Waves

Question 1

Describe a longitudinal wave

Answer 1

The direction of oscillation is parallel to the direction of travel Made of compressions (high pressure) and rarefactions (low pressure) Molecules move back and forth from equilibrium position

Question 2

Draw a labelled wave

Answer 2

Question 3

Define Wavefront

Answer 3

A line or surface joining points which are at the same state of oscillation, i.e. in phase

Question 4

Define Coherence

Answer 4

Two waves are coherent if the phase difference between them is constant. For this to be the case they must have the same frequency.

Question 5

Define superposition

Answer 5

When two or more waves of the same type meet at a point, the resultant displacement of the waves is equal to the vector sum of their individual displacements at that point.

Question 6

Define destructive interference

Answer 6

When the superposition of two or more waves leads to zero amplitude because the waves are exactly out of phase

Question 7

Define constructive interference

Answer 7

When the superposition of two waves that are perfectly in phase - this causes the amplitudes to double.

Question 8

Define Phase

Answer 8

Phase is the position of a point in time on a waveform cycle

Question 9

Give the equation linking path difference and phase difference

Answer 9

ΔX=λ⋅Δϕ/2π

Here, ΔX is path difference, Δϕ is phase difference.

Question 10

Describe how standing waves are formed

Answer 10

Superposition/interference between waves travelling in opposite directions (from open end and wave reflected at closed end)

Waves have same frequency /wavelength

Nodes and antinodes are produced

At node the waves are in antiphase, so there is destructive interference

At the antinode they are in phase so there is constructive interference

At an antinode there is maximum amplitude

At a node there is zero amplitude

Question 11

Define Nodes and Antinodes

Answer 11

At node the waves are in antiphase, so there is destructive interference so there is zero amplitude

At the antinode they are in phase so there is constructive interference so there is maximum amplitude

Question 12

Define Amplitude

Answer 12

The maximum displacement of a wave from the position of equilibrium

Question 13

Define frequency

Answer 13

Number of oscillations per second

Question 14

Define Time Period

Answer 14

The time taken for one oscillation

Question 15

Define Wavelength

Answer 15

The distance between two successive points of maximum or minimum displacement

Question 16

Give the wave equation

Answer 16

v = fλ

Question 17

Define Refraction

Answer 17

Refraction is the bending of light as it moves from one medium to another

Question 18

Give the equation for refraction index

Answer 18

n1 sin θ1 = n2 sin θ2

Since the refractive index of air is approximately 1

n2 = sin θ1 / sin θ2

n = sin i / sin r = c/v

Question 19

Draw a ray digram for refraction

Answer 19

Question 20

Draw a wavefront diagram for refraction

Answer 20

Question 21

Wave Particle Duality Model Answer

Answer 21

Particle theory

Reference to E=hf or quanta of energy /packets of energy/photons

Increased f means more energy of photon

Release of electron requires minimum energy /work function

One photon releases one electron

Greater energy of photon means greater KE of electrons

More intense light means more photons, therefore more electrons

Wave theory

Wave energy depends on intensity

More intense light should give greater K.E of electrons

Energy is spread over the whole wave

If exposed for long enough photons eventually released, doesn’t happen.

Double Slit

The waves superpose Or diffraction at the double slits

Where they are in phase Or

when path difference is a whole number of wavelengths constructive interference takes place

Where they are in antiphase / when path difference is an odd number of half wavelengths destructive interference takes place

Bright bands are when waves are in phase / when path difference is nλ / constructive interference

Or reverse for dark bands

Question 22

Give the equation for the energy of the photon

Answer 22

E = hf

Question 23

Model Answer for Total Internal Reflection

Answer 23

. The direction of travel of light is denser to rarer medium

Angle of incidence is greater than the critical angle

light is reflected not refracted/no light emerges

Question 24

Define Critical Angle

Answer 24

The angle beyond which total internal reflection (of the light) occurs

Question 25

Give the equation used to find the critical angle

Answer 25

n1sinθ1 = n2sinθ2

Not light passes through therefore θ2 = 90 => sinθ2 = 1

n1sin θ1 = n2

sin θ1 = n2/n1

Since n2 is usually air/vacuum then n2 = 1 (dense to less dense so must be n2)

sinθ1 = 1/n1 in some cases

Question 26

Describe how to measure critical angle

Answer 26

Draw a vertical line

Place glass block perpendicular to line

Shine ray box directly up line

Rotate raybox until TIR occurs

Mark where the light enters the block

Mark position where light should exit block

Join the two positions

Measure angle between vertical line and the line just drawn

Question 27

Draw a Huygen’s Diagram

Answer 27

Question 28

Explain the double slit experiment

Answer 28

When in phase constructive interference/superposition occurs Or when path difference is nλ constructive interference/superposition occurs

When in antiphase destructive interference/superposition occurs Or when path difference is (n + ½ ) λ destructive interference/superposition occurs

Light band forms when in phase Or path difference is nλ Or constructive Or Dark band forms when in antiphase Or path difference is (n + ½ ) λ Or destructive

Question 29

Explain the photoelectric effect

Answer 29

Only one photon can transfer energy to a single electron Or a photon transfers all of its energy to a single electron

Photon energy depends on frequency Or photon energy = hf

Therefore photons must have a frequency greater than or equal to the minimum frequency (threshold frequency) in order to provide sufficient energy Or photon energy must be greater than work function

Question 30

Explain Threshold Frequency

Answer 30

Idea that one photon is absorbed by one electron

Photon energy given by E = hf Or photon energy increases with frequency

The idea that there is a minimum energy needed for emission of a (photo)electron (So) emission of electrons only occurs if frequency of light greater than the threshold frequency Or threshold frequency is the minimum frequency for the emission of (photo)electrons

Question 31

Define Work Function

Answer 31

The (minimum) energy required to remove one/an electron from the surface of the metal

Question 32

Give the equation involving work function

Answer 32

hf = Work Function + KE

Question 33

Explain why the photoelectric effect gives evidence for the particle theory of light

Answer 33

• Increasing the intensity (of light) increases the number of electrons emitted(per sec) Or number of electrons emitted(per sec) depends on the intensity (of light)
• One photon releases one electron
• Intensity determines number of photons

OR

• Increasing the intensity (of light) does not increase the energy/speed of the electrons
• One photon releases one electron
• Energy of photon determined by/depends on frequency (not intensity) Or E = hf

OR

• Below a certain frequency / threshold frequency no electrons emitted Or above a certain wavelength no electrons emitted
• Energy of photons increases with / depends on frequency Or E = hf
• Each photon needs a minimum amount of energy / work function Or One photon releases one electron

OR

• Electron emission starts at once (even for low intensity)
• One photon releases one electron
• Wave theory would allow energy to build up

OR

• Increasing the frequency (of light) increases the energy/speed of the electrons Or Increasing the frequency (of light) increases the stopping potential
• Energy of photon determined by/depends on frequency Or E = hf
• One photon releases one electron Or Wave theory would allow energy to build up