Transcript q 1 - Helios

Electromagnetic Waves
Physics 202
Professor Lee Carkner
Lecture 19
PAL #18 EM Radiation
Acceleration of lightsail craft
F = ma = prA
a = prA/m
pr = 2I/c
I =
pr = (2)(1379)/(3X108) = 9.2X10-6 N/m2
a = (9.2X10-6)(2.25X108)/5000 = 0.41 m/s2
Time to get to moon
d = ½at2
t =
t = 43054 sec ~ 12 hours
PAL #18 EM Radiation
How are Earth, Moon, Sun lined up?
Sunlight only pushes in one direction

How do you get back?
You are moving very fast away from
the Earth and you can’t brake or
reverse thrust
Possible answers

Moonbase sends out a spaceship to
stop you
Wait ½ month and sail back to Earth
Polarization

 The plane containing the E
vectors is called the plane
of oscillation
 EM waves in which the E
vector are preferentially
located in specific planes
are polarized

 Any given wave has a
random plane of oscillation
Polaroid

 Polaroid is a sheet of material
that will only pass through
the components of the E
vectors in a certain direction

 If you put a horizontal
Polaroid sheet on top of a
vertical Polaroid sheet no
light gets through
Polarization and Intensity

The sum of all of the y components should be
equal to the sum of all of the z components

I = ½ I0
This is true only when the incident light is
completed unpolarized
What about polarized light hitting Polaroid?
Incident Polarized Light
 For polarized light incident on a
sheet of Polaroid, the resultant
intensity depends on the angle q
between the original direction of
polarization and the sheet
 The new electric field becomes:
 Since I depends on E2 it becomes:
I = I0 cos2 q

 For unpolarized light that pass
through two polarizing sheets, q
is the angle between the two
sheets
Means of Polarization
A sheet of Polaroid has long molecules embedded
in it all aligned in one direction

A similar effect is seen in light passing through
interstellar dust clouds

Light can also be polarized by reflection
Reflection and Refraction

 The normal line is a line
perpendicular to the interface
between the two mediums
 Angles
 Angle of incidence (q1):
 Angle of reflection (q1’):
 Angle of refraction (q2): the angle of
the refracted ray and the normal
Laws
 Law of Reflection

 Law of Refraction
 The angle of refraction is related to the angle of incidence by:
n2 sin q2 = n1 sin q1

 n is always equal to or greater than 1

 Larger n means more bending
General Cases
 n2 = n1

 q2 = q1

 n2 > n1

 q2 < q1

 n2 < n1

 q2 > q1

Total Internal Reflection
 Consider the case where q2 =
90 degrees

 For angles greater than 90
there is no refraction and the
light is completely reflected

n1 sin qc = n2 sin 90
qc = sin-1 (n2/n1)
 This is the case of total internal
reflection, where no light
escapes the first medium
Chromatic Dispersion

 In general, n is larger for shorter
wavelengths

 Incident white light is spread out
into its constituent colors

Polarization By Reflection
 Light reflected off of a
surface is generally
polarized

 When unpolarized light
hits a horizontal surface
the reflected light is
partially polarized in
the horizontal direction
and the refracted light
is partially polarized in
the vertical direction
Brewster Angle

At qB the reflected and refracted rays are
perpendicular to each other, so
qB + qr = 90

qB = tan-1 (n2/n1)
If we start out in air n1 = 1 so:
qB = tan-1 n

Next Time
Read: 34.1-34.6
Consider a dust grain near a star. If the
grain is perfectly balanced between
light pressure out and gravity in, what
happens to the grain if the mass
doubles (but the size stays the same)?
A) Goes in
B) Goes out
C) Stays put
Consider a dust grain near a star. If the
grain is perfectly balanced between
light pressure out and gravity in, what
happens to the grain if the mass
doubles and the surface area doubles?
A) Goes in
B) Goes out
C) Stays put
Consider a dust grain near a star. If the
grain is perfectly balanced between
light pressure out and gravity in, what
happens to the grain if the distance
from the star doubles?
A) Goes in
B) Goes out
C) Stays put