1. General Physics L19_EMwaves Chapter 24
Light
This lecture takes lots of demonstration apparatus:
Transmission overhead projector with
Diffraction grating after focusing lens
Slit mask
Square mask
CMY filters
RGB filters
RGB filter wheels
Vertical-slit lamp and variac
Class set of diffraction gratings
R, G, B projectors
Chapter 24

2. General Physics L19_EMwaves
Objectives
Describe the transverse “medium” of electromagnetic waves.

3. Maxwell’s pivotal insight
Last Piece of EM Theory
Maxwell’s pivotal insight
A changing electric field acts as a current
Thought experiment: charging parallel plates E B + – I
Charges accumulate on the plates
The E field between the plates increases
The changing E field creates a virtual current
A changing E field creates a B field!

4. Electromagnetic Fields
Faraday’s law: a changing magnetic field creates an electric field
Virtual Current: a changing electric field creates a magnetic field DB DE

5. AC Creates Oscillating B Field
General Physics L19_EMwaves
AC Creates Oscillating B Field
Observe here

6. General Physics L19_EMwaves
The End Result
Electric and magnetic fields are perpendicular:
to each other, and
to the direction of propagation.

7. General Physics L19_EMwaves
Question
Are electromagnetic waves transverse, longitudinal, or a combination of both?
Transverse.
Longitudinal.
A combination of transverse and longitudinal.

8. General Physics L19_EMwaves
Speed of Light
(in vacuum)
c =  108 m/s

9. Electromagnetic Wave Energy
General Physics L19_EMwaves
Electromagnetic Wave Energy
Non-classical result:
E = hf
E = energy
f = frequency
h = Planck constant =  J s

10. Electromagnetic Spectrum
General Physics L19_EMwaves
Electromagnetic Spectrum
Demonstrate
Overhead projector with grating over focusing lens. Light from slit is diffracted into separate colors in both horizontal directions.

11. General Physics L19_EMwaves

12. Temp Influences Spectrum
General Physics L19_EMwaves
Temp Influences Spectrum
Higher T  greater power
P/A = sT4
s = 5.6710–8 W m–2 K–4
Higher T  higher peak frequency
lmax = b/T
b = 106 nm K
Demonstrate
Vertical slit with variac, students each have diffraction grating. Show that light is dimmer and redder at lower temperatures.
Source: M. A. Seeds, Exploring the Universe

13. General Physics L19_EMwaves
Poll Question
How do “warm” colors (red, orange, yellow) and “cool” colors (green, blue) relate to temperature?
At higher temperatures, more of the light emitted is “warm”-colored.
At higher temperatures, more of the light emitted is “cool”-colored.

14. General Physics L19_EMwaves
Something to Ponder
Why do hotter objects emit a greater fraction of their energy at short wavelengths (high frequencies)?

15. Next Time
Color
How we see color
Why things are colored