Two opaque pens are tightly clamped side by side. When you look at the light, when you pass through the gap between two pens in a position parallel to the light, you will see alternating color stripes. Because the frequencies of light waves are the same, diffraction will occur. When the size of the slit (or the size of the obstacle) is close to the wavelength, there will be obvious diffraction phenomenon. If the slit is wide, and its width is much larger than the wavelength, the wave will basically travel in a straight line after passing through the slit, and the diffraction phenomenon is not obvious. The slit between two pencils is quite close to the wavelength of light wave, and diffraction occurs.

Film interference, colorful soap bubbles in the sun, colored stripes on the water surface on the roadside after rain, and the metal surface after high temperature treatment presents beautiful blue. Diffraction, have you ever seen the shadow of a tree? Do you see the shadow of the wire? This is a diffraction phenomenon.