# Finding optical power of corrective lenses?

#### astupidummydumstupid

The question is as follows:
"A person with presbyopia has a near point of 40 cm. This person wears glasses to correct their vision such that they can comfortably read the screen of their mobile phone when it is held 25 cm from their eyes.

If this person wears their glasses such that they sit 1.2 cm from their eye, what is the optical power of the corrective lenses in their glasses? (to 2 s.f and in D)"

I am using Peye = 1/di + 1/do

and thus

Peye = 1/0.230 + 1/-0.388

this = 1.77 => 1.8D, which isn't correct.
I subtracted the retina-lens distance from both di and do to get my values. I have been doing a bunch of these questions and surprise I've got them all wrong. Could anyone help me?

Thank you

-I

#### Woody

Don't assume that the notation you use will always be familiar to everyone.
A lot of the notation we use is just what was chosen (pretty much at random) by the first person to consider the problem.
Sometimes this notation becomes adopted by enough people to become a standard convention,
but this should not be assumed.
It may be that the notation you are using is particular to your tutor, or text book.

What is the definition of di, do, Peye ?

As an aside have a quick glance at this Wikipedia article <Wikipedia : Light Speed Notation>
Here it explains that even (arguably) the most famous symbol "c" for the speed of light is not completely universally uniquely defined.

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1 person

#### jje9046

Notations

Hi,

di means image distance
do menas object distance
Peye means Power from the eye