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I get the creation of alternate dark and bright fringes in the double slit light interference experiment. Where I am confused is there will be points on the screen where condition for constructive as well as destructive interference is not met. Example where path difference = 1/3 (wavelength). On those points there should be some light? how come we see only alternate dark and bright fringes and nothing in between?

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    $\begingroup$ The light doesn't oscillate on and off discretely - the intensity varies in a sinusoidal-like way. Unfortunately, our eyes aren't very good at seeing very quantitatively this continuous variation in intensity. $\endgroup$ – Garf Sep 15 '18 at 15:49
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It's because of the way your eyes work.

The intermediate areas are actually intermediate. The boundary between the light and dark areas is not a sharp line. But your eye doesn't pick up on that.

What you see is intensity. Leaving out the details, intensity is proportional to the sine squared of something-or-other. (It would be simpler if you projected the light onto the inside of a cylinder instead of a flat surface.)

If you look at the graph of sine-squared, it's exactly like the graph of a sine function except the frequency is doubled and the range is 0 to 1 instead of -1 to 1.

So the places where the light is maximum or minimum change slowesr, and the places in between change fastest.

So the in-between areas will look thin. Byt they're still there and you can see them if you watch for them. Your eye tends to de-emphasize threm.

intensity

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