Solving a problem in physics section 55 point 64 optics

Solving a problem in physics section 55 point 64 optics

64. In the diffraction grating normally incident beam of light from the discharge tube filled with helium. On what line λ2 in the spectrum of the third order is imposed red line of helium (λ1 = 670 nm) of the spectrum of the second order?

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Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)