Photoelectric Effect and Spectra

Question 1

What is a photon?

Answer 1

A discrete packet of energy formed of electromagnetic radiation

Question 2

What is the photoelectric effect?

Answer 2

When photons cause electrons to be emitted from the surface of a metal

Question 3

What is the typical wavelength for radio waves?

Answer 3

1500m (long wave radio) or 24m (short wave radio)

Question 4

What is the typical wavelength for microwaves?

Answer 4

3cm

Question 5

What is the typical wavelength for visible light?

Answer 5

750nm (red light) or 400nm (violet light)

Question 6

What is the typical wavelength for ultraviolet light?

Answer 6

400nm to 1nm

Question 7

What is the typical wavelength for x rays and gamma rays?

Answer 7

Less than 1nm

Question 8

How does the gold leaf electroscope demonstrate the photoelectric effect?

Answer 8

Initially, the whole thing is negatively charged, meaning the gold leaf is repelled, but when ultraviolet light is shone at the zinc plate, the gold leaf collapses. The only way this could happen is through loss of electrons - these are the photoelectrons that are lost from the surface of the zinc

Question 9

What happens when different sorts of light hit the electroscope?

Answer 9

Nothing, only ultraviolet light has an effect.

Intense uv causes the leaf to fall rapidly, but low intensity uv light causes the leaf to fall at a slower rate.

Question 10

Why does light of varying intensities not causing the photoelectric effect conflict with classical wave theory?

Answer 10

In wave theory, intensity of a wave is proportional to amplitude squared, so we would expect that intense red light would provide enough energy to eject electrons from the surface of the metal, but this is not the case.
The photoelectric effect is explained using the theory that electromagnetic radiation is made up of quanta - discrete packets of energy - which then is the cause of emitting electrons

Question 11

Why is uv light able to remove an electron from the surface of the metal?

Answer 11

There is a minimum energy required to remove an electron from the clean metal, which is the work function, so if the energy of the incoming photon is greater than the work function an electron will be emitted. UV radiation is of sufficient frequency that this can occur

Question 12

What is work function?

Answer 12

The minimum energy needed to just remove an electron from the surface of the metal

Question 13

What happens to the electron if hf = Ф?

Answer 13

One photon causes one electron can escape from the surface of the metal but it will not have any KE

Question 14

What happens to the electron if hf > Ф?

Answer 14

One photon causes one electron to be emitted from the surface of the metal with some KE

Question 15

Why does hf = Ф + KEmax only work for surface electrons?

Answer 15

Some KE is used to get the electron to the surface, so electrons not on the surface have unknown KE before they are released and surface electrons have max KE possible

Question 16

What is the threshold frequency?

Answer 16

The frequency of light that can just remove an electron from the surface of a metal without the electron having any KE
hf0 = Ф

Question 17

What is an electronvolt?

Answer 17

Energy gained by an electron when it is accelerated through a pd of 1 volt

Question 18

The photoelectric cell

Answer 18

• Photons arrive at the metal plate (emitter/cathode) and cause electrons (photoelectrons) to be emitted
• Some of these electrons are collected at the collector (anode)
• The flow of electrons sets up and emf between the plates that causes a current to flow (photocurrent)

Question 19

What is stopping voltage/stopping potential?

Answer 19

The stopping potential is the voltage which is just sufficient to stop the emission of photoelectrons from the surface of the metal when light is shone on the metal i.e. reducing the photocurrent to zero

Question 20

Circuit to investigate stopping voltage

Answer 20

• Photoelectric cell, ammeter, voltmeter, potential divider
• As pd across the cell is increased, the emitter becomes more positive and the electric field acts to repel the photoelectrons, reducing the photocurrent, so some of the weaker electrons are attracted back to the emitter
• If pd is increased sufficiently, no electrons reach the collector. These electrons don’t have quite enough energy to overcome the electric field
• eVs = KEmax

Question 21

Determining the work function of a metal

Answer 21

• A photocell, a lamp and calibrated filters and a voltmeter can be used to investigate the variation of stopping voltage with the frequency of incident emf
• The results may then be used to determine the work function of a metal, and also to obtain an estimate of Planck’s constant
• When plotting a graph of stopping potential (eVs) against incident photon frequency, the gradient is Planck’s constant, the x intercept is threshold frequency and the y intercept (when the line is extended below x axis) is the threshold frequency

Question 22

Saturation, in investigating photocurrents

Answer 22

Saturation occurs when the voltage across the photocell is increased to a level such that all the electrons emitted from the negative electrode reach the positive electrode, and the photocurrent reaches a maximum

Question 23

Increasing the intensity of light in investigating photocurrents

Answer 23

If light intensity is increased then the saturation photocurrent is increased as more electrons are released and the stopping potential is unaltered since the maximum energy of the electrons is unchanged
(Constant frequency is maintained)

Question 24

Changing the frequency of the incident light in investigating photocurrents

Answer 24

If the frequency of light is changed then the stopping potential changes, and if intensity remains constant then for lower frequency light there will be more photons emitted and the saturation current will therefore be higher, but the stopping voltage will not be so negative

Question 25

What is the main evidence that indicates light is a wave?

