Nature of Light

Question 1

Describe the Corpuscular theory

Answer 1

Corpuscular theory - proposed by Newton - light made up of corpuscles (or tiny particles). He imagined the light particles to bounce off surfaces, like a ball bouncing off a wall, to explain reflection and suggested that the light particles changed speed when they moved from one material to another to explain refraction.

Question 2

Describe the Wave theory

Answer 2

Wave theory - proposed by Huygens - light consists of waves

Question 3

Define Photon and Quantum

Answer 3

Photon - a discrete bundle (quanta) of energy. One photon (if it has enough energy) will release one photo electron.

Quantum - a fixed indivisible amount.

Question 4

Explain the Complimentary principle

Answer 4

The complimentary principle says that sometimes electrons have the properties of particles and sometimes the properties of waves, but never both.

Question 5

Describe the Photoelectric effect and how it can be demonstrated

Answer 5

The photoelectric effect can be demonstrated by means of a negatively charged zinc plate. (If it was positive the transmitted photoelectrons would be immediately attracted back by the charge)
The ultraviolet light causes the gold leaf electroscope to discharge (leaf goes down). The uv light is absorbed and the energy is used to eject the electrons from the metal surface.

Question 6

Define Photoelectrons

Answer 6

Photoelectrons - the electrons near the surface of zinc that gain enough energy to escape from the attraction of the positive charge on the nucleus

Question 7

What is the formula (in words) that explains this effect?

Answer 7

Energy from UV light = Energy needed to remove electrons from metal surface + KE of photoelectrons

Question 8

What is the condition necessary for photoelectrons to be emitted?

Answer 8

Electron would only be emitted if a single quantum of the light had enough energy for the electron to escape. This is because one photon will only release one photoelectron.

Question 9

What are the observations from the photoelectricity experiment? (3)

Answer 9

1. Using visible light we do not get any photoelectrons, even if the light is left on for a long time. This is in contradiction with the wave theory, as it suggests that eventually, enough energy should be given to an electron to remove it from the metal surface. This does not happen. No photoelectrons emitted from the surface.
2. Increasing brightness (intensity) and hence the energy of the waves should produce photoelectrons, according to wave theory. Using a green laser (very bright) with the zinc does not produce any photoelectrons. Wave theory: Energy is proportional to share of intesity. This does not happen so this contradicts the wave theory.
3. Increasing the frequency (using UV instead of visible light) we find a threshold frequency (fo) where photoelectrons start to be emitted. Wave theory suggests the energy should not be different depending on the frequency. So this also contradicts the wave theory.

Question 10

Define Threshold frequency and Work function

Answer 10

Threshold frequency - the frequency that is just large enough to liberate electrons, fo (ø=hfo)
Work function - the bare minimum energy needed to just remove an electron from the surface of a metal with no kinetic energy.

Question 11

What is the Einstein’s photoelectric equation?

Answer 11

hf = ø + 1/2 mv^2 max
hf - energy of photons; ø - work function; 1/2 mv^2 max - maximum kinetic energy of photoelectrons 
hf = hfo + 1/2 mv^2
h(f-fo) = 1/2 mv^2
h(f-fo) = eVstop

Question 12

Describe different types of photcells

Answer 12

A phototube is the name given to a particular type of photocell that generates photoelectrons when light falls on a specially coated metal cathode. The other types of photocells are photovoltaic photocells, in which an emf is generated by the presence of light across the boundary of two semiconducting materials, and photoconductive cells or light-dependent resistors (LDRs). An LDR is a semiconductor whose resistance decreases when it is exposed to electromagnetic radiation (it becomes a good conductor). This is because the photon energy release more electrons to act as charge carriers: n increases in I=nAvq.

