Waves to Microwave

7/27/2019 Waves to Microwave

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Light fantastic - guidance notes equipment list at the end

Visualising vibrations_________________________________________________ LIGHTTo understand how light behaves, it is useful to look at the behaviour of other forms of energy,especially those that have wave properties. We will look at real effects, and at some models oranalogies.

Light, microwaves, radio signals, mobile phone signals, and sound all travel from place to

place as waves. There are some important differences, but the similarities are more important.

Although we speak of sound waves, we dont see them in everyday life we only hear them,or even feel them!

Connect the power signal generator to the vibration generator (2 x 4mm plug leads).Set amplitude at minimum, frequency range 1 to 11Hz, main control at 3Hz.Switch on and increase the amplitude.

Cant hear it? The sound is at too low a frequency, only 3 vibrations per second.Change range 10 to 110Hz.

Now it is audible, at 30 vibrations per second (hertz).

Switch off, reselect 1 to 11Hz range.

Connect the red rubber cord to the moving centre pillar of the vibration generator.Tie the red cord to a heavy retort stand. Put the cord under slight tension.

Explore the resonances: 1 wave, 2 waves, 3 waves, 4, 5 and 6 waves.Need to change range to 10 to 110Hz.

As the frequency increases, the wavelength decreases. This is true for all types of waves:

radio, light, and sound.

With a steady pattern of six waves, show that the nodes can be touched (lightly)without destroying the wave pattern. Touch in two places (nodes 5 and 6, countedfrom the vibration generator). The pattern remains.

Move the fingers at node 6, to the left or right. Comments?Remove fingers to let the pattern re-establish, then repeat the trick.

Imagine the waves are waves of red light, arriving at a piece of plastic. If the molecules in theplastic are at the right spacing, then the red light goes through. My fingers are the piece of

plastic and it is letting red light go through. If I change the spacing of the molecules, then thered light is stopped.

If I change the frequency of the vibration, the wavelength changes this is a different colour oflight. My fingers must be at a different spacing to let the light waves through. Perhaps myfingers are now a green filter, letting only green light through?

We will return to this effect later.


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Ripple tank making waves__________________________________________ DARKRipple Tank set up to project downwards.

Drip water at centre = person shouts in a field, sound wave travels away from them, slightechoes from the walls, OR a flashbulb goes off in the middle of a room and the light wavestravel outwards, and are reflected by the walls.

Attach a single dipper to the wave generator,

run slow = person singing a steady note, sound waves travel away,run faster = person singing a higher note.

Increased frequency, decreased wavelength, just as before.

Change to plane dipper.Run to make plane, or straight waves = a beam of light, or microwaves, or soundwaves travelling in one direction, at one frequency, one wavelength.

Change to two dippers.Run to make a pattern = two sources at the same frequency gives us

an interference pattern.

The waves coming from one source meet those from the other source. Sometimes the crestscoincide and we get a super crest, or two troughs coincide and we get a super trough.Sometimes a crest and a trough coincide and we get . . nothing . . flat water.

In the pattern, you can see the waves, apparently going in certain directions, like fingers, withareas between in which there are no waves. This is an interference pattern.If it was sound, no waves means no sound! If it was light, that would mean no light!

To show you that this can really happen, we must go back to some real sound.


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Interference large scale____________________________________________ LIGHTCheap stereo speakers connected together to the same source.

Connect the power signal generator to a pair of small stereo speakers on long leads.Set frequency to 1 to 1.1kHz and frequency control at 1 giving 100Hz.Turn up the amplitude to give a tolerable hum.

In the ripple tank, you saw two dippers creating identical circular waves, at the same

frequency. The waves met and created an interference pattern.

Here we have a similar situation. Two speakers creating identical sound waves, that spreadout in the space around us. You cant see the waves, but you can hear them.

Invite people to move around and listen (one ear only!) carefully.They should find loud and quiet zones (raise one hand when you find a quiet zone).

The room, like the ripple tank, is full of waves. Sound waves from two sources.Sometimes the crests and troughs coincide and we get close to silence.This is a real, audible interference pattern.

If I change the frequency . . increase it from 100Hz to 200Hz, what should happen to theinterference pattern?The wavelength will be shorter, so the distance between silent zones will be smaller.

This is also true for light and microwaves. There will be dead zones, and changing thefrequency will change where they happen.


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From waves to microwaves___________________________________________ LIGHTConnect the Microwave Transmitter (Tx) to its power supply. Nothing audible.

