Answer:
the thickness of the soap film is 127.82 nm or 130 nm
Explanation:
Given the data in the question;
n = 1.33
λ = 680 nm = 680 × 10⁻⁹ m
m = 1 and β = 0
When we see the red fringe, its a point of maximum reflection
hence, for interference with a thin soap film, we say;
2 × n × d × cos( β ) = ( m - 0.5) × λ
so we substitute in our given values;
2 × 1.33 × d × cos( 0 ) = ( 1 - 0.5) × ( 680 × 10⁻⁹ )
2.66 × cos( 0 ) × d = 0.5 × ( 680 × 10⁻⁹ )
2.66 × 1 × d = 3.4 × 10⁻⁷
d = ( 3.4 × 10⁻⁷ ) / 2.66
d = 127.82 × 10⁻⁹ m
d = 127.82 nm ≈ 130 nm
Therefore, the thickness of the soap film is 127.82 nm or 130 nm