Lecture 13 Electromagnetic Waves Chp. 34 Thursday Morning

1
Lecture 13 Electromagnetic Waves Chp. 34Thursday
Morning

• Cartoon -. Opening Demo - Warm-up problem
• Physlet
• Topics
• Light is a Electromagnetic wave
• eye sensitivity
• Traveling E/M wave - Induced electric and induced
  magnetic amplitudes
• Energy transport Poynting vector
• Pressure produced by E/M wave
• Polarization
• Reflection, refraction,Snells Law, Internal
  reflection
• Prisms and chromatic dispersion
• Polarization by reflection

2
Electromagnetic spectrum
3
Eye Sensitivity to Color
4
Production of Electromagnetic waves
5
How the fields vary at a Point P in space as the
wave goes by
6
Electromagnetic Wave
7
Properties of a plane wave of light
E and B are always perpendicular to the direction
of travel E is perpendicular to B E X B the
direction of travel E and B both vary with the
same frequency and in phase. Speed of wave is
independent of speed of observer Wave doesnt
need a medium to travel in.
8
A point source of light is one that emits
isotropically and the intensity of it falls off
as 1/r2
Let P be the power of the source in joules per
sec. Then the intensity of light at a distance r
is I P/4pr2
9
The Poynting vector S
S has units of energy/time per unit area or
Watt/m2
E is perp. to B and in the energy flows in the
direction of the wave. Since BE/c, we get for
the instantaneous power of the wave
Intensity I of the wave is defined as Savg
10

17. The maximum electric field at a distance of
10 m from an isotropic point light source is 2.0
V/m. Calculate (a) the maximum value of the
magnetic field and (b) the average intensity of
the light there? (c) What is the power of the
source?
(a) The magnetic field amplitude of the wave is
(b) The average intensity is
11
Radiation pressure
This is the force per unit area felt by an
object that absorbs light. (Black piece of
paper))
This is the force per unit area felt by an
object that reflects light backwards.
(Aluminum foil)
12
Polarization of light
13
(No Transcript)
14
(No Transcript)
15

35. In the figure, initially unpolarized light
is sent through three polarizing sheets whose
polarizing directions make angles of ?1 40o, ?2
20o, and ?3 40o with the direction of the y
axis. What percentage of the lights initial
intensity is transmitted by the system? (Hint Be
careful with the angles.)
Let Io be the intensity of the unpolarized light
that is incident on the first polarizing sheet.
The transmitted intensity of is I1 (1/2)I0, and
the direction of polarization of the transmitted
light is ?1 40o counterclockwise from the y
axis in the diagram. The polarizing direction of
the second sheet is ?2 20o clockwise from the y
axis, so the angle between the direction of
polarization that is incident on that sheet and
the the polarizing direction of the sheet is 40o
20o 60o. The transmitted intensity is
and the direction of polarization of the
transmitted light is 20o clockwise from the y
axis.
16

35. In the figure, initially unpolarized light
is sent through three polarizing sheets whose
polarizing directions make angles of ?1 40o, ?2
20o, and ?3 40o with the direction of the y
axis. What percentage of the lights initial
intensity is transmitted by the system? (Hint Be
careful with the angles.)
The polarizing direction of the third sheet is ?3
40o counterclockwise from the y axis.
Consequently, the angle between the direction of
polarization of the light incident on that sheet
and the polarizing direction of the sheet is 20o
40o 60o. The transmitted intensity is
Thus, 3.1 of the lights initial intensity is
transmitted.
17
Reflection and refraction
Snells Law
n1 n2
18

47. In the figure, a 2.00-m-long vertical pole
extends from the bottom of a swimming pool to a
point 50.0 cm above the water. What is the length
of the shadow of the pole on the level bottom of
the pool?
Consider a ray that grazes the top of the pole,
as shown in the diagram below. Here ?1 35o, l1
0.50 m, and l2 1.50 m. The length of the
shadow is x L. x is given by x l1tan?1
(0.50m)tan35o 0.35 m. According to the law of
refraction, n2sin?2 n1sin?1. We take n1 1 and
n2 1.33 (from Table 34-1). Then,
The length of the shadow is 0.35m 0.72 m 1.07
m.
19
Dispersion Different wavelengths have different
indices of refraction
20
(No Transcript)
21
(No Transcript)
22
Total Internal reflection