Low power (15 milliwatts), no danger it would take many centuries to cook anything!Connect the Microwave Receiver (Rx) to its power supply.Connect a meter via the lead to the Receiver. Ask someone to observe the meter.

We have a simple indication of the signal being received. A signal we cannot see or hear.

Sound vibrations are in the range from 20 to 20,000 vibrations per second.

These microwaves are much faster, around 10,000 million vibrations per second.But we can make their behaviour more obvious, by adding a tone, and doing away with themeter. The microwaves carry the signal, in the same way that radio waves carry Radio 2 toyou at home. You dont hear the 88 to 91 megahertz you hear Terry Wogan.

We can hear whether the microwave signal is being received.Switch on 1kHz tone and move the Tx and Rx around:

Facing at various distances

Facing via a mirror/reflectorVia two mirrors zig zag fashion

The beam of microwaves is reflected by the aluminium plates.I can do the same with a torch (or a laser), their light is reflected by the aluminium.

Why is the microwave system more like the laser, than the torch?The microwave and laser operate at a fixed frequency, a single wavelength, like a singeralways on the same note.

A torch gives white light, a mixture of all colours as you will soon see: like a choir witheveryone singing different notes!

Lloyds mirror microwaves bounce off the mirror and meet the waves that traveldirectly.

What happens when we move the Tx or Rx slowly back and forth, in a straight line?We get a changing signal. In the right conditions, we get a silent zone.We have an interference pattern again. This time with microwaves.

PolarisationWhen you wear certain types of sunglasses, they do odd things to reflections from watersurfaces. They polarize the light.

Tx facing Rx place polarizing grille between and rotate by 90 degrees

The signal can be reduced by polarization. The grille allows waves to go through like this (upand down) but not like this (side to side).

In sunglasses, this is achieved by Polaroid filters, materials containing minute rows of crystalsthat allow light through as long as the waves are at the correct angle.


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Laser light pure colour______________________________________________ DARKSolid state laser.

Switch on. Avoid audiences eyes.

The laser gives a single colour of light. One frequency, a fixed wavelength.

Compared with the microwave, light waves are vibrating about 10,000 times faster.

If time, bounce through same reflectors as used for microwave Tx and Rx.

If time, project through diffraction gratings.When the laser light strikes the lines of the grating, they all act as though they were a singlesources, generating circular waves. These spread out and interfere with each other.

The interference pattern gives multiple images, with dark zones between like the silentzones, when we were using sound to make an interference pattern.

So, by shining a light into a space, through a diffraction grating, we can create areas of

darkness. Not quite what you would expect.


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White light and Newtons experiment___________________________________ DARKHere comes the choir!

Place White LED Source (WLS) on a sheet of white paper/board.

All colours of light a choir singing different notes!

Whiter than ordinary incandescent lamps and a better mix than a halogen lamp.This is an ideal source for experiments in optics.

Add a slit plate with Blu Tac to the front of the WLS. Select a narrow beam.

Add a cylindrical lens. Collimate the beam into a parallel sided ray.Add a slit plate to create three parallel rays.Place mirror, lenses, perspex block, etc to allow the three rays to go through.

Total blackout is not necessary, as the WLS is quite intense.

When Newton investigated light, he got into arguments with other scientists of the time.

WLS + slit plate + cylindrical lens to make parallel beam+ slit plate to make a single ray.

Add a prism in the focused beam.Add a screen to capture the spectrum.Rotate the prism to achieve best dispersion.

Newton said that white light is made up of all colours. Others said, no, no, the glass is addingcolour to the light, corrupting Gods pure light the same way chandeliers and gew-gaws do.

Newton went away and extended his experiment to answer his critics.

Add a second slit plate, to select only green light.Add a second prism and catch the new image of the green light.

Only green, Im afraid . . Newton had shown that the glass did not add colour, it simply bentdifferent colours of light differently.

This is called dispersion, and relies on light of different wavelengths behaving differently as thelight passes from one medium to another, in this case from glass to air, and back to air.

The WLS can be mounted on a stand and clamp or, with the accessory rod, it can be mountedon a Philip Harris optics bench.

Now that we have taken white light apart, lets have a look at the colours separately.


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A trick of the light?__________________________________________________ DARKSwitch on the LED Array.

We have eleven LEDs. White at the top, then a full spectrum from red through to violet.Very pretty, but what can we do with it?

Colour filters distribute

Please select the red filter you can easily check by noting the colour of the top LED.Because it is white, part of its light will pass through any of the filters.

