1. Light What does light look like? We can detect it, but that doesn’t
mean we know what it looks like
As it turns out, light is a form of
electromagnetic radiation

2. Electromagnetic Radiations
Consider a vertical wire; if an EMF is put across
the wire, current will flow; this current generates
a magnetic field
More general : an EMF will produce an electric field
across the wire; the electric field produces a
magnetic field
If the EMF is reversed, the electric field will reverse;
a continuously changing
EMF produces a continuously
changing electric field

3. The continuously changing electric field produces a continuously changing magnetic field, oriented perpendicular to the electric field
The continuously changing magnetic field produces a
continuously changing electric field
The effect is self-sustaining; once generated at the antenna,
it will generate itself from then on
James Clerk Maxwell, in late 1862, derived four
equations governing the effect; Maxwell’s equations

4. Maxwell’s Equations (1862)
James Clerk Maxwell

5. Net result: an electromagnetic wave that is:
- transverse and sinusoidal
- can travel through a
vacuum; does not need a
medium
has an electric field component and a magnetic field
component, oriented perpendicular to each other and to
the direction of propagation 1
μo = 4π x 10-7 Tm/A
εo = 8.85 x C2/Nm2
- speed of propagation =
μoεo
c = x 108 m/s

6. 528 revs per sec
Michelson, 1924 : x 108 m/s

7. c = f λ Since EM radiations are waves, they follow v = f λ
For v, use c
c = f λ
c = 3.00 x 108 m/s
EM radiations can have many different frequencies (and
wavelengths and energies); different frequencies result in
different properties; organized in the
Electromagnetic Spectrum

8. A range of frequencies of EM radiations, all of which:
are transverse and sinusoidal waves
can travel through a vacuum
come from our sun
travel at the same speed, c, through a vacuum

9. Radio Waves
Very long λ, lowest frequency and energy
Used for communication
AM (amplitude-modulated)
- affected by electricity (lightning,
power lines)
- can bounce off the ionosphere of Earth
FM (frequency-modulated)
- not affected by electricity;
cleaner signal
- shorter ranges

10. Microwaves Long λ, low frequency and energy
Used for communications (military) and radar
In microwave ovens, the cathode emits electrons,
which are accelerated by magnetron, producing
microwaves
Absorbed by water molecules
Energy absorbed by water molecules; makes molecules
vibrate faster; energy comes out as heat; so food heats up

11. Infrared (IR)
Moderately long λ, moderate frequency and energy
We feel this radiation as heat; you feel this when you
put your hands next to a fire

12. Remember, all EM radiations:
are transverse waves
can travel through a vacuum
come from our sun
travel at the same speed, c, through a vacuum

13. Visible Light Moderately short λ, moderately high frequency and energy
Causes chemical reactions in our eyes; so we can
detect it
Narrow band; our sun puts out its most intense
radiation in this region
More on this later

14. Ultraviolet (UV) Short λ, fairly high frequency and energy
Causes a chemical reaction in your skin;
production of melanin causes you to “tan”
Can cause melanoma, or skin cancer
Harmful to vegetation
Absorbed by O3 (ozone) in the atmosphere
Causes certain minerals to fluoresce

15. X-rays Short λ, high frequency and energy
Absorbed by bone; partially absorbed
by flesh
Personal exposure regulated by
government
Discovered by accident by William Roentgen

16. Remember, all EM radiations, including
microwaves, visible light, and ultraviolet:
are transverse waves
can travel through a vacuum
come from our sun
travel at the same speed, c, through a vacuum

17. Gamma Rays Shortest λ, highest frequency and energy
Produced in nuclear reactions
Damages DNA if absorbed by cells
Produced in solar atmosphere (corona)
Should not look at
eclipse directly

18. Polarized Light EM radiation (light) has two components, oriented
perpendicular to each other
Can absorb one component by forcing it to go through a
“polarizer”, or sort of picket fence

19. Polarized Light EM radiation (light) has two components, oriented
perpendicular to each other
Can absorb one component by forcing it to go through a
“polarizer”, or sort of picket fence

20. Polarization Mini-Lab
Using a polarizing filter as an analyzer, determine if the
following light is polarized. (Yes or No)
______ (a) Light from the room lights
______ (b) Light from an incandescent bulb
______ (c) Light from a light bulb reflected from a mirror
______ (d) Light from a light bulb reflected by glass
______ (e) Light from a light bulb reflected from water
______ (f) Light from a light bulb reflected from the floor
______ (g) Light from the blue sky
______ (h) Light reflected from a cloud or the Moon
______ (i) Light from a CRT computer monitor ( the old kind )
______ (j) Light from a flat-screen monitor
______ (k) Light from a cell phone screen
______ (l) Light from an Ipod screen
______ (m) Light from a calculator display no no no
yes
yes
yes
yes no no
yes
yes
yes
yes

21. Light is polarized upon reflection from a
non-metallic surface

22. “Glare” is light that is partially polarized upon reflection;
polarized so that it is mostly horizontal
Polarizing axis in polarizing sunglasses is oriented
vertically to absorb the glare