A diffraction grating has 500 lines per mm. Its slit spacing or grating element will be equal to ____________________?

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A diffraction grating has 500 lines per mm. Its slit spacing or grating element will be equal to ____________________?

A. 500 mm
B. 5 x 10-3 mm
C. 2 x 10-5 mm
D. 2 x 10-3 mm