Question 13

Describe how a phototube can be used to find the work function

Answer 13

We apply a stopping voltage (reverse voltage) to an evacuated tube containing two electrodes. One electrode is irradiated with light of a known wavelength (so we can calculate its frequency)
The stopping potential is increased until the photoelectrons just fail to reach the negatively charged electrode (the photoelectric current goes to zero)
Therefore the work done on the photoelectrons by the electric field is equal to the KE of the photoelectrons. eVstop = 1/2 mv^2 max
so hf = ø + eVstop
and Vstop = (h/e)f - (ø/e)
y = mx + c
So graph of Vstop against Frequency

Question 14

Define the Electron-volt

Answer 14

An electron-volt is the work done on (or the energy gained by) an electron when it moves through a potential difference of 1 volt.
It is used to avoid using powers of 10^-19 but as it is not an SI unit, it needs to be converted in the end result.

Question 15

What is the expression for energy of a photon and this expression but in the emission spectrum

Answer 15

E = hf; E - energy of a photon; h- Planck constant; f - frequency of the radiation

hf = E2-E1

Question 16

Describe the atomic line spectra in terms of transitions between discrete energy levels

Answer 16

Gases are poor conductors of electricity. To make them conduct we need to have low pressure and high voltage. The electrons passing through the gas collide with the atoms, or molecules, of the gas and cause them to become excited so that they give out light. We can examine the emitted light using a spectrometer and a diffraction grating. We find that only certain wavelengths of light are emitted so that we see a line spectrum.
This means that the hydrogen atom can only have particular levels, and not “any” energy level, otherwise we would see photons of all wavelengths and frequencies (ie all energies)
This means that the energy levels are quantised. Only certain energy levels are allowed.
If we consider the wave-like properties of electrons as we can consider the energies that the electron may have as corresponding to standing waves in the hydrogen.

Question 17

Why are the energy levels all negative?

Answer 17

When the electron is infinitely far away from the proton (so it feels no force of attraction) and is not moving (so it has no KE) it seems reasonable to say it has no energy. Therefore we define the zero of energy to be infinitely far away from the hydrogen atom. This means that all the energy levels in hydrogen are negative (as work has to be done on an electron in the hydrogen atom against the force of attraction between the proton and the electron)

Question 18

What is the difference between Emission and Absorption?

Answer 18

Emission - from high energy state to low energy state

Absorption - from low energy state to high energy state

Question 19

Define Intensity(Radiation flux density) and give a formula for it

Answer 19

Intensity is defined as the energy passing normally (at 90 degrees) through a surface per unit area per unit time/ is the power per unit area.

Radiation flux density = Intensity = Power/Area

I = E/At = P/A

Question 20

Define Efficiency

Answer 20

Efficiency of the system is the ability to transfer the input energy to desired (useful) energy

Efficiency = (useful energy/power output / total energy/power input) * 100

Question 21

Explain the wave-particle duality and the complimentary principle

Answer 21

Wave-particle duality: particles can sometimes behave like waves and waves can sometimes behave like particles. De Broglie equation links waves and particles:
λ=h/p

The complimentary principle says that sometimes electrons have the properties of particles Andy sometimes the properties of waves but never both together.

Question 22

Evaluate the use of solace cells

Answer 22

Solar cells provide electricity from a free, renewable source
Considerations include:
- efficiency of the cells
- cost
- availability of materials
- global energy prices
- impact of continuing use of fossil fuels

Question 23

Explain remote sensing and its uses

Answer 23

Remote sensing is the process of detecting and monitoring the physical characteristics of an area by measuring it reflected and emitted radiation at a distance from the targeted area. Special cameras collect remotely sensed images of the Earth, which help us “sense” things about the Earth. Examples:

• Cameras on satellites and airplanes take images of large ares on the Earth’s surface. allowing us to see much more than we can see standing on the ground (clouds, volcanoes, fires, growth of trees)
• Sonar systems on ships can be used to create images of the ocean floor without needing to travel to the bottom of the ocean
• Cameras on satellites can be used to make images of temperature changes in the oceans