10. (a) We note that ray 1 travels an extra distance 4Lmore than ray 2. To get the least possible L which will result in destructive interference, we set this extra distance equal to half of a wavelength: 420.0 nm452.50 nm288LLλλ=⇒===. (b) The next case occurs when that extra distance is set equal to 32λ. The result is 33(420.0 nm)157.5 nm88Lλ===.
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