Light from two lasers is incident on an opaque barrier with a single slit of width 4.0 x 10-5 m. One laser emits light of wavelength 4.0 x 10-7 m and the other is 6.0 x 10-7 m. A screen to view the light intensity pattern is 2.0 m behind the barrier. What is the distance from the center of the pattern to the nearest completely dark spot (dark for both colors)?

y = 6 10⁻² m

Explanation:

This is a diffraction exercise which is described by the expression

          a sin θ = m λ

we can use trigonometry to find the distance from the inside of the screen to the dark point (y)

          tan θ = y / L

angles are very small in diffraction experiments, so we can approximate

          tan θ = sin θ /cos θ = sin θ

          sin θ = y / L

substituting

         a (y / L) = m λ

Let's find the points where the intensity becomes zero

         y = m L λ / a

λ₁ = 4.0 10⁻⁷ m

m = 1

          y = 1 2.0 4.0 10⁻⁷/ 4.0 10⁻⁵

          y = 2 10⁻² m

m = 2

          y = 4 10⁻² m

λ₂ = 6.0 10⁻⁷ m

m = 1

          y = 1 2.0 6.0 10⁻⁷ / 4.0 10⁻⁵

          y = 3 10⁻² m

m = 2

          y = 6 10⁻² m

if we want a point where the two colors are dark, we set the two expressions equal

          y₁ = y₂

          m₁ L  λ₁ / a = m₂ L  λ₂ / a

          m₁/m₂ =  λ₂/λ₁

          m₁ / m₂ = 6 10⁻⁷ / 4 10⁻⁷

          m₁ / m₂ = 1.5

since the quantities m must be integers, the smallest relation that the relation fulfills is

            m₁ = 3

            m₂ = 2

the distance for this destructive interference is

           y = 3 2 4.0 10⁻⁷ / 4.0 10⁻⁵

           y = 6 10⁻² m

this is the first point where the minimum of the two wavelengths coincide

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