Water and glass allow light to penetrate – but only to a certain extent and not evenly. This tiny difference is the reason why light sometimes appears bent - for example, when a light ray passes through a glass of water.

Because the light ray going through the water appears broken, this phenomenon is known as refraction. It occurs every time light passes from one medium to another medium, in this case from air to water. Refraction is also the reason why water droplets bundle light in a spot. All around the spot there is an absence of light. That’s why they throw a shadow, although they are actually transparent.

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