如何計算繞射出來的波的角度?

The angular spread of the diffraction wave for a circular aperture is given by the relation,
sin(a) = 1.22w/D
where a is the angular half-width of the diffracted spot
w is the wavelength of the wave
D is the diameter of the circular aperture
If the wave passes through a slit of width L, the spread of the wave is given by
sin(a) = w/L
You could refer to the following web-page for detailed mathematical derivation of these formulae
http://www.mellesgriot.com/products/optics/fo_3_1.htm
Example, if yellow light of wavelength 589 nm passes through a circular aperture of 1 mm (=1000 nm) diameter, then the beam of light will be diffracted by
sin(a) = 1.22 x 589/1000
i.e. a = 46 degrees
Thus, the angular spread of the wave is 2x46 defrees = 92 degrees.