Wave particle duality

Question 1

Where is wave like nature observed?

Answer 1

• Diffraction taking place
• When light passes through a narrow slit, the light emerging from the slit spreads out in the same way as water waves spread out after passing through a gap.
• The narrower the gap or longer the wavelength, the greater the diffraction.

Question 2

When is particle like nature observed?

Answer 2

• Photo-electric effect
• When light is directed at a metal surface and an electron at the surface absorbs a photon, the kinetic energy of the electron is increased by hf, the electron can escape if the energy it gains from a photon exceeds the work function of the metal.

Question 3

What is the de broglie hypothesis?

Answer 3

• Matter particles have a dual wave-particle nature.
• Wave like behaviour of a matter particle is characterised by a wavelength, the de broglie wavelength, which is related to the momentum of the particle.

Question 4

What is the de broglie wavelength formula?

Answer 4

De broglie wavelength = h / ( m x v )
( m x v ) = momentum
h = planck’s constant

Question 5

How were electron diffraction patterns observed?

Answer 5

• A narrow beam of electrons in a vacuum tube directed at a thin metal foil.
• A metal is composed of many tiny crystalline regions, each region consists of positive ions arranged in fixed positions in rows in a pattern.
• Rows of atoms cause electrons in the beams to be diffracted.
• Electrons in the beam pass through the metal foil a 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.

Question 6

How is the beam of electrons produced?

Answer 6

• Beam of electrons is produced by attracted electrons from a heated filament wire to a positive charged metal plate, which has a small hole in the centre.
• Electrons that pass through the hole form the beam.

Question 7

How can the speed of electrons be changed?

Answer 7

• Increasing the potential difference between the filament and the metal plate.

Question 8

How does speed of electrons affect diffraction ring?

Answer 8

• Increased speed, due to increased potential difference, makes the diffraction rings smaller.
• This is because the increase of speed makes the de broglie wavelength smaller.
• So less diffraction occurs and the rings become smaller.
——
• If momentum is greater, wavelength is shorter, and the spread of lines is smaller.

Question 9

How does the size of rings vary?

Answer 9

• According to wave theory, spread of lines in the diffraction pattern increases if the wavelength of the wave is greater.
• A smaller accelerating voltage, slower electrons, gives more widely spaced rings.

Question 10

How does mass affect diffraction pattern produced?

Answer 10

• if particles with a greater mass, were travelling at the same speed as the electrons, they would show a more tightly packed pattern.
• This is because a neutrons mass, and therefore its momentum, is much greater than an electrons, so a neutron has a shorter de broglie momentum.

Question 11

What is required for diffraction?

Answer 11

If a particle interacts with an object about about the same size as its de broglie wavelength.