Light

what is it?

Light

• what is it? moving energy• wave or particle?

Light

• what is it? moving energy• wave or particle?

• how do we decide?

Light

• what is it? moving energy• wave or particle?

• how do we decide?

• if a wave, what is waving?

(waving even in a vacuum?)

Light

• what is it? moving energy• wave or particle?

• how do we decide?

• if a wave, what is waving?

(waving even in a vacuum?)

Electric & Magnetic Fields

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 < cFrom experiment, we find that the wave

prediction works!(Here c stands for the speed of light in vacuum,

which is 300,000,000 meters/second, or about 670 million miles per hour.)

Property 2: Color

• experiment ?

• particle (photon) ?

• wave (E&M) ?

Property 2: Color

experiment ?visible order:

• red• orange• yellow• green• blue• violet

Property 2: Colorexperiment ?

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) ? frequencyamong different types of “light”: low frequency is radio (AM is 500-1500 KHz)

high frequency is x-ray & gamma rayin 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, orλ = 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) ?

• wave (E&M) ?

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

lightdark dark

Light and Shadows

Consider what we would expect from

wave theory: shadows NOT sharp

lightdark darkdimdim

crest

crest

crest

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.)

Diffraction: single slit

How can we explain the pattern from light going through a single slit?

w

screen

L

x

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.

w

screen

L

x

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.

lensD

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. lensD

s’

x’

s

x

Limits on Resolution:

• Imperfections in the eye (correctable with glasses)

• Rayleigh Criterion due to wavelength of visible light

• Graininess of retinal cells

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