Lecture 2: Electromagnetic Radiation (aka
The Nature of Light)
stationary charge has electric field
accelerating charge produces ripples (waves) in electric field
propagation direction
perpendicular to E, B fields
(E and B perpendicular too)
H = B field in this plot
Five Fundamental Properties
a) speed of propagation, c
b) direction of propagation
c) wavelength
! points)
d) polarization (direction E
light from source generally mixture of waves with different b), c),
and d) properties
however, c is always 3 × 1010 cm/sec in vacuum
e) intensity
! and B
! grow
grows as magnitudes of E
one measure is energy/volume:
1 " "
" · B)
"
(E · E + B
energy density =
8π
1
(E 2 + B 2 )
=
8π
another measure is energy flux (energy crossing in some
direction/area/time):
c
! × B)
! Poynting’s formula
(E
f! =
4π
f = cEB/4π
flux = c(energy density)
Sidebar: Vector Arithmetic
!
A
!·B
! = ABcosθ
A
θ
dot product
!
B
project length A onto direction B and multiply result
commutative :
!·B
! =B
! ·A
!
A
! ⊥ B,
! what does A
!·B
! =?
If A
Sidebar: Vector Arithmetic
!
A
!×B
!
A
θ
! × B|
! = ABsinθ
|A
cross product
!
B
magnitude = area of parallelogram, direction given by
right-hand rule
!×B
! "= B
! ×A
!
not commutative: A
! ! B,
! what does A
!×B
! =?
If A
The Nature of Light (cont.)
How does a charged particle respond to light?
! B
!
force exerted on charge q moving at !v through E,
!
v
! + × B)
!
Lorentz’s formula
F! = q(E
c
highest a for electrons
from F! = m!a, get !a ∝ q/m
electron oscillates sympathetically with passing EM wave
electron’s oscillation generates waves that reinforce
or interfere (refraction or reflection)
why are mirror surfaces made of metal?
Telescopes
observations by collecting and analyzing light
a photon's journey...
• emitted by a source (such as a star)
• perhaps absorbed by interstellar material along the way,
perhaps re-emitted...
• crashes through atmosphere, perhaps absorbed (sunsets
are red), perhaps scattered (the daytime sky is blue)
• falls onto telescope mirror (bounces on several mirrors)
• is seen by our eye, photographed, or digitally imaged
key points of detecting radiation
• atmosphere limits light that reaches group by absorbing and
scattering it, and also blurs image
• telescopes collect light
• optics are used to reflect, refract, or disperse light
• at present, large optical telescopes can improve resolution up to
few tenths of an arcsecond
• instruments (computers and detectors) enable photometry and
spectroscopy, large and sophisticated on modern telescopes...
• and running them and telescope is computer-intensive activity...
• other ground based observing facilities in infrared and radio
wavelength regimes...
• space-based observatories remove constraints of atmosphere
(blurring and blocking of certain wavelengths of light)
ground-based image
Hubble Telescope image
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1/r Laws
gravity, electric force, light
let’s look at a radiating source
same amount of energy/time, or luminosity (L)
crosses successive spheres at larger r
energy/area/time (energy flux) at distance r = L/4πr
or, luminosity/surface area
standard candles --> distances
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