# Why doesn't a double slit act like a single slit?

Well, a single slit can be considered a continuous array of sources, and thus its spectrum is different than that of a double slit. But why is a double slit so different from a single slit if it is just 2 single slits. Also, if the waves coming from the slits of the double-slit setup act like the wave from a single slit how can they form different patterns?

The double slit is missing the waves coming from the blockage between the slits. So yes, it's an array of sources, but it's not the same array.

• Hi, Thanks for the response. Well, I get your point. Kindly clarify why is it that we see secondary waves impacting the pattern in the single-slit experiments but these secondary waves originating from the primary waves aren't even talked about in the double-slit experiments. Commented Dec 21, 2022 at 16:13
• @UpasnaSingh I'm not sure what you mean by secondary waves. Perhaps you're referring to the fact that textbook explanations of single slit diffraction assume a slit wide compared to the wavelength, while for the double slit the assumption is a slit that's narrow compared to the wavelength. The wider the slit, the more wiggles the diffraction pattern will have. Commented Dec 21, 2022 at 16:17
• well by secondary waves I mean "Huygen's principle states every point on the current wavefront acts as a source of secondary spherical waves. These secondary waves propagate outwards, in the forward direction, and a common tangent (an envelope) to all these waves constitute the new wavefront." something like this. Basically, when we are explained about single slits then these secondary waves are always mentioned but this effect of secondary waves is never factored in the double slit experiment. Commented Dec 22, 2022 at 15:30
• @UpasnaSingh See my comment above about the slit width in textbook presentations. Commented Dec 22, 2022 at 15:41
• so that means the difference in the single slit and double slit is all due to the slit width? Commented Dec 22, 2022 at 16:32

...a single slit can be considered a continuous array of sources.

In the context of Young's experiment, the word "slit" implies an extremely narrow opening. Ideally, the same as, or less than the wavelength of the light. If the width of the slit were many times the wavelength, then it would indeed behave like a "continuous array," and most of the light would pass through it as plane waves, forming a narrow pencil beam. Only a tiny fraction of the light would be diffracted at the edges of the slit. As you make the slit narrower, the more significant the diffraction at the edges becomes, which is what you want.

See this page, http://labman.phys.utk.edu/phys136core/modules/m9/diffraction.html, and search down for pictures of "Water waves in a ripple tank."

...thus its spectrum is different than that of a double slit.

When we talk about "spectrum" we usually are talking about a continuous range of frequencies and we are measuring the amount of energy present in each arbitrarily small "band" of those frequencies. But Young's experiment can only be realized by using a single frequency of light, and the slits do not change the frequency.

• „Only a tiny fraction of the light would be diffracted at the edges of the slit.“ It follows that a slit is not necessary for the deflection of photons at the edges, a single edge is sufficient. And it will work fine with single emitted photons. I have endless doubts about the interaction with itself. It would be better to study the edge-photon interaction. For example, quantised phononic phenomena and a resulting intensity distribution on the screen from quantised deflections. Commented Dec 22, 2022 at 5:32

The main reason is interference.

When considering the single-slit and double-slit experiments, it is useful to consider its analogy to water (or sound). Imagine you have water waves incident on a single gap in a wall. You would see a source of waves emanating from the gap. With two gaps in the wall, you now have two sources, which will interact. The pattern you see from a double-slit experiment, or in water waves as the water passes through two gaps that are close to each other, results from constructive and destructive interference. That is to say that the waves will collide with one another and in some places, both waves will have positive amplitude, creating a resultant wave with positive amplitude, and vice-versa. In some places, they cancel out exactly and you have zero amplitude. These are the gaps you see in a double-slit experiment. Check out this graphic for a visual. There are also some fairly simple mathematics to describe this behaviour.

You can also see this with sound, when you have two speakers playing the same frequencies at the same time. You can walk around the room and hear the volume change.