Unit 2.7: Photons

Question 1

Other than behaving as waves, how else can light behave?

Answer 1

As a stream of particles

Question 2

Energy in a light wave is not a continuous stream but rather is quantized into what?

Answer 2

Little bundles of energy known as QUANTA or PHOTONS

Question 3

What did concept did Max Planck introduce?

Answer 3

That energy of a photon is a fixed amount
Concept of quantisation of energy

Question 4

What did Einstein conclude from the quantisation of energy?

Answer 4

That the energy of a photon is proportional to the frequency of radiation

Question 5

What is a photon?

Answer 5

A discrete packet of energy

Question 6

What is the equation for the energy of a photon?

Answer 6

Ephot=hf or E=hc/λ

Question 7

What is one electron volt(eV)?

Answer 7

One electronvolt is the energy of an electron accelerated by a potential difference of 1 Volt

Question 8

What unit is used to describe the energy of a single photon?

Answer 8

Joule

Question 9

What is 1eV in Joules

Answer 9

1eV=1.6x10^-19J

Question 10

What is the typical wavelength for Gamma waves?

Answer 10

10^-12m

Question 11

What is the typical wavelength for X-ray waves?

Answer 11

10^-10m

Question 12

What is the typical wavelength for UV waves?

Answer 12

10^-7 or 10^-8

Question 13

What is the typical wavelength for visible waves?

Answer 13

4x10^-7m to 7x10^-7m
4=Blue
7=Red

Question 14

What is the typical wavelength for infrared waves?

Answer 14

10^-5m

Question 15

What is the typical wavelength for Microwaves?

Answer 15

10^-2

Question 16

What is the typical wavelength for radio waves?

Answer 16

10^2

Question 17

What is the typical photon energy for Gamma waves?

Answer 17

10^6 eV

Question 18

What is the typical photon energy for an X-ray wave?

Answer 18

10^4 eV

Question 19

What is the typical photon energy for UV?

Answer 19

10^1 eV

Question 20

What is the typical photon energy for visible light?

Answer 20

3.1eV to 1.8eV
blue red

Question 21

What is the typical photon energy for infrared waves?

Answer 21

10^-1eV

Question 22

What is the typical photon energy for microwaves?

Answer 22

10^-4eV

Question 23

What is the typical photon energy for radio waves?

Answer 23

10^-8eV

Question 24

What is the photoelectric effect?

Answer 24

When light or UV radiation of short enough wavelength falls on a surface, electrons are emitted from the surface

Question 25

What is work function(Φ)?

Answer 25

The work function is the minimum energy required to remove an electrons from the surface

Question 26

What are the units for Work function?

Answer 26

Joules or electronvolts

Question 27

What is threshold frequency (f0)?

Answer 27

The minimum frequency of a photon that will cause the emission of an electron from a particular metal surface

Question 28

What happens when EM radiation of enough energy strikes the cathode surface?

Answer 28

Electrons are ejected

Question 29

What are photoelectrons?

Answer 29

Ejected electrons from the surface of a cathode

Question 30

During the photoelectric effect experiment, where do the photoelectrons travel and what does this create in the circuit?

Answer 30

The photoelectrons travel across to the anode and a current flows in the circuit

Question 31

During the photoelectric effect experiment, how is current read?

Answer 31

Using an ammeter

Question 32

During the photoelectric effect experiment, the p.d is increased in the circuit it until what happens?

Answer 32

The photoelectrons are being repelled from the anode and at certain p.d the electrons won't have enough energy to reach the anode

Question 33

The cathode and anode have to be in what during the photoelectric effect experiment

Answer 33

They must be in a vacuum otherwise the ejected electrons could collide with molecules in the air

Question 34

State the steps for the photoelectric experiment

Answer 34

1. Shine the light on cathode 2. Adjust p.d until micro-ammeter shows 0 current 3. Read voltmeter 4. Plot a graph, current against p.d

Question 35

If the intensity of light increases, what happens to the number of photons of light

Answer 35

They increase

Question 36

Ek max=Evstop

Question 37

State Einstein's photoelectric equation

Answer 37

Ekmax=hf-Φ

Question 38

If the energy of a photon is just enough to cause an electron to be ejected then te photoelectron will have no Ek. State the photoelectric equation with no Ek.

Answer 38

hf0=Φ

Question 39

Ekmax=hf0-Φ & y=mx+c predicts what on a photoelectric graph

Answer 39

That Ekmax against f will be a straight line

Question 40

How do you find threshold frequency on a photoelectric graph?

Answer 40

Where the line crosses the x axis

Question 41

What idea did Niels Bohr develop in 1913

Answer 41

-Energy in atoms was quantised and the the electrons existed in specific energy levels

Question 42

According to Niels Bohr, when can electrons move between energy levels?

Answer 42

When they are given the correct quantity of energy usually in the form of a photon

Question 43

If an electron want to move up an energy level, it must absorb what particle?

Answer 43

Photon

Question 44

If an electrons wants to move down an energy level is must emit what particle?

Answer 44

Photon

Question 45

What is ionisation energy?

Answer 45

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

Question 46

How does an absorption spectra form in terms of electrons?

Answer 46

When light passes through clouds of gas in the star's atmosphere and certain wavelengths are absorbed as electrons are promoted to higher energy levels

Question 47

What is the emission spectrum?

Answer 47

Electrons collide with atoms in the gas and cause electrons to be promoted to higher energy levels. We get coloured lines.

Question 48

What is electron diffraction?

Answer 48

When a beam of electrons is accelerated through a thin polycrystalline graphite film in an evacuated chamber with a fluorescent screen to see where the electrons come out the other side. This creates a diffraction grating