1. Light
What is it?

2. Light
What is it? moving energy
Wave or particle?

3. Light What is it? moving energy Wave or particle?
If a wave, what is waving? (waving even in a vacuum?)

4. Light What is it? moving energy Wave or particle?
If a wave, what is waving? (waving even in a vacuum?)
Electric & Magnetic Fields
How do we decide between wave and particle?

5. Light
what is it? moving energy wave or particle? if a wave, what is waving? (waving even in a vacuum?) Electric & Magnetic Fields How do we decide between wave and particle? Look at properties of light and see which theory explains the properties the best.

6. Properties of Light speed of light colors reflection shadows
refraction (bending)
energy theory
absorption of light
emission of light

7. Property 1: Speed of Light
particle (photon) ? no prediction
wave (E&M) ? in vacuum, v = c;
in material, v < c
(Here c stands for the speed of light in vacuum, which is 300,000,000 meters/second, or about 670 million miles per hour.)
From experiment, we find that the wave prediction works!

8. Property 2: Color Experiment ? Particle (photon) explanation?
Wave (E&M) explanation?

9. Property 2: Color Experiment: visible order: red orange yellow green
blue
violet

10. Property 2: Color Experiment: invisible as well as visible
total spectrum order:
radio
microwave IR
visible UV
x-ray and gamma ray

11. Property 2: Color Particle (photon) ? Amount of energy per photon
determines “color”

12. Property 2: Color Particle (photon) ? amount of energy
among different types:
x-ray - most energy; radio - least
in visible portion:
violet - most energy; red - least

13. Property 2: Color
Particle (photon) ? Amount of energy
Wave (E&M) ?

14. Property 2: Color Particle (photon) ? Amount of energy
Wave (E&M) ? Frequency
among different types of “light”:
low frequency is radio (AM is KHz)
high frequency is x-ray & gamma ray
in visible spectrum:
red is lowest frequency (just above IR)
violet is highest frequency (just below UV)

15. Wavelength and Frequency
“Nice” sine waves have a simple relation for wavelength and frequency: λ*f = v
where λ is the wavelength (distance from one crest to the next one),
where f is the frequency (how many times one location goes up and down a second),
and where v is the speed of the wave (how fast the crest of the wave moves). λ v

16. Light
For light in vacuum, the speed of the light wave is 300,000,000 meters/sec, or about 670 million miles/hour. We use the symbol “c” to denote this value. Therefore for light in vacuum, we have: λ*f = c .
Example: for a radio wave of frequency 100 MHz, the wavelength is:
λ * (100 * 1,000,000 Hz) = 300,000,000 m/s, or
λ = 300,000,000 m/s / 100,000,000 Hz = 3 meters.

17. Nanometers
The wavelength of visible light is in the range of meters to meters. This is an awkward way to write these numbers. In Scientific Notation, this becomes 4 x 10-7 m to 7 x 10-7 m. This is still somewhat awkward, so we often use the unit of nanometers (nm) which is 10-9 m; this gives the range for the wavelengths of visible light to be 400 nm to 700 nm.

18. Colors: frequencies & wavelengths (in vacuum)
AM radio  1 MHz ’s of m
FM radio  100 MHz m’s
microwave  10 GHz cm - mm
Infrared (IR) x1014Hz mm nm
visible 4x x nm -400nm
Ultraviolet (UV) 7.5x nm - 1 nm
x-ray &  ray > 1017 Hz < 1 nm

19. Property 3: Reflection Particle (photon) explanation?
Wave (E&M) explanation?

20. Property 3: Reflection Particle (photon) ? bounces “nicely”
Wave (E&M) ? bounces “nicely”
bounces nicely means angle incident = angle reflected

21. Property 4: Light and Shadows
Consider what we would expect from
particle theory: sharp shadows
dark
dark
light

22. Light and Shadows Consider what we would expect from
wave theory: shadows NOT sharp
crest
crest
crest
dark
dark
dim
light
dim

23. Light and Shadows What DOES happen? Look at a very bright laser beam
going through a vertical slit.
(A laser has one frequency
unlike white light.)
slit
pattern
screen

24. Diffraction: single slit
How can we explain the pattern from light going through a single slit?
screen x w L

25. Diffraction: single slit
In fact, we can break the beam up into 2n pieces since pieces will cancel in pairs. This leads to: (w/2n) sin(n) = /2 ,
or w sin(n) = n for MINIMUM.
screen x w L

26. Diffraction: circular opening
If instead of a single SLIT, we have a CIRCULAR opening, the change in geometry makes the single slit pattern into a series of rings; and the formula to be: n = D sin(n)

27. Diffraction: circular opening
Since the light seems to act like a wave and spreads out behind a circular opening, and since the eye (and a camera and a telescope and a microscope, etc.) has a circular opening, the light from two closely spaced objects will tend to overlap. This will hamper our ability to resolve the light (that is, it will hamper our ability to see clearly).

28. Rayleigh Criterion: a picture
The lens will focus the light to a fuzzy DOT
rather than a true point.
lens D

29. Rayleigh Criterion: a picture
If a second point of light makes an angle of
limit with the first point, then it can just be resolved.
lens D x x’ s’ s

30. Limits on Resolution:
Imperfections in the eye (correctable with glasses)
Rayleigh Criterion due to wavelength of visible light [A point of light going through an opening can only be focused to a finite fuzzy dot rather than a point. The wider the opening, the smaller the fuzzy dot and so the better the ability to “resolve” the image.]
Size (graininess) of retinal cells [Each retinal cell can only tell how much light is hitting it – it can’t tell if more light is hitting one part of it and less light hitting another part or if the light is hitting it evenly – in other words an individual cell can’t read]

31. Pixels
e e
The bigger the size of the dots, the less we can “resolve”. As you can see, with this first size of dots and this size of the letter, we couldn’t tell that the letter was an “e”. With the second, smaller, size dots, we can start to tell that the letter is an “e”. .0

32. Limits on Resolution: further examples
hawk eyes and owl eyes
cameras:
lenses (focal lengths, diameters)
films (speed and graininess)
shutter speeds and f-stops
Amt of light  D2 t
f-stop = f/D
f-stops & resolution: resolution depends on D

33. Limits on Resolution: further examples
other types of light
x-ray diffraction (use atoms as slits) IR
radio & microwave
surface must be smooth on order of 