How does light pass through a flat lens?
The same way it passes through glass.
If a piece of glass is flat on both sides, it is not a lens. It can displace rays but not bend them.
Light passes through a flat lens primarily due to the phenomenon of refraction. When light travels from one medium to another, such as air to glass or water to glass, it changes direction due to the change in its speed. This change in direction is determined by the refractive index of the materials involved.
In the case of a flat lens, when light approaches the lens surface, it refracts (bends) as it enters the lens material. The extent to which it bends depends on the refractive index of the lens material. This bending allows the lens to focus or spread out the incoming light depending on its curvature.
If the lens is convex (thicker at the center), it causes light to converge, which means the light rays come together at a certain point known as the focal point. This type of lens is used for focusing light, like in a magnifying glass or camera lens.
If the lens is concave (thinner at the center), it causes light to diverge, which means the light rays spread out after passing through the lens. This type of lens is used for spreading light, such as in a corrective eyeglass lens for nearsightedness.
So, to summarize, light passes through a flat lens by entering the lens material, refracting as it changes speed, and bending based on the curvature of the lens. This bending determines how the light is focused or spread, allowing the lens to perform various optical functions.