optics fields? the observer measures the e and b field ... · colors of light? i could bring rays...

OPTICS

Electromagnetic radiation---waves of oscillating E&B fields (these are tied together). What makes electromagnetic fields?

If you wiggle a charge back and forth, then you are also making a changing current.

The observer measures the E and B field

changes as tied together. I can't do one without the other----

Electromagnetic Radiation!

One part of the electromagnetic spectrum is at a "wiggling" or wave frequency of visible light. Higher frequencies carry more energy (more work to make that faster wiggling wave--try it with a slinky).

Whether x-rays, UV, IR, Micro-wave, Radio wave, ---all of these are

different frequencies/wavelengths/"colors" of electromagnetic spectrum---They all have similar properties to visible light.

Some materials absorb or emit different colors of visible light,Some materials absorb micro-waves well--others don'tSome materials absorb x-rays well, others don't. etc.

We use light (often visible) to interact with atoms, chemicals, and do

spectral analysis.

THE SPEED OF LIGHT

Notes 11--Optics-start

The speed of light is one of the most important properties in general

(speed of waves).

Non-things, can go faster. ○

"c"--the speed of light--Nothing goes faster than this.•

In matter, light may travel slower than this.•How much slower depends on material properties--like the index of refraction

•

Light has both WAVE and also PARTICLE properties•

Geometric means particle like---○

light travels in straight lines. ○

Geometric, and Wave optics---we start with Geometric•

Properties of light and EM Radiation (electromagnetic=EM)

The wavelength of visible light, , ranges from about

350nm to 700nm (rough range)

The frequencies are in the ballpark of 1015 Hz (Hertz=cycles/second)

Long

wavelength

Short

wavelength


We want to be able to control and redirect light-•Control light distribution---"locations"---spatially•Also control when light reaches points-----"temporally"•

Goals of optics

Reflection•Refraction•Interference•Diffraction•

•

We can redirect light and control it using

Intensity, •Direction•Color/Frequency/Wavelength•Polarization•Speed •

•

Other properties of light

Why do we see "visible" light---why can't we see--say x-rays, like super-

girl, or IR--like Arnold (I mean--the T-1000).

The sun seems to peak somewhere in the mid visible spectrum, around yellowish.

○

We seem to adapt pretty well to the light that is "abundant"○

What light is around?•

Those wavelengths/colors/frequencies have higher energy. In

fact --those energies destroy organic bonds (Tanning booths---bad).

○

So, it is not a good idea to have optics (lens of our eye) focus

harmful light on our retina---Nature does not make these bad adaptations (and survive).

○

What if I could see say UV or X-Rays (even though there is less of it)•

IR, these are longer wavelengths---the aperture (pupil) scatters light via ---diffraction---for longer wavelengths--we need big apertures. Some animals have this (not much bigger).

○

To see radio waves we need eyeballs the sizes of meters to km to see long waves clearly ---radio waves require eyeballs this big-----https://www.youtube.com/watch?v=RFOw79Oohv8

○

What if I could see IR or Radio waves---well----•


https://www.youtube.com/watch?v=RFOw79Oohv8

Geometric Optics--Refraction and Reflection

PHET Refraction/Reflection-plane surfacehttps://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

PHYSLET---lens, mirror, beams--Optics Bench (zoom your web page in "ctrl +" for best use, "ctrl -" gets you back out)https://www.compadre.org/Physlets/optics/illustration35_1.cfm?NOH=1

Consider how to measure the speed of a wave (like light, or sound).Sound first.---Take a trip to stone mountain---stand way back (500 meters or so), and shout (a brief Yelp as loud as you can). Time how long the echo takes to come back from that big granite surface.

If we know the distance (measure it) and can measure the time from

"yelp" to receiving the echo, then we have the speed of sound.

v=distance round trip/time round trip

The speed of sound in air is ~340 m/s by the way. Lighting (light travels very fast) strikes where I hear thunder ~3 seconds after lighting---is only about 1.00km away. If 1 second, then only ~3 football fields.

Either a really accurate clock for measuring short times for that

"echo" (reflection)

1)

Or a really long distance path for light to travel and come back

(Astronomical distances).

