3.6- WAVE-PARTICLE DUALITY

Question 1

What is light apart of?

Answer 1

EM spectrum of waves

Question 2

What did the theory of EM waves predict?

Answer 2

existence of EM waves beyond visible spectrum,

Question 3

What was the subsequent discovery that confirmed the predictions of the theory of EM waves?

Answer 3

discovery of X-rays + radio waves - nature of light has been settled

Question 4

What did many scientists in the late 19th century reckoned all aspects of physics could be explained using?

Answer 4

Newton’s law of motion + theory of EM waves

Question 5

How did 19th century scientist believe few minor problems areas, i.e. photoelectric effect would be explained?

Answer 5

using Newton’s law of motion + Maxwell’s theory of EM waves

Question 6

However, how the photoelectric effect really explained?

Answer 6

when Einstein put forward radical theory that light consists of photons

Question 7

What are photons?

Answer 7

particle-like packets of EM waves

Question 8

What sort of nature does light have?

Answer 8

dual

Question 9

Why does light have a dual nature?

Answer 9

can behave as a wave or a particle, according to circumstances

Question 10

How is the wave-like of light observed?

Answer 10

diffraction of light

Question 11

Example of diffraction of light

Answer 11

when light passes through a narrow slit

light emerging from slit spreads out in same way as water waves spread out after passing through gap

Question 12

What can increase the amount of diffraction of light? (2)

Answer 12

narrower the gap

longer the wavelength

Question 13

How is the particle-like nature of light observed?

Answer 13

photoelectric effect

Question 14

Example of the photoelectric effect

Answer 14

when light directed at metal surface + electron at surface absorbs photon of frequency f, K.E. of electron increased from negligible value by hf
electron can escape if energy it gains from photon exceeds work function of metal

Question 15

How can electrons in a beam be deflected?

Answer 15

by a magnetic field

Question 16

What is electrons in a beam being deflected by a magnetic field evident for?

Answer 16

electrons having a particle-like nature

Question 17

Who and when considered the idea that matter particles also have a wave-like nature first?

Answer 17

Louis de Broglie in 1923 (de Broglie hypothesis)

Question 18

By extending the ideas of duality from photons to matter particles, what did hypothesis did de Broglie put forward? (2)

Answer 18

matter particles have dual wave-particle nature

wave-like behaviour of matter particle characterised by a wavelength, its de Broglie wavelength

Question 19

What is the de Broglie wavelength related to?

Answer 19

momentum of the particle

Question 20

What is the equation for the de Broglie wavelength?

Answer 20

λ= h
——
mv

Question 21

How can the de Broglie wavelength of a particle be changed?

Answer 21

change velocity of the particle

Question 22

How and when was the wave-like nature of electrons discovered?

Answer 22

3 yrs after de Broglie put forward hypothesis, demonstrated beam of electrons can be diffracted

Question 23

What is a metal composed of?

Answer 23

many tiny crystalline regions

Question 24

What does each region of a metal consist of?

Answer 24

positive ions arranged in fixed positions in rows in a regular pattern

Question 25

When a narrow beam of electrons in a vacuum tube is directed at a thin metal foil, what happens?

Answer 25

row of atoms in metal causes electrons in beam to be diffracted, just as beam of light diffracted when it passes through slit

Question 26

In what directions are the electrons in a beam diffracted when they pass through the metal foil?

Answer 26

in certain directions only

Question 27

When the electrons in a beam are diffracted from the metal foil, what is formed and on what?

Answer 27

pattern of rings on a fluorescent screen at the end of the tube

Question 28

What is each ring formed on the fluorescent screen at the end of the tube due to? (electron diffraction)

Answer 28

due to electrons diffracted by same amount from grains of different orientations, at same angle to incident beam

Question 29

How is the beam of electrons produced? (electron diffraction)

Answer 29

by attracting electrons from heated filament wire to positively charged metal plate, which has a small hole at its centre

electrons that pass through hole form beam

Question 30

How can the speed of electrons in the beam be increased? (electron diffraction)

Answer 30

increasing p.d. between filament + metal plate

Question 31

What affect does increasing the speed of electrons have? (electron diffraction)

Answer 31

makes diffraction rings smaller

Question 32

How does increasing the speed of the beam of electrons make the diffraction rings smaller? (electron diffraction)

Answer 32

increase of speed makes de Broglie wavelength smaller

so less diffraction occurs + rings become smaller