Why can waves bypass obstacles?

This is the diffraction definition of light: the phenomenon that light bypasses obstacles and continues to spread forward.

Include single slit diffraction, circular hole diffraction, circular plate diffraction and Poisson bright spot.

When light encounters obstacles or small holes (narrow slits) in the propagation process, it will break away from the straight path and bypass the shadow of obstacles. This phenomenon is called diffraction of light. The bright and dark stripes or rings produced by diffraction are called diffraction patterns.

The condition of diffraction is: because the wavelength of light is very short, only a few tenths of a micron, usually the object is much larger than it, but when the light touches the pinhole, slit or filament, the diffraction of light can be clearly seen. It is better to irradiate with monochromatic light. If illuminated with polychromatic light, the diffraction pattern is colored.

diffraction of light

1. diffraction phenomenon

The phenomenon that light bypasses obstacles and deviates from the straight propagation path into the shadow area is called light diffraction.

The diffraction of light, like the interference of light, proves that light fluctuates.

2. Conditions for obvious diffraction of light

When the size of small holes or obstacles is smaller than the wavelength of light waves, or close to the wavelength, light can be obviously diffracted. Because the wavelength range of visible light is 4× 10-7m to 7.7× 10-7m, obvious diffraction phenomenon of light is rarely seen in daily life.

Any obstacle can diffract light, but the conditions for obvious diffraction are "harsh".

When the size of the obstacle is much larger than the wavelength of light wave, light can be regarded as straight-line propagation. Note that the straight-line propagation of light is only an approximate law. When the wavelength of light is far less than the wavelength of holes or obstacles, light can be regarded as propagating in a straight line. Diffraction is obvious when holes or obstacles can be compared with or even smaller than the wavelength.

3. Diffraction type:

(1) slit diffraction

Let the monochromatic light from the laser shine on the slit. What happens on the screen when the slit becomes smaller 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 along 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 use vernier caliper to adjust to the minimum distance that can be recognized by naked eyes, and then look at the light source through this slit.

(2) Pinhole diffraction

When the radius of the hole is large, the light propagates along a straight line, and a bright circular spot with the same size calculated according to the straight line propagation is obtained on the screen; If the radius of the hole is reduced, the image of inverted light source calculated by linear propagation will appear on the screen, that is, pinhole imaging; Continue to reduce the radius of the hole, and a circular diffraction halo with alternating light and dark will appear on the screen.