Electromagnetic Waves Physics 202 Professor Lee Carkner Lecture 21
Dec 20, 2015
Electromagnetic Waves
Physics 202Professor Lee
CarknerLecture 21
PAL #20 EM Radiation Acceleration of lightsail craft
F = ma = prA a = prA/m pr = 2I/c I = Ps/4r2 = (3.9X1026)/((1.5X1011)2) = 1379 W 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 = (2d/a)½ = [(2)(3.8X108)/(0.41)] ½
t = 43054 sec ~ 12 hours Problems
Sunlight only pushes in one direction How do you stop or go back?
Gravity and inherited motion also important
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 inB) Goes outC) 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 inB) Goes outC) 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 inB) Goes outC) Stays put
Polarization
The plane containing the E vectors is called the plane of oscillation
Most light sources are unpolarized 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 between the original direction of polarization and the sheet
E = E0 cos
I = I0 cos2
For unpolarized light that pass
through two polarizing sheets, is the angle between the two sheets
Multiple Sheets
Sheet Angles
Means of Polarization
This alignment permits only the components in that direction to pass
The dust grains are partially aligned by the galactic magnetic field and so the light is partially polarized
Light can also be polarized by reflection
Reflection and Refraction When light passes from one medium to
another (e.g. from air to water) it will generally experience both reflection and refraction
Refraction is the bending of the portion of the light that does penetrate the surface
Geometry
Angles Angle of incidence (1): the
angle between the incident ray and the normal
Angle of reflection (1’):
Angle of refraction (2): the angle of the refracted ray and the normal
Laws Law of Reflection
Law of Refraction
n2 sin 2 = n1 sin 1
Where n1 and n2 are the indices of refraction of the mediums involved
Index of Refraction
Every material has an index of refraction that determines its optical properties
n is always greater than or equal to 1
Large n means more bending
General Cases
n2 = n1 2 = 1 e.g. air to air
n2 > n1 2 < 1 e.g. air to glass
n2 < n1 2 > 1 e.g. glass to air
Total Internal Reflection Consider the case where 2 = 90
degrees
For angles greater than 90 there is no refraction and the light is completely reflected
n1 sin c = n2 sin 90
c = 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
Chromatic dispersion with raindrops causes rainbows
Chromatic Dispersion
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 B the reflected and refracted rays are perpendicular to each other, so
B + r = 90
B = tan-1 (n2/n1) If we start out in air n1 = 1 so:
B = tan-1 n
Next Time
Read: 34.1-34.6 Homework: Ch 33, P: 37, Ch 34, P:
7, 9, 10, 11, 12, 13, 15