Unit 2: Light and Lasers

Question 1

What are photons?

Answer 1

Photons are packets of light

Question 2

What is the equation for the energy of a proton?

Answer 2

Energy = hf

Question 3

What is the photoelectric effect?

Answer 3

The photoelectric effect is the release of electrons off the surface of a metal by shining light on it. The electrons being called photoelectrons.

Question 4

What happens if you increase the intensity of light?

Answer 4

The number of photoelectrons emitted increases as the number of photons increases therefore increasing the rate of the photoelectric effect.

Question 5

What is an electron volt?

Answer 5

An electron volt is the energy gained by an electron through a potential difference of 1V.

Question 6

How is the energy of a photon found?

Answer 6

E = hf or E = hc/λ

Planks constant x Frequency

Question 7

Describe the gold leaf experiment

Answer 7

We must have a negatively charged. system, as when it is negative there is a surplus of electrons.

This causes the gold leaf to repel away from the system due to everything being the same charge (negative charge).

When the system is hit with light called the light of incident, the photons deliver energy to knock the electrons off, as it will give them energy in order to be ejected off the surface of the metal.

This will cause the system to become neutrally charged causing the leaf to drop.

Question 8

Describe the vacuum photocell

Answer 8

You have a circuit which has a metal surface and opposite it a collecting electrode within a vacuum glass the circuit will also have a variable power supply and a micro-ammeter in order to find the current.

You shine light onto the metal surface and this will cause the electrons to be ejected off the surface of the metal and be attracted to collecting electrode (made positive by the battery) and this will form a current due to the electrons flowing through the circuit. This is because the electrons are emitted with enough kinetic energy to reach the collecting electrode.

We can then increase the pd between the metal surface and collecting electrode causing the collecting electrode to become negative until it prevents the electrons from reaching it causing the current to drop to 0. This voltage is called the stopping voltage.
This is represented with the equation:
Kinetic-Energy(max) = e x Stopping Voltage

Question 9

What is Einstein’s photoelectric equation?

Answer 9

Ke(max) = hf-Φ

The maximum kinetic energy an electron can have after being ejected is the energy from the photon takeaway the work function

Question 10

What is the work function?

Answer 10

The work function is the minimum amount of energy required for an electron to be ejected from the surface of a metal.

Therefore the hf must be at least as big as the work function.

Question 11

What is the ionisation energy?

Answer 11

The energy required to remove an electron from the atom in its ground state

Question 12

What is excitation?

Answer 12

Excitation is when an electron absorbs a photon and goes up an energy level

Question 13

What is de-excitation?

Answer 13

De-excitation is when an electron falls down an energy level and emits a photon

Question 14

What is electron diffraction, how it works and what does it prove?

Answer 14

Electron diffraction is a test to prove that particles can behave like waves, one way of proving it is by firing an electron gun towards a very thin slice of carbon and the atomic spacing act like slits this causes the electron to form a diffraction pattern on the carbon sheet with dark and bright areas, concluding that particles do have wave-like properties.

Question 15

How do you calculate photon momentum?

Answer 15

p = h/λ

Question 16

What are the 3 atomic processes

Answer 16

Absorption of light
Spontaneous emission of light
Stimulated emission of light

Question 17

What is stimulated emission?

Answer 17

Stimulated emission is the emission of a photon from an excited electron by passing a photon called the incident photon of energy equal to the gap between the excited level and energy level below causing the photon to hit the electron allowing 2 photons of same frequency and phase to be emitted in the same direction

Question 18

What is population inversion?

Answer 18

Population inversion is when there are more electrons in the upper level than the lower level

Question 19

Why is a population inversion needed?

Answer 19

Electrons in lower states will absorb photons causing them to rise rather than drop as there is a higher chance of absorption occurring rather than stimulated emission

Question 20

How are population inversions obtained?

Answer 20

By a process called pumping, this is where electrons within an atom are promoted to a higher energy level as energy is being fed into the amplifying medium causing them to go up in energy levels

Question 21

Population inversion in a 2-level system

Answer 21

It is not possible to create a population inversion in a 2-level system as the electrons have the same chance of absorption and stimulated emission therefore the best that can be obtained is Electrons(ground) = Electrons(upper)

Question 22

Population inversion in a 3-level system

Answer 22

Electrons are pumped to level 3 which is not stable so the electrons quickly drop to level 2,

Level 2 is meta stable so the electrons will stay in this level long enough for a photon to cause stimulated emission

Question 23

Population inversion in a 4-level system

Answer 23

Electrons are pumped to level 4, which is not stable so the electrons quickly drop to level 3,

Level 3 is metastable so the electron will stay in this level long enough for a photon to cause stimulated emission,

Question 24

What is the issue with pumping?

Answer 24

The pumping light is a greater frequency that the laser making the laser not very efficient

It also takes a lot of energy for pumping to take place

Question 25

What is the typical structure of a laser?

Answer 25

A laser consists of an amplifying medium between 2 Mirrors, one of which reflects 99% of the photons allowing 1% of the photons to be transmitted

Question 26

What are semiconductor lasers?

Answer 26

Conventional lasers are not very efficient
so semi conductor lasers which are smaller, cheaper and more efficient are used

Question 27

How do semiconductor lasers work?

Answer 27

They use electrical current to pump the electrons to produce a population inversion and mirrors used transmit up to 60% of the photons
However they cannot produce high power focussed beams that conventional lasers do

Question 28

What is the advantage of a level 4 laser

Answer 28

Level 4 lasers are more efficient as they require less energy to achieve a population inversion