out of phase,
resulting in destructive interference.
On the other hand, constructive interference may also occur.
The complete explanation of patterns observed in Young's experiment
depends on both diffraction and interference effects.
When light passes near the edge of an opaque object and then onto a screen, there appears to be some illumination in the area of the geometrical shadow. In other words, light fails to travel in straight lines when passing sharp edges. This phenomenon is called diffraction and occurs because of the wave nature of light.
There are basically two categories of diffraction effects. The first is Fraunhofer diffraction, which refers to diffraction produced when both the light source and screen are an infinite distance from the given obstacle. Fresnel diffraction is the second type and refers to diffraction produced when either the source or screen or both are at finite distances from the obstacle. We can observe Fraunhofer diffraction experimentally by using a collimated light source and placing the viewing screen at the focal plane of a double convex lens located behind the obstacle.
Since light does not travel in straight lines near a sharp lines,
there are many different path lengths from the obstacle to the screen.
If there is sufficient difference in path length,
two waves reaching the same point may be 180 out of phase,
resulting in destructive interference.
On the other hand, constructive interference may also occur.
The complete explanation of patterns observed in Young's experiment
depends on both diffraction and interference effects.
When we use the incandescent source in studying diffraction, we get an interference pattern, but since the interference patterns depend on the wavelength of the light and since white light contains light of various wavelengths, the patterns are not sharp. The coherence and monochromaticity of laser light makes it ideal for this experiment.