Combining Waves

Question 1

What is a Stationary Wave

Answer 1

A wave which transfers no energy and whose positions of maximum and minimum amplitude are constant

Question 2

How are stationary waves produced

Answer 2

A stationary wave is produced from the superposition fo 2 progressive waves travelling in opposite directions

Question 3

What is Interference

Answer 3

The superposition of waves from two coherent sources of waves

Question 4

What are the conditions for a stationary wave to be produced

Answer 4

• The waves must be of the same frequency, wavelength and amplitude (cohernet)
• They must be travelling in opposite directions
• This normally happens when a wave is reflected back onto itself

Question 5

What does Coherent mean

Answer 5

When two waves have a fixed phase difference and same frequency

Question 6

What are the Different Types of Interference

Answer 6

• Constructive interference - when 2 waves have displacement in the same direction
• Destructive interference - when one wave has positive displacement and the other has negative displacement

Question 7

Describe Destructive and Constructive Interference Diagram

Answer 7

Question 8

What is Total Destructive Interference

Answer 8

If the waves have equal but opposite displacements so they cancel each other out

Question 9

Describe Young’s Double Slit Experiment

Answer 9

• Shine a coherent light source through 2 slits about the same size as the wavelength of the laser light so the light diffracts
• Each slit acts as a coherent point source making a pattern of light and dark fringes
• Light fringes are formed where the lights interfere constructively, this occurs where the path difference between waves is a whole number of wavelengths (nλ, where n is an integer)
• Dark fringes are formed where the light interferes destructively, this occurs where the path difference is a whole number and a half wavelengths ((n+½)λ)

Question 10

Describe Young’s Double Slit Experiment Diagram

Answer 10

Question 11

Describe what happens using White Light rather than Monochromatic in Young’s Double Slit

Answer 11

Gives wider maxima with a central white fringe with alternating bright fringes (violet is closest to the central maximum and red furthest)

Question 12

Explain what happens to the amplitude of two wave superposing when the frequency of them decreases

Answer 12

• If the frequency decreases then their wavelengths will decrease
• So the path difference will get closer and closer to one wavelength
• Therefore the ampltiude will get bigger as the waves become more in phase

Question 13

How would you increase the accurancy of finding the wavelength of Young’s Double Slit Experiment

Answer 13

• Increase the distance from the screen to the slits - when the distance is greater the fringe spacing is greater so the maximas are easier to read off
• Measure across 2 maximas and taking the mean - reduces percentage error

Question 14

What is the FringeSpacing Equation

Answer 14

w = λD/s

Fringe Space (m) = Wavelength(m) x Distance from slits to screen(m) / Slit Spacing (m)

Question 15

Describe the Safety Procuations of Using Lasers

Answer 15

• Wear ​laser safety goggles
• Don’t ​shine the laser at​ reflective surfaces
• Never shine the laser at a person

Question 16

Why do we use a lazer to show iterference and diffraction

Answer 16

Lazers produce monochromatic light so diffraction and interference patterns are more defined

Question 17

How was the knowledge and understanding of the nature of electromagnetic radiation changed over time

Answer 17

• Young’s double slit experiment showed that light can diffract and have interference
• These are wave properties so proved that​ electromagnetic radiation must act as a wave​ (at least some of the time).
• At the time some theories suggested light was formed of tiny particles, however this experiment disproved that theory

Question 18

Describe the Different Orders Diagram

Answer 18

Question 19

What is a Node and an Antinode

Answer 19

• Node - a point on a stationary wave where the displacement is 0
• Antinode - a point on a stationary wave with maximum displacement

Question 20

What is a Harmonic

Answer 20

A mode of vibration that is the multiple of the first harmonic

Question 21

Describe Three Lowest Harmonics in a Diagram

Answer 21

Question 22

Describe the Three Lowest Harmonics of a Stationary Wave

Answer 22

• First Harmonic- two nodes at either end and an antinode in the middle, wavelength is 2L
• Second Harmonic - three nodes and two antinodes, wavelength is L
• Third Harmonic - four nodes and three antinodes, wavelength is 2L/ 3

Question 23

What is the Fundamental Mode of a Stationary Wave

Answer 23

The Lowest Harmonic

Question 24

What is First Harmonic Equation

Answer 24

f = 1/2 x L x square root T/u

Frequency (Hz) = 1/ 2 x Length (m) x Tension (N) / Mass per unit length (kg m-1)

Question 25

What is Path difference

Answer 25

The difference in the distance travelled by two waves

Question 26

What is Diffraction

Answer 26

When waves spread around an obstacle or through a gap

Question 27

When is there Maximum Diffraction

Answer 27

When the gap is the same as the wavelength

Question 28

Describe the Single Slit Diffraction Graph

Answer 28

A ​bright central fringe​, which is double the width of all other fringes, with alternating dark and bright fringes on either side

Question 29

Describe how can you vary the width of the central maximum

Answer 29

• Increasing​ the ​slit width​ - ​decreases the amount of diffraction ​so the central maximum becomes ​narrower ​and its intensity ​increases​
• Increasing​ the light ​wavelength​ - ​increases the amount diffraction​ as the slit is closer in size to the light’s wavelength, so the central maximum becomes ​wider​ and its intensity ​decreases

Question 30

What is a Diffraction Grating

Answer 30

A slide containing many ​equally spaced slits

Question 31

Why would you use a diffraction grating

Answer 31

The interference pattern is much sharper and brighter ​than it would be in Young’s Double Slit Experiment this is because there are many more rays of light reinforcing the pattern

Question 32

Describe the Single Slit Diffraction Graph with 50 slits compared to 20 slits

Answer 32

Question 33

What is the Diffraction Grating Equation

Answer 33

d sin(𝛳) = n λ

Distance between Slits(m) x Sin (angle between 0th and n order maxima) = Order of the Maxima x Wavelength (m)

Question 34

Explain where the formual dsinθ = nλ is derived from

Answer 34

• For the first order maximum, the path difference between two adjacent rays of light is 1λ and the angle between the normal to the grating and the right ray is θ
• A right angled triangle is formed, with side lengths d and λ, the upper angle is θ
• For the first maxima sinθ = λ/d this rearranges to dsin = λ
• For other maxima the path difference between the two rays of light is nλ, where n is the order of maxima

Question 35

How would you calculate the distance between the slits using the number of lines per metre

Answer 35

You divide the number of lines per metre by 1 (for example if you know there are 300 lines per metre the distance between the slits would be 1/300 m)

Question 36

What are the Applications of Diffraction Grating

Answer 36

• Line Absorption Spectra - splits up light from stars and is used to identify elements present in stars
• X-ray Crystallography - crystal sheet acts as diffraction grating the X-ray passes through, used to find the spacing between atoms