First of all, rain is a transparent liquid. When light passes through raindrops, it will be refracted. According to the refraction law of light, when light enters another medium (such as raindrops) from one medium (such as air), it will change its propagation direction. This means that the light will be deflected when it enters the raindrops.
Secondly, the surface of raindrops will reflect and scatter. When light enters the raindrop, some light will be reflected by the raindrop surface, and these reflected light will interfere with the incident light to form a rainbow. Another part of the light will pass through the raindrop and scatter inside the raindrop, and these scattered lights will leave the raindrop at different angles.
In this process, the wavelength of light will affect the refraction and scattering effect of light. Normal yellow street lamps emit light with longer wavelength, while pink light with shorter wavelength. When light is refracted and scattered by raindrops, yellow light with longer wavelength will scatter more, while pink light with shorter wavelength will scatter less. So when we observe the light under the street lamp, we will find that the yellow light is weakened, while the pink light is relatively enhanced, thus making the whole light pink.
In addition, the size and density of raindrops will also affect the light scattering effect. When the raindrops are larger or denser, the light will scatter more with the raindrops, thus enhancing the pink light. On the contrary, when the raindrops are small or the density is low, the scattering effect of light is weak, and the pink light is relatively less.
To sum up, the normal yellow street lamp will turn pink after rain, which is caused by the refraction and scattering of rain. This phenomenon is the result of the interaction between the wavelength of light and the size and density of raindrops, which makes the yellow light weaken, while the pink light is relatively enhanced, thus showing a pink effect.