AS2 2.5 Quantum Physics

Question 1

What is a photon?

Answer 1

A photon is an indivisible packet of electromagnetic energy.

Question 2

What did Einstein theorise about light?

Answer 2

Light is made up of photons

Question 3

What does the energy of a photon depend on?

Answer 3

The frequency of the photon, NOT the intensity

Question 4

What is photon energy proportional to?

Answer 4

Light energy

Question 5

What is the equation for energy per photon?

Answer 5

E = hf

E = energy per photon (J)
f = frequency of light (Hz)
h = Plank’s constant (6.63 x 10^-34) (Js)

Question 6

what is the equation for frequency? (Quantum Physics)?

Answer 6

f = V/ λ

f = frequency (Hz)
V = speed of light (3 x 10^8) (m/s)
λ = wavelength (m)

Question 7

What is an electron volt and what is it equal to in joules?

Answer 7

It is another way of writing energy
1 electron volt = 1.6 x 10^-19 Joules

Question 8

What observations can’t be explained using the energy of light?

Answer 8

1. Emission of electrons from metal surface does NOT take place if the frequency of the EM radiation is below the threshold frequency?
2. If the frequency is < threshold frequency, no matter how intense the radiation, no emissions will take place
3. photoemission occurs without delay (provided f > threshold frequency)

Question 9

What is the photoelectric effect?

Answer 9

When light is incident on a metal surface, an electron volt 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 photo

Question 10

When can an electron leave the metal surface in the photoelectric effect?

Answer 10

An electron can leave the metal surface if the energy gained from a single photon exceeds the work function of the metal

Question 11

What is the work function?

Answer 11

The minimum energy a photon must have to cause the emission of electrons?

Question 12

What is the threshold frequency?

Answer 12

The minimum frequency a photon must have to cause the emission of electrons?

Question 13

What is the equation for the energy of a photon?

Answer 13

E(photon)= Φ + Ek
1/2mv^2(max) = hf - hfo
Ek = E(photon) - Φ

Φ = work function

Question 14

How can the number of electrons released can be increased (assuming f >fo) ?

Answer 14

1. increase the intensity of the radiation - if we visualize more photons landing per unit area, clearly more photons will be released.
2. increase the frequency of the radiation - work function is the energy required to free electrons from the surface. Φ increases, fo increases with depth. increasing frequency increases the energy allowing the photons to go deeper.

Question 15

2 conditions required for photoelectrons to be emitted from the surface of metal?

Answer 15

1. The photons must be absorbed b y the electrons
2. Photons must have energy greater than or equal to the work function of the metal.

Question 16

what are the characteristics of energy levels?

Answer 16

• Electrons exist in energy levels
• Energy levels are said to be discrete. this means they have exact values
• Typically, electrons exist in the ground state
• The energy of an electron shell is constant.

Question 17

What is the ground state?

Answer 17

the lowest possible energy level or most stable energy level

An orbit with an energy where an electron can remain

Question 18

What is excitation?

Answer 18

when an electron moves up an energy level by absorbing energy.

Question 19

What are the two ways that excitation can occur?

Answer 19

1. When the atom collides with another particle
2. When the electron absorbs a photon with the exact amount of energy it needs to jump a level.

Question 20

What is relaxation (De-Excitation)?

Answer 20

• The electron configuration in an excited atom is unstable. The vacancy is filled by an electron from an outer shell.
• When an electron falls, it loses energy and the atom emits a photon. The energy of the photon is the exact energy that is lost.

Question 21

What is the Absorption Spectra?

Answer 21

• white light is a mixture of photons on different colour “frequencies”. When the light is viewed after passing through the gas we can see black absorbed lines. These lines represent the frequency of photons which have been absorbed to excite electrons.

Question 22

What is the Emission Spectra?

Answer 22

Each possible drop between energy levels in an atom corresponds to the emission of one specific frequency of photon. This results in one line in an element’s emission spectrum.

Question 23

What does Line Emission mean?

Answer 23

The emission of light from elements proves that there are discrete energy levels in that atom.

Question 24

What are the 6 energy levels of hydrogen?