When you look through the red filter what happens to the other LEDs?If time - Voting procedure to create a sort of graph that describes the filtersbehaviour. This graph is the transmission characteristic for the material.

Pass on share - Try a blue filter instead.

For each filter we can see that it allows certain colours through, but blocks the others.

Like my fingers, right back at the start, when the vibration generator was driving the red cord.Fingers at the right spacing the waves go through. Wrong spacing, the waves are blocked.The filters respond to the mixture of wavelengths, and allow some light through because theirmolecules have a particular size or spacing.

Take back the colour filters.Issue diffraction gratings 300lines/mm.

Hold close to the eye, label at the top and look at the LED Array.The central image is coming directly through the grating. On either side is a first order image,and possibly a second order image.

Notice that the white LEDs light is spread out into a spectrum, just as the prism did earlier.

Look at the images, at either side, of the other LEDs.Is each LED a pure colour?Is ANY LED a pure colour?

Pass on and share.

You can see that each LED appears to be a single colour to our naked eye, but gives a rangeof colours when you look through the diffraction grating.

Each colour of light has a different frequency, a different wavelength. When the differentwaves pass through the diffraction grating, they behave slightly differently and give thepatterns you see.

By doing some simple measurements, knowing the line spacing on the diffraction grating,using a ruler and a calculator, we can work out the wavelength for each colour of light.

Using a voltmeter, we can relate the energy levels in each LED to the wavelength of light. Thistakes us into quantum physics and something called Plancks Constant.


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Colour mixing______________________________________________________ DARKColour mixer + control box no diffuser at first.

Switch on and set 20% brightness to show inner and outer LEDs, and controllability.

Turn to face the screen/wall, i.e. away from the audience.

Light the wall/screen in red, then blue then green.

What happens if we mix colours?

Add diffuser to inner LEDs, adjust to get the wall/screen looking white.

Add the aperture screen, to get the classic overlapping text book diagram.

Turn the LEDs off and on, to show mixing by overlapping.

So, now we know the rules about mixing, lets look at some shadows.

Hand in between the mixer and screen.

How do I make the yellow shadow disappear?Discuss - turn off blue.

Now, lets look at some familiar objects.

Coloured cards, coloured ball, Kit Kat or Coke Can.

We perceive colour because the different wavelengths of light interact with the rods and conesin our retina. The colours we see depend on the colour of light that is shining on the object,

and the reflective properties of the object we are looking at.

Seeing is believing . . but are you sure?


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Light fantastic workshop - UNILAB and Philip Harris product guide

For a copy of the notes describing the demonstrations, please e-mail [email protected]

Visualising vibrations______________________________________________________________________Spacesaver Power Signal Generator H10579Vibration Generator H31134Vibration Generator Accessory Kit H30920

Ripple tank making waves________________________________________________________________Ripple Tank kit H25730Ripple Tank PSU L01077

Interference large scale__________________________________________________________________Spacesaver Power Signal Generator H10579Pair of small stereo speakers on long leads buy cheaply from a car boot sale and wire them yourself usingcheap loudspeaker wire.

From waves to microwaves________________________________________________________________Microwave Transmitter J73478Microwave Receiver J73479

Aluminium reflector 210x210mm -Aluminium reflector 210x60mm -Hardboard/semi-reflector 210x210 -Polarisation grille J73480 for the kit

Crystal microphone H27581Basic Student meter H30981Multiplier 1V and 5V for meter H31006Earphones any normal Walkman or iPod phones will work.

Laser light pure colour___________________________________________________________________Solid state laser + key H83651Diffraction gratings various 80 lines/mm A46267 300lines/mm A46279 600lines/mm A46280

White light and Newtons experiment_________________________________________________________

White LED Source L87348Slit plates G98647Lenses, cylindrical +10D or +7D L27303 or L27297Prisms, Extra Dense Flint glass (EDF) A46413

Holders for slit plates and small white screens + a large sheet of white paper/cardA selection of lenses and mirrors from a typical raybox kit. H25857

A trick of the light?________________________________________________________________________LED Array L75279Diffraction gratings various 80 lines/mm A46267 300lines/mm A46279 600lines/mm A46280

Colour filters RGB CMY cheap ones are ok A46012

Easy-Read meter20V attachment4mm leads + crocodile clips

Colour mixing________________________________________________________________________Colour Mixer & accessories L75267

Diffuser greaseproof paper or tracing filmObjects in various colours sheets of card, ball, Kit Kat, fruit, etc.
mailto:[email protected]:[email protected]

Waves to Microwave

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