An air wedge is formed between two glass plates in contact along one edge and slightly separated at the opposite edge.

When the plates are illuminated with monochromatic light from above, the reflected light has 85 dark fringes.

Calculate the number of dark fringes that appear if water (n=1.33) replaces the air between the plates.

m = 113.05

Explanation:

given,

number of dark fringe, m = 85

refractive index of air, n = 1

refractive index of water, n = 1.33

The condition for the destructive interference in the wedgeshaped film is

2 n t = m λ

for air refractive index, n = 1

2 t = 85 λ ... (1)

For water n = 1.33

2 t x 1.33 = m λ

now, putting value of 2 t from equation (1)

85 λ x 1.33 = m λ

m = 1.33 x 85

m = 113.05

Hence, 113 dark fringes are observed when water is replaced by air between the glass plates.