Physics__Waves and the Particle Nature of Light

Question 1

Snells Law

Answer 1

n1 sin() = n2 sin()

Question 2

What’s the refractive index of air?

Answer 2

about 1

Question 3

What’s the grating equation and how can it be derived?

Answer 3

d sin() = n lambda

Question 4

What’s the wavefront?

Answer 4

A line joining all points on a wave that are in phase

Question 5

A polarising filter is placed in front of a laser. When the laser is switched on a red spot is seen on a screen. The filter is then rotated through 180*. As the filter is rotated, the intensity of the red spot falls to almost zero and then returns to original intensity. Explain these observation

Answer 5

Lasers emit plane polarised light.
Whenever the transmission axis of the filter is parallel to the plane of polarisation of the laser, light is transmitted.
Whenever the transmission axis of the filter is perpendicular to the plane of polarisation of the laser light, no light is transmitted.

Question 6

Explain the difference between plane polarised and unpolarised light.

Answer 6

Unpolarised light - has oscillations in many planes.
Plane Polarised light - oscillations are in a single plane
Plane includes the direction of propagation

Question 7

What’s diffraction?

Answer 7

The process of waves spreading when they pass through a gap or barrier

Question 8

Describe longitudinal waves in terms of pressure variation and the displacement of molecules

Answer 8

Longitudinal waves have areas of high pressure called compressions, and low pressure called rarefactions.
Particles oscillate parallel to the direction of the propagation of the wave and direction of energy transfer

Question 9

Describe transverse waves?

Answer 9

Particles oscillate in the direction of energy transfer but perpendicular to the direction of the propagation of the wave.
Transverse waves show areas of crests (peaks) and troughs.

Question 10

What are the different ways of representing transverse waves on a graph?

Answer 10

Displacement-distance graphs, displacement-time graphs

Question 11

What are the different ways of representing longitudinal waves on a graph?

Answer 11

Displacement-distance graphs

Question 12

What are the different ways of representing standing waves on a graph?

Answer 12

Standing waves can be transverse or longitudinal so can be be plotted on a graph as thus

Question 13

What’s coherence?

Answer 13

Interference is observable only if produced by a coherent source.
Waves are said to be coherent if they have:
A constant phase difference and the same frequency

Question 14

What’s the principle of superposition

Answer 14

Waves from two sources meet and occupy the same region.
Total displacement is the vector sum of their individual displacements at that point

Question 15

What’s interference?

Answer 15

When multiple waves combine to produce a resultant wave with a new amplitude

Question 16

What’s constructive interference?

Answer 16

When waves from two sources superpose at a point in-phase, constructive interference occurs.
The resultant wave has a larger amplitude than any of the individual waves

Question 17

What’s destructive interference?

Answer 17

When waves from two sources superpose at a point anti-phase, destructive interference occurs. Resultant wave has a smaller amplitude than individual waves

Question 18

What is phase?

Answer 18

2 points on waves are in phase when they are at the same point in their wave cycle.
Angle between their wave cycles is the phase difference.

Question 19

What is path difference

Answer 19

The difference in distance travelled by two waves from their sources to the point where they meet

Question 20

How can we tell if waves are in-phase or anti-phase?

Answer 20

When path difference is an integer no. of wavelengths, then in-phase.
When path difference is an odd no. of half wavelengths, then antiphase.

Question 21

Explain how a stationary wave is formed. Then describe the key features of the stationary wave?

Answer 21

When two progressive waves of equal frequency and amplitude travelling in opposite directions superpose.
Nodes where there is zero amplitude.
Antinodes where amplitude is maximum.
Destructive superposition at nodes/Constructive superposition at antinodes.
Energy is not transferred along string - instead it’s stored

Question 22

What are nodes and antinodes?

Answer 22

Nodes: points of zero amplitude on standing wave.
Antinodes: points of maximum amplitude on standing wave

Question 23

What is the fundamental frequency?

Answer 23

The lowest frequency that can produce a standing wave

Question 24

What is the equation for the speed of a transverse wave on a string?

Answer 24

Where v is wave speed, T is tension, and M is mass per unit length

Question 25

What is the focal length?

