Nature of Waves

Question 1

Polarised Light

Answer 1

Vibration of transverse light waves in only one plane perpendicular to the direction of travel.

Question 2

Non-Polarised Light

Answer 2

Vibration of transverse light waves in numerous planes perpendicular to the direction of travel.

Question 3

Coherence

Answer 3

Waves that are in-phase and have equal frequency.

Question 4

Standing Wave

Answer 4

Interference caused by the superposition of two or more waves travelling in opposite directions with equal amplitude, frequency and wavelength. System of nodes and antinodes.

Question 5

Transverse Wave

Answer 5

Wave which propagates energy through a medium, particles oscillate perpendicular to the direction of travel.

Question 6

Longitudinal Wave

Answer 6

Wave which propagates energy through a medium, particles oscillate parallel with the direction of travel.

Question 7

Multimode Fibres

Answer 7

Large range of path lengths therefore times.
Pulses therefore take different paths and times, pulse broadening. This causes pulses to disperse and overlap, arriving out of order.

Question 8

Amplification

Answer 8

An incident photon causes stimulated emission of another in-phase and coherent photon, causing an excited electron to drop to a lower energy level. This occurs repeatedly as photons reflect back and forth, travelling across the amplification medium each time, intensifying the light as photons increase in number.

Question 9

Population Inversion

Answer 9

Achieved by pumping, there will be more excited electrons in the higher orbital than the lower, making stimulated emission more probable and absorption less probable.

Question 10

Path Diff.

Answer 10

At any fringe, path difference is n x lambda.

Question 11

Check for Polarity

Answer 11

Place polarising filter in front of light source and a light sensor behind the filter. Rotate filter through 180 degrees.
If signal strength detected at sensor changes during rotation, light is polarised.

Question 12

Diffraction Grating

Answer 12

Sheet with an array of equally spaced slits, each of equal width.

Question 13

Grating > Young

Answer 13

Slits are spaced much closed in a diffraction grating than in Young’s model, causing the fringes to have more clarity and be further apart.

Question 14

Refraction

Answer 14

As waves pass from a lower to a higher refractive index medium, their velocity decreases, therefore decreasing their wavelength when in the 2nd medium. If the waves enter the 2nd medium at an angle, they bend towards the normal when passing between the two media.
[Reference diagram]

Question 15

Standing wave / Progressive wave

Answer 15

Wavelength is 2xinternodal distance, wavelength is distance between two crests/troughs
Different amplitude on every point, same amplitude everywhere
Particles between nodes in phase(phase reversal between particles outside internodal), not in phase.
Does not propagate energy, propagates energy

Particles oscillate in both

Question 16

Progressive wave

Answer 16

Particle oscillations through a medium across a distance that carries energy without transporting matter, where there is constant amplitude.

Question 17

Monomode Fibres

Answer 17

Smaller core than multimode fibres, therefore only allows one path light can take. This means all light beams take the same amount of time to pass through the fibre, so they can’t overlap and there is no pulse broadening. Pulses reach the end in order without getting muddled, so data being transmitted can be translated.

Question 17

Slit Width Relation to Diffraction

Answer 17

If slit width&raquo_space; wavelength, the wave will pass undisturbed
As slit width gets closer to wavelength, diffraction will occur until the wave spreads across 180 degrees.

Question 18

Antiphase

Answer 18

Waves that are lambda/2 out of phase/phase diff. will arrive exactly out of phase and destructively interfere.

Question 19

Principle of Superposition

Answer 19

The resulting displacement of two or more interfering waves is the algebraic sum of the individual displacements of those interfering waves.

Question 20

Fibres

Answer 20

Light beams are internally reflected along the core and there is no loss due to large angles the light reflects off the sides to the normal

Question 21

Potential Difference Increase

Answer 21

Increasing pd in a circuit accelerates electrons faster, increasing drift velocity and current.
This means electrons have more energy that is transferred to atoms when moving electrons collide with them, transferring energy to them as vibrational energy. This increases temperature and therefore makes collisions more frequent, increasing resistance.

Question 22

Resistance Decrease

Answer 22

As resistance decreases, current increases.

Question 23

Semi-conductor Lasers

Answer 23

• More efficient than regular laser (~60% of photons emitted rather than 1%)
• Cheaper to produce
• Used in CD or DVD players

Question 24

Superconductors

Answer 24

Metals have a transition temperature at which their resistance quickly drops off. Liquid nitrogen used to cool metals to their transition temperature. Used in high power lines, MRIs and the LHC

Question 25

Electron DIffraction

Answer 25

An electron gun with high voltage shoots an electron beam through a thin crystal foil. This creates a concentric circle pattern, an interference pattern.

Question 26

Photoelectric Effect Method

Answer 26

[Labelled diagram] Circuit is completed with a variable DC battery to promote electron flow. Light is shone on the emissive surface, increase wavelength incrementally. Increase p.d using the variable battery until the microAmmeter reads 0A for each wavelength of light shone. This is the stopping voltage. Plot a graph of Vstop x e to get E k max over f Gradient is h Intercept is -Φ

Question 27

Stimulated Emission

Answer 27

An incident photon of specific energy travels through an atom. This causes an excited electron to drop to a lower energy level. As it does this, it releases a coherent, in-phase photon with the incident. 1 photon input causes 2 photon output.

Question 28

Absorption

Answer 28

An incident photon of specific energy is absorbed by an atom. The photon disappears and an electron in the atom is excited to a higher energy level.

Question 29

Spontaneous Emission

Answer 29

Electron drops to a lower energy level randomly with the release of a photon.

Question 30

Pumping

Answer 30

Supplying amplifying medium with energy to excite electrons to a higher energy level / to create a population inversion. So that there are more electrons on a higher energy level than lower, making stimulated emission more likely than absorption when incident photons pass across the amplifying medium's atoms, which is essential for amplification.

Question 31

4 Level Population Inversion

Answer 31

We need more electrons in the third level than second. Pump electrons to unstable fourth level, they drop to third quickly, spontaneously, to estabilish a population inversion between 3 and 2 more effectively. Once electrons drop to second level due to stimulated emission, they will quickly drop to G, keeping 2 empty almost at all times in order to keep 3 more populated than 2.

Question 32

Amplification

Answer 32

After a population inversion is estabilished in the amplifying medium of the laser cavity, photons are introduced to the cavity that pass across the medium's atoms. This causes stimulated emission, meaning the atoms release coherent photons in the direction of the incident photons, increasing the number of photons present in the cavity (1 photon input, 2 photon output). The number of photons keeps increasing exponentially as the photons will keep bouncing off the reflective sides of the laser cavity and passing across the amplifying medium over and over. This increases the intensity of the light from the laser as the coherent photons constructively interfere.