The odd question
Can light slow down?
Light has a famous speed limit, but that is not the whole story.
Here's the surprise
Light travels more slowly through water or glass than through a vacuum.
What's really going on?
The speed of light in vacuum is a fundamental constant, approximately 300,000 kilometres per second.
In materials, interactions with matter make a light wave propagate at a lower effective speed.
This change in speed helps explain refraction: why a straw appears bent in a glass of water.
Think about it
Does light exceed its vacuum speed when it leaves glass?
Reveal the explanation
No. In vacuum, light propagates at its usual vacuum speed; passing through a medium does not let it exceed that limit.