THE STORY OF OPTICS LENS

Principles of the Optics

VOL.005 We have discussed on the Light observing various phenomena which are familiar to us through our daily life and we understood its various character. Now, we would like to study various equipment and instruments using its character of the light which passing through and reflecting through lens or prism.

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1. Outline of Single Lens:
1-1. Kinds of Single Lens:
We have generally two groups of single lens which are Convex Lens and Concave Lens. Focal distance of convex lens is plus and the same of concave lens is minus.
Convex lens concentrates light and concave lens disperses it.

bi-convex lens 　　　　　　　　convex-meniscus lens
plane-convex lens plane-concave lens
bi-concave lens 　　　　　　　concave-meniscus lens

The focal distance of thin lens without thinking its thickness can basically be figured out as under:

A. In case of Plano-convex Lens or Plano-concave Lens;
In case one side of surface is plane surface and the radius of curvature is "r";

r ( Plano-convex lens r > 0 )
Focal distance "f" = ---------------
N - 1 ( Plano-concave lens r < 0 )

In case the refraction ratio of glass is n　≒　1.5 ,
f = 2 r ie the focal distance is two times of the radius of curvature.

B. In case bi-convex lens and bi-concave lens with equal curvature ratios,
r ( bi-convex lens r > 0 )
f = -------------------------------
2 ( n - 1 ) ( bi-concave lens r < 0 )

In case of n　≒ 1.5, f = r is computed.
Focal distance shall be about equal with radius of curvature.

2. Outline of loupe:
We use a loupe to observe the object with our naked eyes through the virtual image as
result of enlargement with it.
Stated quite simply, we can observe the object as illustrated below, if the loupe is
supposed a thin convex lens. If we keep the object PQ vertically on the axis of the light just a little bit inside ( lens side ) of the focal point F in front of the loupe lens, we see the enlarged P ' and Q ' in front of the lens. This P' and Q' is the imaged one and it is the enlarged object standing normally. Our eyes can feel it is enlarged while we look at through the lens.

When our eyes keep just on the focal point of the loupe, enlarged ratio is ;
mo = 250/f
This distance of 250mm is called as the least distance of distinct vision and it has been traditionally mentioned the distance we can see without any big tiredness.

3. Telescope;
Excepting for that both lens are concave lenses, telescope can be made if we select properly the distance between both lenses.

a. Kepler Telescope: focal point
The kinds of telescope with two convex lenses are the headstand type one. This type of telescope is used as a prism telescope that the object is viewed to stand normally by keeping a normal-stand prism between both convex lenses.

In case of Kepler telescope or binoculars ( prismatic glasses ) , it is indicated its optics characteristic on it or in the catalogue as under;
Power x effective diameter of objective lens field of view.
For example, it is indicated as 8 x 30 7.5°
( power 8 x effective diameter 30mm field of view 7.5°).
b. Galilei Telescope; The telescope using convex and concave lens is normal stand type telescope. Since this type telescope has small power, it is applied to simple telescopes such as opera glasses. There are kinds of telescope using reverse arrangement of convex and concave lenses to Galilei type telescope. It is often used for finders of cameras.

To be continued………….

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If you have interest of past issues, please visit to our web site reception@sankyou-kogaku.co.jp VOL.001.The Bases of the Optics
VOL.002.The Bases of the Optics
VOL.003.The Bases of the Optics
VOL.004.The Bases of the Optics