The Setup

I’m trying to focus a green laser in between two slits. The slit width is 0.2mm and gap between the slits is also 0.2mm. I measured the laser’s minimum spot size. It’s elliptical with major and minor axis are 0.1mm and 0.09mm respectively. This spot size suggests me that I should be able to focus the laser in between two slits. But when I tried to do so, I’m getting something like the following pictures (pictures are not actual, just drawn to explain) when I projected the light beam on a white paper. Pattern 1 appears when the slits are further away from the focusing lens and pattern 4 appears when the slits are closest to the lens. My understanding is that if the laser is focused on the gap between the slits, there should not be any green light on the projection screen. Can anyone please help me to understand why I’m getting these patterns?

enter image description here

  • $\begingroup$ When you say the gap between slits is 0.2 mm, do you mean the distance between the centerlines of the gaps, or the distance from the edge of one gap to the nearest edge of the next gap? How did you measure the beam diameter? There may be light outside of the 0.1 or 0.09 mm diameter that is causing some of the light to reach the projection screen. $\endgroup$ – JB2 Apr 22 at 12:30

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