Diffraction Calculator

Including which fringes cannot exist at all.

Including which fringes cannot exist at all. Diffraction angles and fringe spacing from a slit or grating, plus the Rayleigh resolution limit for an aperture.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Wavelength

Slit separation

µm

Fringe order

Distance to screen

Telescope or lens aperture

Angle to the order-1 fringe

14.478°

Orders up to 4 are visible with this geometry

Diffraction angle14.4775°

Highest visible order4

Fringe spacing on the screen250 mm

Rayleigh resolution limit6.1000 × 10^-6 rad

In arcseconds1.258″

Constructive interference happens where the path difference is a whole number of wavelengths: d sin θ = mλ. Because the sine cannot exceed 1, there is a hard limit on how many orders exist — beyond it the fringe simply is not there, and a formula applied blindly will return an impossible angle for it. The same physics sets the resolution of every optical instrument. The Rayleigh criterion, 1.22λ/D, says two points are just resolvable when the first dark ring of one falls on the centre of the other — which is why bigger telescopes see finer detail, and why they are built large for resolution as much as for light. Diffraction is also why stopping a camera lens down past about f/11 starts to soften the image. The aperture becomes small enough that the diffraction blur exceeds what the extra depth of field gains.

How the Diffraction Calculator works

Diffraction angles and fringe spacing from a slit or grating, plus the Rayleigh resolution limit for an aperture. The maximum order is reported because beyond it the fringe genuinely does not exist.

Also known as: double slit fringe spacing · diffraction grating angle · telescope resolving power · why does f22 look soft

Where to go next

The Diffraction question rarely arrives on its own. These are the ones that usually come with it:

Wavelength & Frequency Calculator — With the photon energy that makes the spectrum mean something.
Thin Lens Calculator — A negative image distance is a magnifying glass, not an error.
Critical Angle Calculator — Only exists going from dense to less dense.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.

Sources

Frequently asked questions

What is the diffraction formula?

d sin θ = mλ. Constructive interference happens where the path difference between adjacent slits is a whole number of wavelengths.

Why is there a maximum order?

Because sine cannot exceed 1. Once mλ exceeds the slit spacing there is no angle that satisfies the equation, and that fringe simply is not there — a formula applied blindly will return an impossible angle for it.

What is the Rayleigh criterion?

1.22λ/D — the angular separation at which two point sources are just resolvable through an aperture of diameter D. It is why bigger telescopes see finer detail as well as fainter objects.

Why does stopping down a camera lens soften the image?

Diffraction. Past about f/11 on a small sensor, the blur from the aperture exceeds what the extra depth of field gains, so overall sharpness falls.

How does a diffraction grating split light?

The angle depends on wavelength, so each colour emerges in a different direction. Gratings separate light far more strongly than prisms, which is why spectroscopes use them.

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