Answer 25

Distance between the optical centre of the lense and the focal point (principle focus). f is measured in metres, it depends on how curved a lense is (the more curved, the shorter the length)

Question 26

How to calculate the power of a lense?

Answer 26

f is focal length in metres

Question 27

What is a real image?

Answer 27

It is formed by the convergence of rays of light (the rays meet). Real images can be projected onto a screen

Question 28

What's a virtual image?

Answer 28

A virtual image is seen but not formed on a screen The rays of light have not met, they have been perceived by the eye e.g.: An image viewed through a magnifying glass is a virtual image

Question 29

What is the equation for magnification

Answer 29

Where v is image height, and u is object height

Question 30

Huygen's construction

Answer 30

(You need to be able to draw this) Those point sources which pass through the gap create new wavelets on the other side, leading to the characteristic curved shape of the diffracted wave

Question 31

What happens to a wave when it meets a slit?

Answer 31

Some energy is dissipated as a wave is diffracted through the gap, effecting the amplitude

Question 32

What happens as a wave meets an obstacle?

Answer 32

The wave diffracts around the edges of the obstacle, and a "shadow" forms where no part of the wave reaches behind.

Question 33

What factors effect diffraction?

Answer 33

Diffraction is most prominent when the size of the gap or obstacle is the same or smaller than the wavelength of the wave. As the size of the gap or obstacle increases, the effect gradually gets less pronounced. When the gap is much larger than the wavelength, the waves are no longer spread out.

Question 34

How do diffraction experiments provide evidence for the wave nature of electrons?

Answer 34

Electron diffraction using an electron gun suggests that particles can demonstrate wave like properties. Electrons are accelerated through vacuum towards a crystal lattice Where they interact with the small gaps between atoms. Electron diffraction produces the following pattern. If electrons didn't have a wave nature, then when they pass through a slit, they would be unaffected - a single bright region would be formed

Question 35

What factors does the de Broglie wavelength depend on?

Answer 35

Mass and Velocity

Question 36

How does pulse echo technique work?

Answer 36

Wave pulse transmitted Transmitted and reflected at the boundary between 2 media Returning wave echo is detected Speed and time taken are used to calculate distance to object

Question 37

What may limit the amount of info that can be obtained by pulse-echo technique

Answer 37

Wavelength of radiation (must be short to reduce diffraction) Duration of pulse (must be shorter pulses so that reflections from nearby interfaces don't reach transducer before pulse has ended - i.e. reduce interference)

Question 38

Which 2 models are used to describe EM radiation

Answer 38

The wave model The particle model

Question 39

What provides evidence for the particle model?

Answer 39

The photoelectric effect

Question 40

What is the photoelectric effect?

Answer 40

Photons of a certain frequency are fired at a metal plate Photon transfers all energy to an electron. If the frequency of the photon is high enough, the electrons will gain enough energy to overcome the metal's work function Electrons are then emitted from the surface of the metal. One photon liberates one electron

Question 41

What is the threshold frequency?

Answer 41

The minimum frequency of a photon that is required to liberate an electron from the surface of a particular metal.

Question 42

If the intensity of light being shone on a metal increases, how does the energy of photoelectrons change?

Answer 42

Energy is unaffected, An increase in intensity means that there are more photons per area. Therefore more photoelectrons are emitted.

Question 43

Why are photoelectrons emitted with a range of kinetic energies?

Answer 43

Electrons are at different depths within the metal. So, require different amount of energy to be liberated. After liberation, the excess energy from the photon is the kinetic energy of the photoelectron.

Question 44

How to convert between eV and J

Answer 44

1eV = 1.6 x 10-19J

Question 45

How do electrons transition between energy levels?

Answer 45

When electrons absorb radiation, they move to a higher energy level. When electrons emit radiation, they move to a lower energy level.

Question 46

Why can only certain frequencies of radiation be absorbed by an atom to cause an electron to transition energy levels?

Answer 46

Electrons exist in discrete energy levels. The energy of the photon must be the exact amount of energy required to cover the difference between the two discrete energy levels (the bandgap)

Question 47

What is evidence for the wave theory of EM radiation?

Answer 47

Young's Double Slit experiment.

Question 48

What is meant by a real image?

Answer 48

Light rays converge to a point where the image is formed