Human beings typically have a capability to see ranging about 25 cm to infinity. 25 cm is usually called as near point (punctum proximum ) and infinity is the far point (punctum remotum).
When one reads, he will put the reading about 25 cm to be able to read clearly. At this time the eyes’ lens are thickened to focus the near object to the retina. We called the eyes in its maximum accomodation. One whose near point further than 25 cm is called hyperopia. In order to read an object at the normal distance, he can be helped by spectacles with positive lens.
Without the lens, the image of the object at 25 cm will falls behind the retina.
Applying lens to the eyes can help the them to focus the object to the retina.
The measure of the power of the lens can be determined as follow
Where P is the power of the lens expressed in diopter , PP is the punctum proximum in cm.
In order to see a distant object, one will set his eyes’ lens thin to focus it onto his retina. But in some cases, one fails to do that. This kind of eyes in called myopia.The image of the object falls in front of the retina.
This eyes can be helped using a negative lens.
The power of the lens needed can be determined as follow
P is the power of the glasses’ lens, PR is the punctum remotum (far point), that is maximum distance of an object that still can be seen by the man.
The image of the object for glasses always virtual, so S’ has negative values.
In general, we can summarize the formula of the power of the lens with
Where P is the power of the lens, x is the preferred distance to see, PP or PR is the punctum proximum (near point) and punctum remotum (far point) respectively.
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