關於light wave 既diffraction問題

The alternate bright and dark fringes are due to interference of light.

This only occurs when the slit width is larger than the wavelength of light. Under such situation, there are a number light rays entering into the slit at any one time. Because of diffraction effect, these light rays are brought to interfere with one another, producing alternate bright and dark interference fringes.

If the slit is narrow enough (equal to or smaller than the wavelength of light), there will not be any interference pattern observed. Only a wide spread of light intensity is seen, as a result of diffraction effect only..