Explanation:

Actually, flat lenses can focus light if they are built properly. Let us look at the basics of lenses and then at different types of flat lenses.

A traditional lens is essentially a curved piece of glass that bends light through a process known as "refraction". Whenever a ray of light travels from one transparent material, such as air, into another transparent material, such as glass, it changes direction (except in the special case of the light's direction being perfectly perpendicular to the interface). This change of direction occurs right at the interface between the two materials, and not deep in one material. The light bends because one material has electrons that interact more strongly with the light passing through it than the other material, causing the light to slow down and bend. The ability of a material to bend light is quantified by its "refractive index". A material with a higher refractive index, such as diamond, will bend light more. When light goes from a standard low-index material (such as air) into a standard high-index material (such as glass), it always bends towards the normal to the surface. In contrast, when the light goes from a standard high-index material (such as glass) into a low-index material (such as air), it always bends away from the normal. This is illustrated in the diagram.

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