Light What is it?

Light What is it? moving energy Wave or particle?

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

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?

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.

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

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!

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

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

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

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

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

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

Property 2: Color Particle (photon) ? Amount of energy Wave (E&M) ? Frequency among different types of “light”: low frequency is radio (AM is 500-1500 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)

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

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.

Nanometers The wavelength of visible light is in the range of 0.000000400 meters to .000000700 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.

Colors: frequencies & wavelengths (in vacuum) AM radio  1 MHz 100’s of m FM radio  100 MHz m’s microwave  10 GHz cm - mm Infrared (IR) 1012 - 4x1014Hz mm - 700 nm visible 4x1014 - 7.5x1014 700nm -400nm Ultraviolet (UV) 7.5x1014 - 1017 400 nm - 1 nm x-ray &  ray > 1017 Hz < 1 nm

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

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

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

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

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

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

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

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: 1.22 n = D sin(n)

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).

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

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

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]

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

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

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 