2.4 Superposition, Interference and Diffraction

Question 1

What is the principle of superposition?

Answer 1

When waves interfere, the resultant displacement of the medium at any point is the vector sum of the displacements that each wave would cause at that point in time

Question 2

Define constructive interference

Answer 2

when the crests and troughs of the waves overlap in phase, producing a wave of greater amplitude

Question 3

Define destructive interference

Answer 3

when the crest of one wave coincides with the trough of the other, the displacements are in opposite directions

Question 4

Define a standing wave

Answer 4

A wave produced by the interference if two waves, of the same type, wavelength, amplitude and speed, but moving in opposite directions

Question 5

What apparatus can be used to show standing waves

Answer 5

Melde’s apparatus

Question 6

How does Melde’s apparatus work?

Answer 6

A signal generator is used to generate vibrations up and down w small amplitude

• waves travel along the string
• when the waves meet at pulley end of string they are reflected back, creating 2 similar waves travelling in opposite directions

Question 7

Define a node

Answer 7

a point along a standing wave at which the resultant displacement is always zero

Question 8

Define an antinode

Answer 8

midway between two nodal points where the amplitude of vibration is a maximum

Question 9

Define coherence

Answer 9

Coherence means that there is a constant phase difference between the waves

Question 10

Define coherent light

Answer 10

light waves that are in constant phase

Question 11

eg. of a source of coherent light

Answer 11

laser light

Question 12

Define incoherent light

Answer 12

random stream of energy bursts

Question 13

eg. of source of incoherent light

Answer 13

incandescent sources eg. hot filament

Question 14

define diffraction

Answer 14

the spreading out of waves as they pass thru a small aperture

Question 15

As the size of the gap increases, diffraction decreases/increases

Answer 15

decreases

Question 16

For the same slit width, does blue or red light have a narrower diffraction pattern, and why?

Answer 16

Blue light has a narrower diffraction patter because it has a smaller wavelength

Question 17

As the width of the slit thru which diffraction takes place decreases, what 2 effect are observed?

Answer 17

width of diffraction pattern increases ( fewer maxima observed)
heights of the maxima decrease (less energy arrives on the screen)

Question 18

Compared to subsidiary maxima, central maximum is (2)

Answer 18

• many times more intense

- twice the width

Question 19

Name three precautions for laser usage

Answer 19

• never look directly into ANY laser beam
• pin a notice to lab door ‘LASER IN USE: DO NOT ENTER’
• remove any unnecessary shiny, reflective surfaces ( so laser light not reflect into eye)

Question 20

In terms of energy, describe the difference between a stationary wave and a progressive wave

Answer 20

In a progressive wave there is a flow of energy in the direction of propagation; no point is permanently at rest.
In a stationary wave, there is no flow of energy; certain pts called nodes are permanently at rest

Question 21

What are the necessary conditions to produce a standing wave?

Answer 21

Stationary waves are produced by the superposition of two waves, of the same type and having the same amplitude, wavelength and speed but moving in opposite directions