Why does a tubelight look brigther when observed through a transparent bottle filled with water as compared to looking it through an empty bottle ?
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When you say “tubelight”, you mean like a fluorescent tube?
A bottle of water acts like a lens. This is because the water has an optical density greater than air.
Optical density is the tendency for light to bend as it enters a medium (such as water). A vacuum and air both have an optical density of about 1; water has an optical density of about 1.33; glass has an optical density of about 1.5.
The bending that occurs when a light beam moves from a medium with one optical density to a medium with a different optical density is called refraction. Google “optical density” or “refraction” to find out more.
So the short answer is: a bottle filled with water acts like a lens. A bottle filled with air doesn’t.
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