physics 神手...有問題關於double slit同

The position of bright fringes formed by a diffraction grating is,

a.sin(theta) = m入
where a is the line separation,
(theta) is the angular position of a spectral line
m is an integer (the order of bright fringe)
入 is the wavelength of light used.

When (theta) is small, the equation reduces to,
a.(theta) = m入
or, (theta) = m入/a
Thus, the angular position of the fringes is proportion to the order m. The fringes are evenly separated.

But when the angle (theta) is large, m is proportional to sin(theta), thus the fringes are not evenly separated.

That is to say, fringes close to the central bright fringes are more or less evenly separated, but those far away are not.

The same phenomenon also applies to bright fringes formed by ouble slits.