Diffraction And The Nature Of Light

Question 1

What is diffraction

Answer 1

Diffraction is the spreading out of a wave as it passes through a gap

Question 2

What criteria must be met for a maximum de fraction to occur

Answer 2

The size of the gap must be of the same magnitude as the wavelength of the wave

Question 3

What happens if the gap is much smaller than the wavelength of the wave

Answer 3

The wave will be reflected

Question 4

State the diffraction grating equation

Answer 4

N λ=dsinø

Question 5

What does electron diffraction provide evidence for

Answer 5

The wave nature of electrons. It suggests that particles can demonstrate wavelike properties

Question 6

Describe the diffraction patter produced by electrons

Answer 6

Concentric circles of bright and dark fringes from a central bring point

Question 7

If electrons didn’t have a wave nature, describe the pattern that would be produced when they pass through a slit

Answer 7

The electrons would be unaffected by the gap and pass straight through it. A single bright région would be formed

Question 8

What is the name given to the wavelength of a particle

Answer 8

De broglie wavelength

Question 9

What two factors does the de broglie wavelength depend on

Answer 9

1) mass
2) velocity

Question 10

State the equation used to calculate the de broglie wavelength

Answer 10

λ=h/mv

Question 11

What can ‘mv’ be replaced with in the de broglie wavelength equation

Answer 11

P (momentum)

Question 12

What is the basic process of a pulse-echo technique?

Answer 12

-wave pulse emoted
-it is transmitted and reflected at a boundary
-the returning wave is detected
-the speed and time taken are used to calculate the distance

Question 13

Suggest two things that may limit the amount of information that can be obtained by a pulse-echo technique

Answer 13

1) the wavelength of the radiation
2) the duration of the pulse

Question 14

What are the two models that can be used to describe electromagnetic radiation

Answer 14

1) the wave model
2) the particle model

Question 15

Which model does the photoelectric effect provide evidence for

Answer 15

The particle model

Question 16

Outline the photoelectric effect

Answer 16

-Light is shone on a metal plate
-if the light is above a certain frequency, electrons are emitted from the surface of the metal
-if the frequency is too low, no electrons are emitted

Question 17

What are the particles of light used to explain the photoelectric effect called

Answer 17

Photons

Question 18

How do you calculate the energy of a photon

Answer 18

E=hf

Question 19

Explain how a photon can liberate an electron

Answer 19

One photon interacts with one electron and transfers all its energy to it. If this energy is greater than the metals work function, the electron will have sufficient energy to be released

Question 20

What is threshold frequency

Answer 20

A metals threshold frequency is the minimum frequency that a photon requires to liberate an electron from its surface

Question 21

If the intensity of light being shone on a metal increases, how does the energy of the photoelectrons change

Answer 21

The energy remains unaffected. An increase in intensity means more photons per area and so more photoelectrons are emitted

Question 22

Why are photoelectrons emitted with a range of kinetic energies

Answer 22

The electrons are at different depths in the metal so require different amount of energy to be liberated. The excess energy from a photon once an electron has been liberated, in the kinetic energy of the electron

Question 23

State the equation for the maximum kinetic energy of a photoelectron

Answer 23

0.5mv^2=hf-ø

Question 24

What is the conversion factor between eV and J

Answer 24

1eV=1.6x10-19J

Question 25

What happens when electrons transition between energy levels

Answer 25

-if electrons move to a higher energy level, radiation must be absorbed
-if electrons move to a lower frequency, radiation must be emitted

Question 26

Why can only certain frequencies of radiation be absorbed by an atom to cause an electron transition

Answer 26

The electrons can only exist in discrete energy levels. The energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levels