Bcz speed of light changes when it travels from rarer to denser or denser to rarer. thats why light ray seems to bend . it bend towards normal when entrs from rarer to denser and away frm normal when it enters from denser to rarer.
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Light bend when it travels one medium to another due to the rarer and denser medium through which it travels. This property of bending of light is known as "Refraction of Light "