Waves

Question 1

Define wavefront

Answer 1

The surface made up of all the points of the wave that are in phase with each other.

Question 2

Define displacement

Answer 2

How far a wave has travelled from its undisturbed position.

Question 3

Define phase

Answer 3

The measurement of a certain points position along the wave cycle.

Question 4

Define longitudinal

Answer 4

Oscillations occur parallel to the direction of travel

Question 5

How does a standing wave form?

Answer 5

Two waves are travelling in opposite directions and superimpose continuously to form a standing wave.

Question 6

What are the conditions for waves to be able to form a standing wave

Answer 6

They need to be coherent (same frequency and a constant phase relationship)

Question 7

Do standing waves transfer energy?

Answer 7

No they don’t

Question 8

Where does the node occur when a standing wave is formed between two points

Answer 8

At each end

Question 9

Define diffraction

Answer 9

The spreading out of waves as they pass through an obstruction

Question 10

How does Huygens explain diffraction

Answer 10

Huygens suggested that every point on a wavefront can be considered to be a point source of secondary waves (which he called wavelets)

Question 11

Define wave front

Answer 11

A line that marks point all in phase

Question 12

How does the electron gun experiment work?

Answer 12

Electrons are accelerated in an electron gun to a high potential and are then directed through a thin film of graphite. The lattice structure of the graphite acts like the slits in a diffraction grating.
The electrons diffract from the gaps between carbon atoms and produce a circular pattern on a fluorescent screen made from phosphor.

Question 13

Define the critical angle

Answer 13

The angle at which the angle of refraction is 90 degrees

Question 14

What are the properties of an image formed by a converging lens

Answer 14

• Real
• Inverted
• Magnifies

Question 15

What are the properties on a diverging lens image?

Answer 15

• Upright
• Diminished
• Virtual

Question 16

What is polarisation?

Answer 16

The filtering of an electromagnetic wave so that it only oscillated in one plane (direction)

Question 17

Can sound be polarised?

Answer 17

No only transverse light can be polarised

Question 18

What is the evidence for the wave properties of light?

Answer 18

• Interference
• Polarisation
• Diffraction

Question 19

What is the evidence of particle properties of light?

Answer 19

• Travels in a straight line (shadows)
• Photoelectric effect

Question 20

Define work function

Answer 20

The minimum energy of incident photons to release electrons from the surface

Question 21

What is the photoelectric effect

Answer 21

When light is incident on a surface, electrons absorb photons so they have enough energy to be released from the surface

Question 22

Define thermionic emission

Answer 22

A metal will release electrons when it is heated

Question 23

True or false:
Two points λ/2 apart oscillate with the same amplitude

Answer 23

True

Question 24

What effect does the speed of electrons have on the diameter of rings in the electron gun experiment.

Answer 24

Inversely proportional.
(Decreasing speed increases diameter)

Question 25

State 2 ways to improve accuracy when measuring the refractive index of glass.

Answer 25

- Use a large angle of incidence - Use a narrow ray - Work in a dark room - Repeat and calculate a mean

Question 26

Why do atoms only emit a certain frequency of radiation?

Answer 26

- Electrons exist in discrete energy levels - Energy is proportional to frequency so frequency is also discrete - Eventually all atoms will return to a lower state/ ground state as this is the most stable - Atoms won’t absorb electrons/energy outside the discrete numbers - As the atoms deexcites a photon is emitted

Question 27

Why is there a bright spot equidistant from two slits?

Answer 27

- The distance is the same so path difference is zero - So the waves will arrive in phase - So constructive interference occurs

Question 28

For the equation m = v / u explain what v and u represent.

Answer 28

V is the distance of the image to the lens U is the distance of the object to the lens

Question 29

Explain a method to find the focal length of a converging lens

Answer 29

- Focus an image of a far object onto a screen - Measure distance of lens to the screen

Question 30

Why is a lens with a shorter focal length thicker at its centre?

Answer 30

- Light gets refracted by a greater amount - So rays converge at a point closer to the lens

Question 31

What does the thin metal foil experiment show?

Answer 31

The diffraction experiment indicates electrons have wave nature. Therefore electrons behaviour has wave-particle duality.

Question 32

What is an infinite source?

Answer 32

Light from the source is parralel

Question 33

What is a real image

Answer 33

An image that can be projected onto a screen