A liquid drop is spherical in shape due to surface tension. But why does it appear as a vertical line under the free-fall due to gravity? (E.g. During a rain - falling raindrop) Is there a specified length for the line or does it vary with the size of drops?
Answer
A drop that is free falling in vacuum is spherical. This is because free falling in a gravitational field is the same thing as being at rest with no gravitational field present: the gravitational field and the acceleration cancel each other out.
Rain drops falling to the earth can have various shapes depending on their size, although I am not aware that they can become elongated (can you provide a source?). These shapes are due to the air flowing past them, in a rather intuitive way: roughly, air flow causes the bottom to become flatter.
Edit: Regarding the appearance of raindrops (as opposed to their physical shape), consider taking a photograph of falling rain. The camera integrates the incoming light over the exposure time $t$, during which the drop travels a distance of $v t$, where $v$ is the velocity. If we are close to the ground this will be the terminal velocity, which is about $2_{m/s}$. If we use an exposure time of $t=1/60_s$ (say we are using a flash), the drop will trace a line of length $\sim 3_{cm}$. The apparent line on the photograph then has to account for distance from the drop, etc.
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