Electromagnetic radiation and quantum phenomena

Question 1

When will a metal emit electrons?

Answer 1

When you shine radiation of high enough frequency onto the metal’s surface (frequency greater than or equal to threshold =emitted)

Question 2

What is the frequency that metals emit electrons?

Answer 2

Usually in the UV range

Question 3

What happens when you shine radiation on a metal?

Answer 3

• Free electrons on/near the surface will absorb the radiation’s energy, making them vibrate.
• If the electrons absorb enough energy, its bonds will break and it’ll be released.

Question 4

What are emitted electrons called?

Answer 4

Photoelectrons

Question 5

What conclusions can you get from the photoelectric effect?

Answer 5

• No electrons emitted below the threshold frequency
• photoelectrons emitted with a variety of Kinetic Energies (0 to Max). It increases with frequency.
• Intensity is the amount of energy hitting an area per second ( Max KE is unaffected by intensity)
• Number of photoelectrons emitted per second is proportional to intensity.

Question 6

Explain what wave theory says about threshold frequency. Why is this wrong?

Answer 6

• Wave theory says for a particular frequency of EM wave, energy carried is proportional to intensity.
• Energy is evenly distributed over the wavefront.
• This means that if shone on a metal each electron would gain a bit of energy and would eventually gain enough to be emitted (lower frequency = longer to leave)
• However, this doesn’t happen as no electrons emitted below the threshold frequency.

Question 7

Explain what wave theory says about Kinetic Energy of photoelectrons. Why is this wrong?

Answer 7

• Higher intensity = more energy to electrons
• This means KE of electrons should increase with intensity
• However, wave theory cant explain why KE is dependent on frequency in the photoelectric effect.

Question 8

What did max planck suggest about EM waves?

Answer 8

They are only released in discrete packets/quanta

Question 9

What is the equation for energy of an EM wave?

Answer 9

E = hf = hc/wavelength

Question 10

What did Einstein suggest about EM waves?

Answer 10

• EM waves (and their energy) only exist in discrete packets called photons
• And that they have a 1 on 1 interaction with the electrons on a metal’s surface
• They would transfer all their energy to that electron
• This is used to explain the photoelectric effect

Question 11

Learn electroscope with metal diagram

Answer 11

Uv shone on metal = electrons leave gold leaf and gold leaf falls towards negative electroscope

Question 12

What is the work function?

Answer 12

The minimum energy gain to be emitted (breaking attractions to metal)

Question 13

What is a photon?

Answer 13

Discrete wave packet of em waves

Question 14

Why are photoelectrons emitted with a range of KEs?

Answer 14

Because of energy losses (work done to get to surface)

Question 15

What is KE affected by?

Answer 15

Unaffected by intensity

Increases with frequency

Question 16

What is no emitted per second proportional to?

Answer 16

Intensity

Question 17

Threshold frequency?

Answer 17

Work function divided by h

Question 18

What happens when external pd applied?

Answer 18

Electrons lose there energy by doing work against pd

Question 19

What is stopping potential?

Answer 19

Pd needed to stop electron with Ek(max)

eVs = Ek(max)

Question 20

What is an eV?

Answer 20

The KE carried by an electron after being accelerate from rest through a pd of 1 volt (energy gain = accelerating voltage)

Question 21

What happens when electron dexcited?

Answer 21

Emits photon equal to the difference between energy levels (vice versa with excitation)

Question 22

White light through a cool gas?

Answer 22

Cool gas has electrons in ground state at low temps, they are excited by photons.
If put through a prism produces a continuous spectra ( absorbed wavelength are black lines)

Question 23

Fluorescent tube through a prism?

Answer 23

Line emission spectra (bright lines = specific frequency of light)

Question 24

What are line emission spectra and continuous spectra evidence for?

Answer 24

Discrete energy levels (only certain photons absorbed)

Question 25

Fluorescent tube?

Answer 25

1) FT with mercury vapour, high voltage applied
2) free electrons are accelerated and they ionise some mercury atoms producing more free electrons
3) free electrons collide with mercury atoms exciting there electrons
4) when they de excite they emit high energy photos in uv range
(Range of energies and wavelengths due to different energy level transitions)

Question 26

What shows that light acts as a particle and a wave?

Answer 26

Photoelectric effect and electron diffraction

Question 27

Electron diffraction?

Answer 27

High velocity electrons in diffracted by spaces in graphite crystals:

1) greater spread of greater wavelength if wave theory is correct but it increases as accelerating voltage decreases
2) particles with same speed but greater mass = more tightly packed pattern

Question 28

Why are electron microscopes good for looking at DNA?

Answer 28

Electrons waves don’t blur images as much (as they have a short wavelength and so less diffraction occurs)