Effect of Source Size on Diffraction Pattern Visibility
A non-point light source emitting spatially incoherent waves affects the visibility (sharpness) of the diffraction pattern.
Since individual points of an extended light source emit spatially incoherent light, the diffraction patterns from these points do not interfere with each other — their intensities simply add up.
This superposition of slightly shifted diffraction patterns results in a gradual blurring of the overall pattern as the size of the light source increases.
Experimental apparatus: Mercury spectral lamp with ballast, adjustable slit, spectral filter (optional), double-slit slide, screen (implemented using a DSLR without lens).
Detail of the apparatus.
Diffraction patterns in monochromatic light
Diffraction pattern in monochromatic light. Wider slit.
Diffraction pattern in monochromatic light. Even wider slit.
Diffraction pattern in monochromatic light. Widest slit.
Diffraction pattern in monochromatic light.
Diffraction patterns in full lamp light – unfiltered by spectral filter. Shorter wavelengths (e.g., violet) bend less than longer wavelengths (e.g., green).
Diffraction pattern in “white” light. Narrow slit.
Diffraction pattern in “white” light. Wider slit.
Diffraction pattern in “white” light. Even wider slit.