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Diffraction principle of light
The principle of light diffraction phenomenon is as follows:

The phenomenon that light waves will deviate from the propagation law of geometric optics when encountering obstacles. Geometrical optics shows that light propagates in a uniform medium according to the law of straight line, while light propagates at the interface between two media according to the laws of reflection and refraction.

However, light is electromagnetic wave. When a beam of light passes through a perforated fence, its intensity can spread to the geometric shadow area defined by the linear propagation law, which also makes some dark spots or dark lines appear in the geometric illumination area.

Diffraction effect makes the light intensity distribution in the space behind obstacles different from that given by geometrical optics and that when light waves propagate freely. Diffraction intensity redistributes, and diffraction makes all geometric shadow circles lose the edge of Ming Rui.

Brief introduction of slit diffraction;

Monochromatic light emitted by the laser shines on the slit, which appears on the light screen when the slit is gradually narrowed from very wide.

When the slit is very wide, the width of the slit is much larger than the wavelength of light, and the diffraction phenomenon is extremely inconspicuous. Light travels in a straight line, producing bright lines equivalent to the width of the slit on the screen.

However, when the width of the slit is adjusted to be very narrow, which can be compared with light waves, the light obviously deviates from the straight-line propagation direction after passing through the slit, and irradiates a quite wide place on the screen, and diffraction stripes alternating light and dark appear. The smaller the slit, the larger the diffraction range and the wider the diffraction fringe. But the brightness is getting darker and darker.

Test: You can adjust the vernier caliper to the minimum distance that can be recognized by naked eyes, and then you can see the light source through this slit.