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    Light with a wavelength 500nm is incident normally on a plane diffraction grating. Using a spectrometer telescope the first order spectrum on opposite sides of the normal A and B. are 88 degrees and 120 degrees respectively. Work out the 1) number of lines per mm on the grating
    2) max number of diffraction orders on one side of the normal

    i don't understand how do to it as there is 2 different angles which do I use ?!

    Welcome to TSR.

    It's possible here that either
    a) the question has been mistyped, or
    b) those are not the angles to the normal but the readings on the telescope scale. The scale is not zeroed at the normal, in other words.

    If b) is the case then you have 120 - 88 = 32 degrees between the two rays.
    Assuming they are equidistant either side of the normal then this is double the angle theta you want.
    So theta for the 1st order would be 16 degrees.

    If neither a) nor b) is the case then I'm puzzled.
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