Waves-interference and diffraction

Question 1

What is constructive interference ?

Answer 1

When 2 waves which are in phase superpose.

Question 2

What is destructive interference ?

Answer 2

When 2 waves which are in antiphase superpose.

Question 3

How are stationary waves produced ?

Answer 3

When 2 progressive waves, that are travelling in opposite directions, with same frequency similar amplitude and constant phase difference superpose

Question 4

On a vibrating string where are the nodes and anti-nodes ?

Answer 4

Nodes are at the points with 0 amplitude.
Anti-nodes are the points with maximum displacement.

Question 5

What is the equation for the first harmonic and what does each letter represent ?

Answer 5

f = 1/2l x (T/μ)^0.5

f - frequency
l - length of string
T - tension is string
μ - mass per unit of length of string

Question 6

If the fundamental frequency of a wire is 120hz what is the frequency for the 2nd 3rd and 5th harmonic ?

Answer 6

2nd = 240
3rd = 360
5th = 600

Question 7

What are the conditions for coherent sources ?

Answer 7

Constant phase difference, same wavelength and frequency and similar amplitude.

Question 8

What is path difference ?

Answer 8

difference is distance traveled by one wave compared to another.

Question 9

What is diffraction ?

Answer 9

When waves spread out after passing through a opening or around a obstacle.

Question 10

Why does light not diffract through a doorway.

Answer 10

Difference between width of door and wavelength of light is very big, the smaller the difference between slit width and wavelength the greater the diffraction.

Question 11

Where do maxima and minima form on a single slit graph

Answer 11

maxima occur where the path is nλ
minima occur where the path is nλ + 0.5

Question 12

What is the formula for fringe spacing and what do the letters stand for ?

Answer 12

w = Dλ/s
w - fringe spacing
D - distance to screen
s - width of slit

Question 13

Compare the single slit diffraction graph of violet light and red light.

Answer 13

Violet light relative intensity would be greater but fringe spacing would be less as the energy is spread over a smaller area. vice versa for red.

Question 14

what is the width of the maxima excluding central maxima ?

Answer 14

W/2

Question 15

What is the relationship between maxima and intensity

Answer 15

exponential decay from central maxima.

Question 16

How does double slit interference work ?

Answer 16

The two waves superpose as they pass through each other and maxima and minima form where the path difference is a multiple of λ and minima at nλ + 0.5.

Question 17

What is the formula for a diffraction grating ?

Answer 17

dsinθ = nλ

d - separation of slits
n - order of the fringe

Question 18

Explain how a minima occurs using a diffraction grating.

Answer 18

A ray has a path distance of 1 and another slit has a path of 2.5 so the path difference is n + 0.5 so they cancel, this happens for each of the slits in the diffraction grating.