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Refraction of Light through a Glass Block.

When a ray of light enters a glass block at an angle other
than the normal, it changes speed, wavelength
and direction as shown below. This also happens in a Lens.

Light showing Refraction through Glass

In going from a less dense medium (air) to a more dense
medium (glass), light bends towards the normal.
This means that i > r  (the angle i is greater than the angle r).
In going from a more dense to a less dense medium (glass to air),
light bends away from the normal.
How much the light bends depends on its colour.

The change in angle of the light ray is the same
when it  enters and leaves the glass.
If the incident ray had continued without changing direction,
then the emergent ray would be parallel to it.

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