# Need help, my text books explanation on de broglies hypothesis is very confusing

1. Apr 5, 2012

### Sofie1990

we have just started to learn a new chapter in my physics class about de broglies hypothesis. In my text book it shows a diagram of a line of electrons being fired at a constant speed, hitting a sheet of thin metal foil and then diffracting.

The explanation in my book says
' A narrow beam of electrons in a vacuum tube is directed at a thin metal foil. A metal is composed of many tiny crystalline regions. Each region or 'grain' consists of positive ions arranged in fixed positions in rows in a regular pattern. The rows of atoms cause the electrons in the beam to be diffracted, just as a beam of light is diffracted when it passes through a slit. The electrons in the beam pass through the metal foil and are diffracted in certain directions only. They form a pattern of rings on a fluorescent screen at the end of the tube. Each ring is due to electrons diffracted by the same amount from grains of different orientations, at the same angle to the incident beam.

To be honest with you, i dont really understand what this means. Are they saying that an electron(particle) hits the foil and then transforms into a wave? or are they saying its a particle throughout but it diffracts? if so, how can a particle diffract? because diffraction is a wave property. I tried to ask my teacher, but he just kept saying that physicists dont actually know for sure, that all they do know is that an electron hits the foil and then all they see is diffraction pattern but dont actually know what happens in the middle stage because electrons are too small and travelling too fast.

Is this true? Does anyone have any more information on this, really would help x

Cheers

2. Apr 5, 2012

### Staff: Mentor

They are saying that an electron has wavelike properties just like light does. Otherwise it would not show diffraction rings and patterns. This is a very confusing situation, as we are not used to seeing wavelike properties of matter in our day to day lives! The simple answer is that an electron is both a particle AND a wave. Or rather, it has particle-like and wave-like properties.

3. Apr 5, 2012

### ErolDynamics

could you say your textbook's name

4. Apr 5, 2012

### Bobbywhy

Here is an excerpt from Wikipedia that may help. I recommend you visit the site and read up on this, as your teacher either does not understand the process, or cannot explain it to you, or just wants you to learn about on your own. If you have further questions or doubts, do come back here and post them!

"In 1927 at Bell Labs, Clinton Davisson and Lester Germer fired slow-moving electrons at a crystalline nickel target. The angular dependence of the reflected electron intensity was measured, and was determined to have the same diffraction pattern as those predicted by Bragg for x-rays. Before the acceptance of the de Broglie hypothesis, diffraction was a property that was thought to be only exhibited by waves. Therefore, the presence of any diffraction effects by matter demonstrated the wave-like nature of matter. When the de Broglie wavelength was inserted into the Bragg condition, the observed diffraction pattern was predicted, thereby experimentally confirming the de Broglie hypothesis for electrons."

http://en.wikipedia.org/wiki/Matter_wave

Last edited: Apr 5, 2012
5. Apr 5, 2012

### sophiecentaur

@Sofie1990
If something behaves in the same way that something else behaves then we justifiably say that they share some of the same properties. There is good evidence that light is a wave. It produces interference / diffraction patterns. Massive particles also can be made to produce diffraction patterns so we say they must have also have wave properties. Massive particles also behave like - well, Particles. They can be seen bashing into things and behaving like billiard balls. Also there are times when a beam of light behaves very much as if it was a stream of particles (photons). Very unsettling, if you let it get to you.

When a stream of electrons produces a diffraction pattern there must be something wavelike happening. If you accept that an electron, whilst you are not actually observing it, can be a wave (because of diffraction effects) then this model agrees with experience. Once you actually observe an electron, though, you find it in just one place and it is behaving like a (hit you in the face) particle. It's not an 'either or' explanation. Both models apply at different times so we have 'duality'. Not easy to accept and even harder (impossible, even) to understand fully but that's why QM is a hundred years old and still giving people sleepless nights.
QM is at least as hard to accept as it was to accept that our Universe is not Geocentric or that the 'Laws' which describe how things behave were not actually Laws laid down by a god but merely descriptions of the way things usually behave. QM is not for the literally minded.

6. Apr 6, 2012

### Sofie1990

AQA physics A - AS Level book

7. Apr 6, 2012

### Sofie1990

So all waves can behave like particles and all particles can behave like waves. Given whatever situation there in determines what they'll behave as. Is that right?
Or is it only certain waves/particles that do it? or does there have to be certain condition i.e high velocity? Its so confusing and ive only got about 5 weeks till my exams.

8. Apr 6, 2012

### sophiecentaur

Even sound travels through crystals in the form of phonons. But don't worry. It's A level and only requires you to know that electrons have a de Broglie wavelength and how to work out the energy of a photon. This forum may make things too hard for you.