2)

I can measure the speed of light in a lab---because I can measure with a good oscilloscope--light traveling about 1 foot, in only 1 nanosecond. …..but what did they do back when----way way back.

3)

Light travels almost a million times faster than sound. So how doe we measure that? To measure the speed of light I need


https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

https://www.compadre.org/Physlets/optics/illustration35_1.cfm?NOH=1

Some of the Early measurements for speed of light. Ole Roemer

Starting with Earth in position 1, I watch the moon "IO" go around Jupiter. I push a stopwatch (some kind of clock) and say "Start"--when IO is at the position shown (against backdrop of stars). I wait for the Earth to move and I push "Stop" when IO is back in its position. For time t2 light needed to travel extra distance, since the Earth moves during that time. "Stop " came late--due to speed of light.

I do this experiment six months later, when the Earth is on the other side of the sun, and now go from time 3 to time 4. Now, my "stop" is called early, since the Earth is moving toward IO.

These two measurements give a result not too far off of the speed of

light---using almost no technology. (around 1670). Remember that Galileo was "under house arrest, and excommunicated (pardoned by

church in mid 1990's) " for saying "Look through my telescope, and see the moons going around Jupiter" (1633). It was a rough time to be a scientist.


We can measure the speed of light--really important--and really fast.

c=~ 3.00x108 m/s

8 minutes for light to reach Earth from the Sun (so if the sun blew up

right now---we won't know for 8 minutes)

•

About 2.5 seconds round trip to the moon and back----so if you notice brief latency/lag times---it's real.

•

A little more than 1/10th of a second for light to travel around the Earth (whether in fiber cables, radio waves to satellites, …whatever)

•

It takes approximately :

GEOMETRIC OPTICS (REFLECTION AND REFRACTION).

Play with the PHET simulation---all aspects PHET Refraction/Reflection-plane surfacehttps://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

When light is incident upon a surface ---light can partially reflect and partially transmit through the surface.

One type of Reflection that can occur is called "Diffuse"

Diffuse reflection means we

can see the spot on the wall (collect light into Eye)

regardless of where we place our Eye.

The light is scattered--in

many directions.

Flat Paints have many "scattering" solids embedded



Specular Reflection (mirror)

The law of reflection indicates that the angle of incidence and angle of reflection are the same (measured from normal).

If we modify or control the shape of a mirror--or reflecting surface---

then we control how reflected light forms an image.

Refraction (OK--You've been to that PHET simulation nowPHET Refraction/Reflection-plane surfacehttps://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html

)

The picture has both a reflected ray (part of light is reflected) and refracted ray (goes into 2nd medium).The picture here is drawn qualitatively as light goes from a "fast" medium to "slow".



medium to "slow".


Redo the refraction picture again---for fast to slow

We will define the index of refraction for a medium as n=c/v index=speed of light in vacuum/ speed of light in medium.

So fast and slow don't mean the medium is moving.

We can list several properties that are qualitatively true for any

refraction event (as long as light does not enter along the normal)


Something called "Snell's Law" relates angles in each medium.

But the way this is usually written is in terms of index of refraction

Recall

It does not matter which medium comes first (fast or slow). I like

labels "fast" and "slow" since they tell us about some physics (much better than 1 and 2). Remember that the angle in the slow medium

is always smaller (unless light enters along the normal where angle is zero in both media).

If you don't like the PHET refraction simulation--then take a cheap pen laser (Wal-Mart-cat toys for ~$5) and point at the pool from different angles. Get a zip-lock bag, put the laser in (at night) and aim the laser at different angles on the surface from underwater (at night so you can see the laser beam in the water or air due to dust). ----other phun physics demos for you---bite a few coils of slinky, drop the rest---back to optics.

So we have Snell's law, and we have index of refraction for various media----


As a wave goes from slow to fast:

Just as a row of cars going from 35 mph, to 70 mph--the cars spread out, ---likewise the wave peaks spread out for light entering a faster medium ( and vice versa)

The frequency is the same in each medium (the peaks tap on the left side of the boundary at some rate, and create the transmitted wave at the same frequency).

