 10 8 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 meters, power of sun 3.9 X 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 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 O 2, N 2 Absorption H 2 O, CO 2 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  Homework, Ch 22, P 21, Ch 24, P 53, 57  Final exam:  Section 1: Tuesday 9 am  Section 2: Thursday 12 noon