In the double-concave lens
is negative,
In the double-convex
is negative,
In the plano-concave either
is infinite, or
is infinite, and
negative; therefore putting
for the single radius
In the plane-convex,
When in the double-concave, or double-convex lens the radii are
equal,
[1]
88.It appears from all this, that the place of the principal focus is the same, whichever side of a lens is turned towards the incident light, and that
The concavo-convex[2] |
![{\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1a4163f7b866b250a0f7c232d9b4e166f7638469) |
make parallel rays diverge.
|
the double-concave
|
and the plano-concave
|
The meniscus |
![{\displaystyle \scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/1a4163f7b866b250a0f7c232d9b4e166f7638469) |
make parallel rays converge.
|
the double-convex
|
and the plano-convex
|
- ↑ If
which is nearly the case in glass,
or the principal focal length is equal to the radius of sphericity.
- ↑ See Fig. 87, for the relation between these different kinds of lens. Those placed together are equivalent.