Answer 24

0 : n = ∞
- 0.54 : n = 5
- 0.85 : n = 4
- 1.51 : n = 3
- 3.40 : n = 2
- 13.59 : n = 1

Question 25

What is represented by transitioning down and transitioning up an energy level?

Answer 25

Emission = transitioning down Absorption = transitioning up

Question 26

why does the hydrogen emission spectrum consist of a limited number of characteristic wavelengths only?

Answer 26

1. Fixed discrete energy levels 2. **Downward transition** between levels increase photons of specific energies , hence characteristic lines.

Question 27

Explain the florescence tube?

Answer 27

The florescent tube is a glass made with a florescent coating on its inner surface. The tube contains mercury vapor at low pressure.

Question 28

What are the 4 reasons as to why a florescence tube emits visible light when it is on?

Answer 28

1. Ionization and excitation of the mercury atom occurs as they collide with each other and the electrons in the tube. 2. The mercury atoms emit UV photons , as well as visible photons and photons of much less energy when they de-excite 3. The UV photons are absorbed by the atoms of the florescent coating, causing excitation of the atoms. 4. The coating atoms de-excite and emit visible photons

Question 29

why is the 6th energy level the ground state?

Answer 29

Electrons always occupy the most stable energy level.

Question 30

Why are all energy levels except the zero energy state negative?

Answer 30

Energy is required to remove an electron from an atom, anything less would be negative.

Question 31

What does laser stand for?

Answer 31

**L**ight **A**mplification by the **S**timulated **E**mission of **R**adiation

Question 32

What are the 3 characteristics of lights produced by lasers?

Answer 32

- coherent - monochromatic - collimated (all parts travel in he same direction)

Question 33

What is stimulated emission?

Answer 33

If a photon of **exactly the same energy** approaches an excited atom, an electron may be induced to fall to a lower level and emit another photon. this photon has the **same phase, frequency and direction of travel** as the stimulating photon.

Question 34

What is population inversion?

Answer 34

When there is more electrons in an upper level than a lower level POPULATION INVERSION CANNOT OCCUR IN A TWO LEVEL SYSTEM.

Question 35

What is optical pumping?

Answer 35

A method of population inversion that consists of illuminating the laser material with light.

Question 36

What happens in population inversion in a **Two-level** system?

Answer 36

The pumping radiation induces stimulated emission from level 2 back down to level 1 since it is of the correct energy

Question 37

What happens in population inversion in a **Three-level** system?

Answer 37

1. Electrons absorb radiation causing them to move up energy levels to an excited state 2. Electrons fall down into the metastable state 3. They stay in this state longer than the excited state 4. This allows population inversion to occur (more electrons in upper energy level than lower level) 5. Once an electron falls to ground state it releases a photon 6. This causes stimulated emission of photons which all have the same energy as the incoming photon.

Question 38

What are some uses of lasers?

Answer 38

- CDs and DVDs - Shop Checkouts - Laser Printers - Laser Pointers - Car Parking Assist

Question 39

How are X-rays used in the industry?

Answer 39

1- Patient is placed in position by radiographer 2- The detector plate (with film inside) is placed below the target area 3- X-Ray tube is positioned above the target area (X-Ray) and detector plate is now stationary.

Question 40

What are the advantages of **X-Rays**?

Answer 40

- Cheaper than a CT scan - Lower radiation values compared to CT scan

Question 41

What are the disadvantages of **X-Rays**?

Answer 41

- Only gives info about bones - Radiation doses (employee safety) Vulnerable people - pregnant women and children

Question 42

What are CT scans?

Answer 42

**Computed Tomography**. The X-Ray tube moves around , the detector moves in phases. CT scans allow flesh and internal organs to be seen clearly in section view of the patient. The data is sent from a scanner to a computer , where complex software builds together the overall picture.

Question 43

What are the advantages of **CT scans**?

Answer 43

- Allows images of organs in high detail - Can detect many life-threatening injuries in accidents - Can detect many heart diseases.

Question 44

What are the disadvantages of **CT scans**?

Answer 44

- Much more costly than a conventional scan - Higher radiation doses than X-Rays - Higher preparation time for patients because of potential allergic reactions? vulnerable people - pregnant women and children.