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Why does light bend when it enters and leaves a lens?

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How lenses and mirrors affect light and vision — lens Explanation of the refraction of light through different lenses.

Light bends when it enters and leaves a lens because of a phenomenon called refraction. This occurs because light travels at different speeds depending on the medium through which it passes; specifically, light travels more slowly in the material of the lens (such as glass) than it does in the surrounding air.1

When a light beam encounters the boundary between these two media, the change in speed causes the light to change its direction of travel. The specific amount and direction of this bending are governed by Snell’s law, which relates the angles of the light ray to the refractive index of each medium.21

Because a lens has precisely curved surfaces, different parts of an incident light beam strike the boundary at different angles. This causes the rays to be refracted by varying amounts, allowing the lens to redirect parallel rays to converge at a focal point or appear to diverge from one. This process of refraction occurs at both the entry and exit surfaces of the lens, enabling the formation of images.12