Let's do a problem. 500nm light is incident upon a glass surface from

air (nair ~ nvacuum ~1). nglass=1.500 The incident angle is 60o. What is the refracted angle, what is the wavelength in the new glass,

what is the frequency in each medium?


What kind of things can happen if we shape the surface of

the refracting surface ---? "the shape of water"---no---but we can easily shape glass and make a lens.

And--is the index of refraction the same for all different

colors of light?

I could bring rays together using a prism, with light from a single source. But the proper shape of a lens does better.

All the rays can come

together at an image point.

Images--movies, glasses, telescopes, microscopes. Wow.

You may recognize the image---(Pink Floyd-right). Yes---blue light has a bigger index of refraction in glass than red light. THE LIGHT DISPERSES---SPECTROSCOPY IS BORN. …..GO RAINBOWS.

So Dispersion refers to the fact that "Light of a different color" has

different speeds. Good optics can reduce the image blur due to chromatic aberration!!!!!!! Good cameras have many lenses.


Rainbows, Blue Sky

Blue sky---shorter wavelengths scatter more off molecules in the

atmosphere. That’s it. Blue light everywhere, red goes through without much scatter.

Remember when I told you (earlier in the notes) to take a laser in a ziplock--underwater (at night) and shine it up on the surface of the water. If you make the angle big enough--something special happens.

Pictures---fast to slow---then later, slow to fast.


T.I.R.--first picture from fast to slow. Or go back to that

PHET sim and change settings.

OK--the above figure has 4 different experiments. The colors mean nothing--all the same color light, just keeping the lines (experiments) straight. We shine light in at angle 1, then bigger 2, 3, 4.

Each experiment, the angle of refraction in the slow medium is smaller than the angle in the fast--refraction went toward normal. But--we make angle bigger in one medium, it gets bigger in the other.

The biggest we can do in the fast medium---is 90o. Light is just

skimming the surface in fast. Then we are at a BIGGEST REFRACTED ANGLE IN THE SECOND MEDIUM. So a fish under water, sees the

entire outside water world from -90 to +90---squished into a smaller angle region. Is the fish just not allowed to look up at a bigger angle from under water. Get your snorkel gear out. Bring the flashlights

and lasers.


The second TIR picture (the real one) ---under the sea---

We are doing the same experiment as before, but now from inside the slow medium.

Same idea as before. Starting in the slow medium. Angle 1 in fast medium is bigger than angle 1 in slow. As we increase our angle (up to us to point the laser) in the slow medium, the fast angle increases too. THERE IS A SPECIAL ANGLE (CASE 3 DRAWN HERE) where we have increased the angle in the slow medium just enough to make the angle in the fast medium 90o--so light is just skimming the surface inside fast.

This special angle is called the critical angle.

For any angle of incidence (from slow medium) less than the critical angle, normal refraction occurs. Nothing new. Snell's law.

BUT IF I USE AN ANGLE BIGGER THAN THE CRITICAL ANGLE--THE LIGHT CANNOT LEAVE THE SLOW MEDIUM (IT GETS BENT EVEN MORE AND NEVER LEAVES--NEVER REFRACTS). THE LIGHT IS REFLECTED. 100% BACK INTO THE INITIAL MEDIUM. TOTAL INTERNAL REFLECTION


Summary TIR

Given two media (both index of refractions)--find critical

angle by setting fast=90o then solving Snell's law for the Critical angle. c

1)

The critical angle is always in the slow medium2)Light incident on boundary from inside slow at less than

critical angle is refracted

3)

Light incident on boundary at angle greater than c

undergoes reflection back into the medium4)

If you have every used high speed internet or watched cable TV you have used this effect TIR


Light tube (fiber optic) video

https://www.youtube.com/watch?v=Lic3gCS_bKo

Oh--and as a final note---Fiber optics are used in all sorts of medical

instruments both to image internally (endoscopes) and to deliver light energy (lasers) for treatment or other diagnostic tools.


https://www.youtube.com/watch?v=Lic3gCS_bKo

optics fields? the observer measures the e and b field ... · colors of light? i could bring rays...

Documents