3.4 Photons

Question 1

What is the value of the Planck constant?

Answer 1

Represented by the letter h, the Planck constant has a value of 6.63x10^-34 Js

Question 2

What are photoelectrons?

Answer 2

This is a term used to describe electrons emitted by the photoelectric effect

Question 3

Define work function

Answer 3

The least energy required to remove an electron from a metal surface, usually express in electron volts

Question 4

What is the photoelectric effect?

Answer 4

If light is shone at a negatively charged plate, the plate will lose its charge. As this phenomenon is not observed with a positively charged plate, this suggests that electrons are given off from the negatively charged plate, for the positive plate this are not given off and are instead attracted back by the plate

Question 5

What are the results of experiments into the photoelectric effect?

Answer 5

• If electrons are emitted there is no measurable time delay
• For any metal there is a characteristic threshold frequency below which no electrons are emitted, no matter the intensity of the radiation
• There is a linear relationship between the maximum kinetic energy of an emitted photon and the frequency which has a constant gradient across all metals
• If electrons are emitted Ekmax is independent of the radiation intensity
• If electrons are emitted, the number of electrons emitted per second is proportional to the intensity of the radiation

Question 6

What was the model Einstein proposed to explain the photoelectric effect?

Answer 6

• EM radiation consists of discrete packets of energy, photons, with the photon energy given by E = hf, where h is a constant called Planck’s constant
• When a photon interacts with an electrons in the metal surface its entire energy is transferred
• An electron only interacts with an individual photon
• There is a characteristic minimum energy called the work function which is needed to remove an electron from a metal surface

Question 7

What is Einstein’s photoelectric equation?

Answer 7

Ekmax =hf - (phi) where (phi) is the work function of the material

Question 8

What is wave-particle duality?

Answer 8

The idea that light has both wave and particle properties

Question 9

Define ionisation energy

Answer 9

The energy an electron must be given for an electron of that element to escape the atom

Question 10

How does an electron diffraction tube work?

Answer 10

• A metal-coil cathode is connected to a low voltage a.c. supply, heating it up and causing it to emit electrons by thermonic emission
• The electrons are accelerated to the anode by an EHT supply
• A beam of electrons emerges from the hole in the anode and hits the graphite target
• The beam is diffracted by the graphite, similarly to light at a diffraction grating. The electron beams hit the phosphor producing a display of circular rings
• With increaased EHT voltage, the rings have decreased radius