I saw a puddle of water on the road and I noticed that I can very clearly see the reflection on its surface in almost 8k high definition of each cars and buses that drove by. I went to the canteen and ordered drinks, I also noticed the reflection of patrons on the surface of the steaming hot coffee. I wonder why do so many liquids displaying this property? I am thinking a mirror has a smooth surface and the molecules don't move around but it is a completely different story for a liquid, the molecules are moving non stop bumping against every other molecules but somehow it can produce a clear reflection, why?
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As noted in the comments, reflections from water are not affected by molecular motion because the size of the molecules and the distances they move are a thousand times less than the wavelength of visible light.
Any flat dielectric interface such as air-water (or air-coffee) has a reflectivity described by the Fresnel Equations, and this reflection is unaffected by features much less than the wavelength $\lambda$ of light. In general, scattering from a sub-wavelength feature with size $d$ falls off dramatically with wavelength, as illustrated by the $d^6/\lambda^4$ dependence of Rayleigh scattering.
A surface with a roughness smaller than $\sim \lambda/2-\lambda/4$ is considered to be "optical quality", and the the best mirrors are flatter than $\lambda/50$. Visible light has wavelengths roughly in the range 380-750 nm. This is more than a thousand times greater than the size and spacing of liquid water molecules, so the roughness due to molecular motion is $<\lambda/1000$. This is far too small to affect reflectivity.
Reflections from liquid surfaces are, however, more susceptible than solid surfaces to large collective molecular movements with dimensions $>\lambda$. For example, reflections from a puddle will be distorted by ripples caused by the wind or a passing truck, and liquid surface mirrors require great care to reduce vibrations.
David Bailey
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