# Distance between interference and diffraction fringes

In problems related to Interference and Diffraction fringes - I often have encountered phrases like "bright fringes 1.5 mm apart" or "distance between first bright fringe and 4th dark fringe". Fringe is some sort of rectangle not a point. we can talk about distance between two points not distance between two say, bands.

I think both interference and diffraction fringes are equal width - determined by wavelength, distance between slits and screen and distance between slits or width of one slit.

in one text book - it is mentioned that in diffraction fringe widths are not same where it is same in interference. Is it right?

• Don't fret too much about all this: as in BMS's answer language is really loose and sloppy with this kind of thing. The only sure fire way to specify fringe spacings and so forth is by a clear diagram or a full description in equations. There are some precise standards to defining fringes in the ISO14999 and ISO10110 standards, but these are only ever talked about in optics manufacturing and you won't find them in physics textbooks, so it would be pointless learning about them. – Selene Routley Feb 5 '14 at 13:12

Fringe is some sort of rectangle not a point. we can talk about distance between two points not distance between two say, bands.

I'm not sure what your question is. However, your statements are examples of loose but commonplace language. It would be more accurate to say the shortest distance between adjacent bands, or the spacing between bands.

I think both interference and diffraction fringes are equal width - determined by wavelength, distance between slits and screen and distance between slits or width of one slit.

The geometry of the fringes are determined by all of the variables you've determined. That doesn't mean that the expressions have to be equal. For example, the two expressions $at$ and $at/2$ both depend on $a$ and $t$, but they don't yield the same result.

in one text book - it is mentioned that in diffraction fringe widths are not same where it is same in interference. Is it right?

If click this Google image search, you'll see the closely spaced interference fringes and the wider spaced diffraction envelope. Using a ruler or your fingers, you may be able to answer this question by directly measuring.