Two narrow, parallel slits are illuminated with yellow light ( = 578 nm) produced by a copper laser and light of an unknown wavelength ’ produced by another laser.

Question

Two narrow, parallel slits are illuminated with yellow light (λ = 578 nm) produced by a copper laser and light of an unknown wavelength λ’ produced by another laser. Find λ’ knowing that the 6 th maximum for λ is superimposed on the 6th minimum forλ’.

Answer 1

Given

λ = 578 nm

λ' = ?

Distance of 6th Bright fringe from center = 6 λD/d

Distance of 6th dark fringe from center =5.5 λ' D/d

Where D = Distance of screen from plane of two slits

d = distance between two parallel slits

As the two have same location

6λ = 5.5λ'

λ' = (6/5) λ

where λ= 578 nm

λ' = 694 nm