3.1 The photoelectric effect Flashcards
What is the photoelectric effect?
» Is where where photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone on it
What is the certain frequency?
» It is different for every type of metal and is called threshold frequency
What is the number of electrons emitted per second proportional to?
» The intensity of the incident radiation, provided the frequency is greater than the threshold frequency
What will happen if the frequency of the incident radiation is less than the threshold frequency?
» No photo electric emission from the metal surface can take place, no matter how intense the incident radiation is
When does photoelectric emission occur?
» As soon as the incident radiation is directed at the surface, provided the frequency of the radiation exceeds the threshold frequency
» And occurs without delay
Why couldn’t the threshold frequency be explained by wave theory?
» it suggests that any frequency of light should be able to cause electron emission as the energy absorbed by each electron will gradually increase with each incoming wave
What did Einstein assume?
» Light is composed of wavepackets or photons
What is the energy of a photon equal to?
» hf
What is h?
» Planck’s constant which is roughly about 6.63 x 10^-34 Joules
What did Einstein say happens when light is incident on a metal surface?
» An electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon
When can an electron leave the metal surface?
» If the energy gained from a single photon exceeds the work function of the metal
What is the work function?
» It is the minimum energy required for electrons to be emitted from the surface of a metal, and is denoted by ϕ
What happens to the excess energy gained by the photoelectron?
» Becomes its kinetic energy
What is the maximum kinetic energy equal to then?
» hf - ϕ
What is the photoelectric effect equation?
» hf = E(kmax) + ϕ
What equations can reflect when emission can take place from a metal surface?
» E(kmax) > 0
» hf > ϕ
What is the equation for the threshold frequency?
» f(min) = ϕ/h
What is the stopping potential?
» The minimum potential needed to stop photoelectric emission
What happens at the stopping potential?
» The maximum kinetic energy of the emitted electron is reduced to zero because because each emitted electron must to extra work equal to e x Vs
What do metal contains?
» Conduction electrons, which move around the positive ions and collide with one another
What does the photoelectric equation show?
» The photoelectric equation is E = hf = Φ + E and it shows the relationship between the work function, maximum kinetic energy, and the frequency of light.
Explain why, when ultraviolet light is shone on a positively charged plate, no charge is lost by the plate?
» Photoelectric effect involves the emission of electrons which are negative
» Any electrons emitted will only make the positive charge greater
Explain why photoelectric emission from a metal surface only takes place if the frequency of the incident ray is greater than a certain value?
» hf = work function + Ekmax
» Ekmax > 0 for electrons to be emitted
» Therefore hf has to be > work function
» So f > Work function / planks constant
» For electrons to be emitted
Why is it important to test any new theory by testing its prediction experimentally?
» If the test does not provide supporting evidence, the prediction is incorrect
» So the theory is incorrect and must be changed
In the photoelectric equation, what is the name of characteristic quantity of the metal that emits photoelectrons?
» Work function
What is meant by an excited atom?
» When an electron promotes to a higher energy level from a lower energy state
Explain the meaning of the threshold frequency?
» Minimum amount of energy required for a photon for the photo electric effect to happen
