Interference

Question 1

Two waves are coherent if they have a

Answer 1

constant phase relationship

Question 2

the conditions of constructive interference

Answer 2

two waves meet crest to crest or trough to trough

Question 3

the conditions of destructive interference

Answer 3

two waves meet crest to trough or trough to crest

Question 4

that a wave DOES NOT experience a phase change when it is travelling in a

Answer 4

more dense medium and reflects from an interface with a less dense medium

Question 5

A coated (bloomed) lens can be made

Answer 5

non reflective for a specific wavelength of light

Question 6

A coated (bloomed) lens can be made

Answer 6

non reflective for a specific wavelength of light

Question 7

Thin film interference

Answer 7

liquids which are less dense than water form a thin film on the water surface. A ray of light is incident on a film, the light will be partially reflected from the surface with a change of phase of π since the ray is travelling into a medium with a higher refractive index. The ray then passing through the film is then partially reflected. There is no change of phase for this ray since the ray is travelling into a medium with a lower refractive index. The two rays combine to produce interference.

Question 8

Wedge interference

Answer 8

Interference by division of amplitude can be produced by light falling on two plates of glass separated by an air gap

Question 9

opd = mλ or (m + 1/2)λ where m = 0,1,2…..

Answer 9

mλ a maximum occurs

(m + 1/2)λ a minimum occurs, where n is an integer

Question 10

Derivation of d = λ/4n

for glass lenses with a coating such as magnesium flouride

Answer 10

there is a phase change of πwhen both rays are reflected
the coating has a thickness of d
optical path difference = 1/2λ for destructive interference
optical path coating = 2nd
thus
1/2λ = 2nd
d = λ/4n

Question 11

Derivation of d = λ/4n

for glass lenses with a coating such as magnesium fluoride

Answer 11

there is a phase change of πwhen both rays are reflected
the coating has a thickness of d
optical path difference = 1/2λ for destructive interference
optical path coating = 2nd
thus
1/2λ = 2nd
d = λ/4n

Question 12

equations for interference of waves by division of amplitude

Δx = λl/2d

d = λ/4n

Answer 12

Δx = width of fringes (m)
λ = wavelength (m)
l = length of wedge (m)
d = height of open end of wedge (m)

d = thickness of lens coating (m)
n = refractive index 
λ = wavelength (m)

Question 13

Young’s slits interference

Explanation of Division of Wavefront

Answer 13

A single source of light is passed through a single then double slit. The double slit produces two sources of coherent waves which produce interference fringes that can be observed on a screen.

Question 14

Why would you take 10 measurements over 1 of a fringe separation

Answer 14

to reduce the value of the uncertainty

so the uncertainty is smaller

Question 15

How does an increase in temperature effect a thin wedge

Answer 15

Δx = λl/2d

as the temperature increases the material expands.
thus the diameter increases as λ and l stay the same

Question 16

In Young’s slit interference how would the fringe pattern change when the wavelength has increased

Answer 16

Δx = λD/d

if λ decreases
and the distance between the screen and slit is unchanged
and the distance between the slits is also unchanged
then the fringe separations must increase

Question 17

Young’s slit interference formula

Answer 17

Δx = λD/d

Δx = fringe separations (m)
λ = wavelength (m
D = distance between the screen and slit (m)
d = distance between the slits

Question 18

how are TWO-DIMENSIONAL patterns of bright spots produced

Answer 18

Pattern produced by interference.

Slits horizontal and vertical or at right angles

Question 19

in Young’s slit modulus the screen is now moved further away from the slits. Explain why this is the most effective way of reducing the uncertainty in the calculated value of the wavelength

Answer 19

Increasing the slit–screen distance spreads out the fringes, reducing the uncertainty in the fringe separation

Question 20

The number of fringe separations is measured by

Answer 20

the gaps NOT the lines.

Question 21

wavelength in the middle of the spectrum is

Answer 21

not reflected

only blues and reds.