Question

With the laser, the diffracted images on the wall and the light source were on opposite sides of the grating. However, for the hydrogen and sodium lights, the light sources and the diffracted images were on the same side of the grating. Do you see any contradiction?

Answer 1

The laser may have been shining a beam intense enough to show what the "light sources" - discharge tubes ? - couldn't show 'cos they were too weak, ie not enough photons flying around.