Interference

Question 1

what happens when 2 waves of the same wavelength combine

what happens when 2 waves out of step add up

Answer 1

add up, yielding a wave twice the amplitude

cancel each other out

Question 2

what is the most important optometric application of interence

Answer 2

anti-reflective coating placed on both surfaces of the lens to reduce reflections

Question 3

what are modern A/R coatings

Answer 3

mulit-layer for maximum suppression of reflections

Question 4

what is a design wavelength

Answer 4

a single layer coating is maximally effective only at one wavelenth

Question 5

what is a wavefront

Answer 5

surface of constant phase

-imagine a point source emitting light waves that travel in all directions

Question 6

how does a wavefront from a point source look

Answer 6

spherical

as we get farther from source and look at a small area of the sphere then the wavefront is essentially planar

Question 7

how can a plane wavefront be created

Answer 7

by putting the light through a collimating lens

Question 8

when will the next bright band occur in young’s double slit after the midline

Answer 8

where the path difference btwn the rays is a whole wavelength

Question 9

where are the dark bands located in regards to the bright band to youngs double slit

Answer 9

dark bands always midway btwn bright bands

Question 10

why does a thin layer of any material have the capability of producing interference

Answer 10

bc it divides incidnet light into multiple reflected and transmitted rays

Question 11

will better cancellation result from reflected or transmitted rays

Answer 11

reflected rays

Question 12

where are observations made in interfernece in a wedge

Answer 12

in reflected light bc to obtain good cancellation for destructive interference the 2 waves have to have nearly equal amplitudes
-easier when bands are very dark

Question 13

where is interference observed in a wedge

Answer 13

btwn the rays that are refelcted back form the surfaces that form the wedge

Question 14

at the line where the 2 plates are in contact the path difference is ____ but there is a dark band ther ebc…

Answer 14

zero

bc one of the rays undergo a 1/2 wavelength phase shift and the other does not, putting them out of phase=dark band

Question 15

why are the dark bands straight lines in an air wege

Answer 15

bc the plates are flat and the path diff btwn the rays is the same all the way across the plates

Question 16

in newtons rings, the central spot is dark and the first bright ring is white, why

Answer 16

bc the small path diff brings most of the wavelenths into register at the same time -after that diff wavelengths are in constructive interference at diff places resulting in a sequence of colored rings

Question 17

why are soap bubbles diff colors randomly

Question 18

how do anti-reflective coatings work

Answer 18

reflections are suppressed by adding a thin transparent layer to the lens surfaces, causing destructive interference btwn the light rays reflected from the 2 surfcaes of this thin layer

Question 19

how does A/R layer thickness work

Answer 19

the thickness must be such that the rays reflected from the 2 side of the anti-reflective layer are 1/2 wavelength out of phase

Question 20

why must the thickness of the A/R layer be 1/4 wavelength

Answer 20

bc one ray makes 2 trips through the A/R layer