A beam of light comprises two wavelengths is passed through a transmission diffraction grating. When viewed at an angle of 21.1° to the incident direction, the second order maximum for one wavelength is overlapped with the third order maximum for the other wavelength. The shorter wavelength is 424nm.
i) Calculate the longer wavelength. (3M)
ii) Determine the number of lines per metre in the diffraction grating. (2M)
iii.) Determine the angle(s) (other than 21.1°) at which overlapping occur. (2M)
For constructive interference, d*sinθ = mλ, m = 0, ±1, ±2...
For destructive interference, d*sinθ = (m + 1/2)λ, m = 0, ±1, ±2...
The Attempt at a Solution
I think my main problem here is not sure what "When viewed at an angle of 21.1° to the incident direction" means. How should I hand this, please?