Mammoth Memory

Refraction concave lens

For a concave lens, where light travels from a fast medium (e.g. air) to slow medium (e.g. glass), the light is bent towards the normal.

Refraction bends the light towards the normal as it enters the concave lens

NOTE: See Mammoth Memory, Refraction, for how to use the term "FAST SOFA" to recall that, when going from a fast medium to a slow medium, the light bends towards the normal.


For a concave lens, when light travels from a slow to a fast medium, the light is bent away from the normal. See below:

Refraction bends the light away from the normal as it leaves the concave lens

NOTE: Again, see Mammoth Memory, Refraction and use the term "FAST SOFA" to mean, here, that when passing from a slow medium to a fast medium, the light bends away from the normal.


The overall impact of refraction on a ray of light is:

Ray of light refracting as it enters and exits a concave lens

We can see that the light has been bent twice. In some books this diagram is summarised as:

Refraction in a concave lens is often depicted as this

But technically this is not correct.

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