Let There Be Light 13 3 Where are
Let There Be Light 13. 3
Where are we? • Exam #3 on Monday Morning – Watch for another of those Webby Things • Today, we continue our study of optics and continue working with the optics kit. • Next week … optics marathon. Start reading chapter 24 after the test
Which one of the following statements concerning electromagnetic waves is false? A) Electromagnetic waves are longitudinal waves. B) Electromagnetic waves transfer energy through space. C) The existence of electromagnetic waves was predicted by Maxwell. D) Electromagnetic waves can propagate through a material substance. E) Electromagnetic waves do not require a physical medium for propagation.
The electric field E of an electromagnetic wave traveling the positive x direction is illustrated in the figure. This is the wave of the radiation field of an antenna. What are the direction and the phase relative to the electric field of the magnetic field at a point where the electric field is in the negative y direction? Note: The wave is shown in a region of space that is a large distance from its source. A) B) C) D) E) +y direction, in phase –z direction, 90° out of phase +z direction, 90° out of phase –z direction, in phase +z direction, in phase Think about the diagram we used yesterday and how the charge moves to produce the wave in the first place!
Glare tends to be A. B. C. D. un-polarized light polarized vertically light polarized horizontally the polarization depends on the conductivity of the material causing the glare
Which one of the following colors of visible light has the highest frequency? A) B) C) D) E) yellow red green blue violet
Like all waves, electromagnetic waves have a wavelength and frequency, related by:
MEASURING THE SPEED OF LIGHT WITHOUT ELECTRONICS … THE OLDEN DAYS The speed of light in a vacuum
24. 3 THE SPEED OF LIGHT Maxwell’s prediction of the speed of light
MORE ON POLARIZATION
FROM CAPACITOR AND COIL WE FOUND The total energy density carried by an electromagnetic wave Must contain moving energy
INTENSITY
E AND B ARE COUSINS! � � Maxwell’s Equations show that the energy density of the magnetic field and the electric field in an electromagnetic wave are the same. Consequently
MORE ON POLARIZATION In polarized light, the electric field fluctuates along a single direction.
IMPORTANT STUFF HERE Polarized light may be produced from unpolarized light with the aid of polarizing material.
MALUS’ LAW intensity after analyzer intensity before analyzer
24. 6 POLARIZATION Example 7 Using Polarizers and Analyzers What value of θ should be used so the average intensity of the polarized light reaching the photocell is one-tenth the average intensity of the unpolarized light? S 0/2
Conceptual Clicker Suppose that a third piece of polarizing material is inserted between the polarizer and analyzer. Does light now reach the photocell? A Yes B No C More info required.
THE EXPERIMENT - MIRRORS Complete the last one first.
REFLECTION
Prepare to Discuss These Problems After The Experiment: 1. A friend is standing 2 m in front of a plane mirror. You are standing 3 m directly behind your friend. 2. How far is your image behind the mirror? 3. What is the distance between you and the image of your friend? 4. Suppose that you are walking perpendicularly with a velocity of +0. 90 m/s toward a stationary plane mirror. What is the velocity of your image relative to you? The direction in which you walk is positive direction.
GET TO WORK ON MIRRORS I Feel Horrible!
- Slides: 24