Answer 25

- During propagation it behaves like a wave; it can be reflected, refracted, diffracted and can show interference - Coherent sources of light will interfere constructively and destructively with each other, which relies on superposition, a wave phenomenon

Question 26

What is the main evidence that light is a particle?

Answer 26

When light arrives at the surface of a metal, the photoelectric effect is observed, when wave theory would predict that increasing the intensity of the light should result in the liberation of electrons with greater energy and if the surface is illuminated for a sufficient time then electrons should eventually gain enough energy to escape, which does not happen

Question 27

What was Einstein's explanation for the photoelectric effect?

Answer 27

Light is quantized i.e. photons are particles carrying a precise amount of energy which depends on the frequency of light, and one photon will liberate one electron. The more intense the light source, the more photons hitting the surface per second so the more electrons released per second

Question 28

What did De Broglie propose?

Answer 28

That electrons can have a wavelength - momentum was linked to wavelength (p x λ= h)

Question 29

What is the evidence for De Broglie's theory?

Answer 29

The diffraction patterns observed when an electron beam is fired at a thin piece of metal foil or graphite. The gaps between the atoms act as sites for diffraction, and the observed diffraction patterns are similar to the ones when x-rays are directed at foil, showing particles can have a wave-like nature.

Question 30

What is the Bohr model of the atom?

Answer 30

(In a hydrogen atom) The electron could only exist in a set of definite orbits based on the electron speed (centripetal force balancing electrostatic attraction), the larger the radius, the less negative the energy of the electron and the lowest orbit is the ground state.

Question 31

What is the problem with the Bohr model of the atom?

Answer 31

Electrons in circular motion are accelerating and should therefore radiate energy, losing energy, so in theory the orbit should diminish, meaning the model is unstable

Question 32

What is the De Broglie model of the atom?

Answer 32

Considered that electrons existed as a stationary wave around the nucleus, and the circumference of the electron orbit should be an integral multiple of the de Broglie wavelengths. Ground state has just 1 wave in orbit

Question 33

What is the Schrodinger model of the atom?

Answer 33

Electrons can have certain discrete energy levels, and the electrons can be anywhere around the nucleus with the probability that it is a given distance from the centre (given by the Schrodinger wave equation), and this is a mathematical model rather than a physical one

Question 34

What happens when an electron absorbs a photon?

Answer 34

It can be made to jump (excited) to a higher energy level when it absorbs a photon with specific energy, governed by the particular energy levels in the atom. It can then relax back into the ground state, releasing a photon of precise energy when it does this, again governed by the particular energy levels permitted for that electron

Question 35

Why is the spectrum for light produced by the sun not indicative of pure white light?

Answer 35

It has gaps (lines) where light of particular frequencies should be present because light leaving the sun has to pass through gas clouds at the surface of the sun and all the photons of particular frequencies are absorbed by electrons in the atoms in the gas.

Question 36

Why are only set frequencies of light absorbed in the sun's spectrum (for example)?

Answer 36

Electrons can only exist in atoms in certain, precise energy states or levels (quanta). To move from one level to the next requires a specific amount of energy, so photons with these amounts of energy are the ones absorbed by the gas, but photons with different energy values are unaffected This leaves gaps (lines) on the spectrum where photons are missing

Question 37

Why are values for electron energy levels given as negative values?

Answer 37

- The electron and the nucleus of an atom attract each other due to their opposite charge, meaning the electron has potential energy - As the electron moves away, the potential energy increases, but the force due to electrical attraction gets smaller - At infinity, the attraction between the electron and the nucleus is zero - there is no force on the electron and the potential energy is zero - So the electron energy levels on the diagram are given a negative value, showing the potential energy at that point is less than it is at infinity

Question 38

Emission of energy after excitation of an electron

Answer 38

- Shortly after becoming excited, electrons drop back to the ground state, losing some energy - They do this by emitting a photon - a packet of energy - The frequency of the photon corresponds to the size of the jump it has to make (hf = E1 - E2) - The more energetic the jumps, the higher the frequency of the photon absorbed or emitted, so the further into the uv end of the spectrum the lines go

Question 39

How can absorption and emission spectra be used to identify an element?

Answer 39

Each atom has different electron structure. so each element can be identified by the frequency or wavelength of the photons it absorbs or emits as the electrons move from one energy level to another. Using gas at low pressure in vapour tubes means that the results are undistorted as when they are involved in bonding the results are more unclear

Question 40

Absorption spectrum

Answer 40

If white light is shone through cold gas, some frequencies of light will be absorbed by the gas atoms, and an absorption spectrum will be obtained - dark lines on a bright background

Question 41

Emission spectrum

Answer 41

A hot gas or discharge tube emits light of well defined frequencies and the spectrum is an emission spectrum as photons are emitted

Question 42

What is dispersion?

Answer 42

The separating of white light into its constituent wavelengths or frequencies. This can be done with a prism, which causes it by the process of refraction - blue light refracts through the largest angle

Question 43

What is light intensity?

Answer 43

The amount of light per unit area passing through a point distant from a source of power

Question 44

Why does light intensity decrease with distance from the source?

Answer 44

Light emitted from a source is spread over a spherical area surrounding the source, the further from the source of power, the larger the spherical area over which it will be spread, therefore the lower its intensity - it follows the inverse square law