# What if we replace the flat screen with a complete sphere? Double slit experiment with photons

I am trying to make this question a generic one. That's why it contains seemingly many questions.

Light source --> slit-plate --> spherical screen

Consider the classical double slit experiment with single photons. Assume that we replace the flat screen, which detects photons, with a complete sphere around the slit-plate and the light source. Also, coat the slit-plate with a special material to prevent reflection. In this scenario, what would we observe on different regions of the spherical detector, i.e. top, right etc? As an example, would the photons form an interference pattern behind the light source somehow?

Additionally, does the "vertical" dimension of the slits have any effect on the result in such configuration? What would we observe on the sphere when:

a. the slits are very long/tall compared to the wavelength, like this: " ||_|| "

b. the slits are very short, i.e. the same height as the wavelength, like this " .._.. " ? (Note that slits are open on top and bottom)

• Comments are not for extended discussion; this conversation has been moved to chat. Sep 20, 2021 at 4:46

Here is what happens...

The source emits photons in the direction of the plate. The plate will block most of the incident photons, but some will pass through the narrow slits.

When a wave passes through a narrow gap in a barrier it spreads out beyond the gap in a circular pattern (narrow here means that the width of the gap is of the same order of magnitude as the wavelength of the wave). Where you have two adjacent gaps, the spreading wave from one gap overlaps the spreading wave from the other. Where the peaks and troughs of the wave from one gap coincide with the peaks and troughs of the wave from the other, the peaks and the troughs become higher and deeper- where the peaks from one wave coincide with the troughs of the other, the waves cancel each other out.

With light waves or photons, this effect produces regularly spread dark and light areas. If the two waves from the two gaps hit a screen you can see the dark and light areas as 'fringes'.

If the screen is flat, the fringes will appear as alternate light and dark lines running parallel to the lengths of the slits and about the same length as the slits.

If you change the shape of the screen, the behaviour of the light itself doesn't change- in 3d space it continues to have light and dark areas as before, but the screen now cuts through the space in a different way, so the pattern of the interference fringes shown on the screen will change. If the screen has spherical curvature, you will still see the fringes as lines, but they will be curved and their spacing will change.

If you increase the height of the slits you will just increase the height of the fringes.

If you decrease the height of the slits, you will shorten the fringes, unless you decrease it so much that the height approaches the width of the slit. At that point, the light leaving the slit will spread out vertically as well as horizontally. Rather than seeing vertical interference fringes you will see a pattern of light and dark spots.

So, in summary, the spreading light from the two openings interferes with itself, regardless of whether any screens are present, producing regions of increased and reduced intensity in a regular pattern. Introducing a screen simply allows the pattern to be viewed- the shape and orientation of the screen with determine how it costs through the regions of light and dark, and thus determines how the patterns appear on it.

• Thanks a lot. The paragraph starting with "If you decrease the height of the slits..." cleared many points. But one thing is not clear to me. Is it possible that we see an interference pattern before the slits, i.e. on the half sphere containing the light source? In particular, does some interference pattern appear behind the 'light source' when the slits are quite short (assuming the light source does not block any photons)? Sep 19, 2021 at 20:38