1. PHYS 221 Recitation
Kevin Ralphs
Week 9

2. Overview
HW Questions
Electromagnetic Waves
Malus’s Law

3. HW Questions
Ask Away…

4. Electromagnetic Waves
Maxwell predicted from Faraday’s law and his correction to Ampere’s law that EM waves exist
They are transverse waves
Originally it was thought that a substance called the luminiferous aether mediated EM waves
Direction of propagation: 𝑘 =𝛼 𝐸 × 𝐵

5. Electromagnetic Waves
They carry energy
This follow from the fact that electric and magnetic fields have energy
The intensity of an EM wave tells you how much energy is flowing across a surface in a span of time
There are 2 ways of calculating the intensity:
From the electric and magnetic fields
𝐼= 𝑢 𝑇𝑂𝑇 ×𝑐= 𝜀 𝑜 𝐸 𝑅𝑀𝑆 𝜇 𝑜 𝐵 𝑅𝑀𝑆 2 ×𝑐= 𝜀 𝑜 𝐸 𝑅𝑀𝑆 2 𝑐
From the power emitted by a source
𝐼= 𝑃 𝐴
I: intensity, P: power emitted, A: surface area the waves propagate over

6. Electromagnetic Waves
They carry momentum
As we shall see when we talk about relativity, momentum and energy are related to one another.
Without considering this Newton’s laws would fail in the presence of electromagnetic waves
𝑝= 𝐸 𝑡𝑜𝑡𝑎𝑙 𝑐
Since they carry momentum, they can apply pressure to an object
𝑃 𝑟𝑎𝑑 = 𝐼 𝑐
Prad: radiation pressure, I: Intensity, c: vacuum speed of light

7. Malus’s Law What does it tell me?
How the intensity of polarized light is affected by a polarizer
𝐼= 𝐼 𝑜 cos 2 𝜃
Your book defines theta to be the angle between the transmission axes of two polarizers
Alternatively, it is the angle between the plane of polarization and the transmission axis of the polarizer
If the light is unpolarized, the intensity if halved