Question

Combinations of Thin Films (MC) Due in 2 hours, 18 minutes The diagram shows light incident from above on a film of thickness d. Each of the three materials in the figure can be chosen to be air, with index of refraction n-1.00, water, with index n-1.33, or glass, with index n-1.50. Under which of the following conditions will the reflected light be completely or nearly eliminated by interference? (lambda2 refers to the wavelength of the light inside the thin film.) (Note: In the case that d<<lambda2, you can assume the thickness is so small that the travel distance in material 2 has negligible affect on the phase of the wave. Treat this as if the two interfaces are almost right on top of each other.) 1 2 3 d<<lambda2, material 1 is air, material 2 is water, material 3 is glass. d lambda2/4, material 1 is air, 2 is water, 3 is glass. d lambda2/4, material 1 is water, 2 is glass, 3 is air. d-lambda2/4, material 1 is glass, 2 is air, 3 is water. d lambdaz/2, material 1 is air, 2 is water, 3 is glass. Submit Answer Tries 0/10

Answer 1

We know that when a ray of light Travels from Rarer to Denser medium there is a phase change of pie...Keeping this point in our mind we will proceed...

There are Two Cases when Reflected Light is Completely eliminated For Normal Incidence.. Of light

1) when 2ND =n×lamda when there is a phase Change of pie between two Rays reflected from two boundaries of Thin Film

2) when 2ND=(2n+1) lamda/2 when there is a phase change of 2pie

where N =Refractive index of Thin Film,

D= Thicknes of Thin film,

Lamda = wavelength of light used

n = order of fringe

In Gives Options Only option (B) is correct because it has phase change of 2pie reason when Light is traveling from Air (Rarer)to water (Denser) medium it has a phase change of pie in reflected Ray.on upper Boundary and When a Refracted ray Of light Travel and incident on Water Glass boundary (LOWER BOUNDARY OF THIN FILM) then It will again beacome a case of Light Traveling Rarer to denser medium because refractive index of water (RARER) is less than Refractive index of Glass(DENSER) ... For formula 2ND = (2n+1) lamda/2 for n=0 and N= 1.33 we will get D= lamda /4N which in turn approximately equal To D=lamda/4

For 5th option to be correct There should be phase change of pie but in option We are getting 2pie so it is also not correct..

HENCE only Option( B) is correct