How thick a lens can be, while still remaining practical, somewhat depends on the application for the lens. However, there are two main reasons why most lenses (regardless of their function) should not be extremely thick. There is a useful equation relating the key physical properties of most types of lenses called the lensmaker's equation (valid only for lenses in air). While there are approximations of this equation with different terms, I will use the...
How thick a lens can be, while still remaining practical, somewhat depends on the application for the lens. However, there are two main reasons why most lenses (regardless of their function) should not be extremely thick. There is a useful equation relating the key physical properties of most types of lenses called the lensmaker's equation (valid only for lenses in air). While there are approximations of this equation with different terms, I will use the following form to demonstrate the first reason why a lenses should not be excessively thick:
`(1)/(f)=(n-1)*((1)/(R_1)-(1)/(R_2)+((n-1)*d)/(n*R_1*R_2))`
In the equation above "f" refers to the focal point of the lens, which is the distance from the center of lens that light is either concentrated at or dispersed from. The focal length of a lens is its most practical dimension. The other terms in the equation are not extremely important for this explanation, so I will define them quickly: "n" is the refractive index of the lens material, "R1" and "R2" are the radii of curvature of the lens surface farthest from and closest to the light source, and "d" is the thickness of the lens. This last term is the one we care about here. The equation above dictates that the thickness of the lens "d" and the focal length "f" are inversely proportional. Thus, as the thickness of the lens increases and becomes extremely large (infinite) the focal point approaches zero. This means that the focal point is in the center of the lens, which is useless for controlled focusing of light because the focal point is physically inside of space occupied by the lens material. A concrete example of this case would be glasses that are so thick that the user's eyes would have to be pushed directly against the lenses (or need to be inside of them, which is impossible) for the user to see a clear image of their surroundings. Exactly what value of lens thickness that results in an impractical focal length depends on the other terms in the equation above, as well as the desired focal length.
The second reason why a lens should not be excessively thick has to do with light transmission and absorption of the material that the lens is made of. While lenses tend to be made of glass or clear polymer that transmits all wavelengths of light very well and absorbs very little of the light, they are still not perfectly ideal optical materials. This means that the lens material does not transmit 100% of the light entering and some energy is lost through absorption and dispersion of light by the molecules in the lens. For most lenses this is never a problem because they are kept sufficiently thin so that the effects of absorption and dispersion of the light do not get in the way. However if a lens were made extremely thick, the light being focused or dispersed may not be bright enough for the application.
The first reason is probably relevant more often than the second, but they are both valid concerns involving thickness when designing lenses for a specific application.
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