What is the near-point of the eye-plus-spectacle?

• gungo
In summary, the conversation discusses the calculation of power for a specific eye using the equation P = 1/.25m + 1/-.75m. The near point of 0.25m is derived from a normal eye, while the exercise deals with an eye with a power of -1.5 d. The final answer for the power is 2.67 d, which is then added to the .75 d of the lens. The same equation is used to find q, which results in an answer of -1.72 m. However, the correct answer for q is 48 cm.
gungo
Homework Statement
A hyperopic eye has a near point of 75 cm. If a lens of power .75 d is placed in front of the eye, what is the near point of the eye-plus spectacle?
Relevant Equations
P= 1/p + 1/q
I found the power of the eye itself by doing P= 1/.25 m + 1/ -.75 m. I used negative .75 m because the image would be on the same side of the object. I got a power of 2.67 d. The I added that to the .75 d of the lens. Then I used the same equation and did 3.42= 1/.25 m +1/q to find q, and got -1.72 m. However, the answer is 48 cm.

Hi,

Where does the 0.25 come from?

What power is really needed for a near point of 0.75 m ?

BvU said:
Hi,

Where does the 0.25 come from?

What power is really needed for a near point of 0.75 m ?
I learned in class that 0.25 m was the near point of a normal eye

gungo said:
the near point of a normal eye
So a normal eye has a power of 4 d. But:
This exercise is about a different eye ! Namely, with a power of ...

1. What is the near-point of the eye-plus-spectacle?

The near-point of the eye-plus-spectacle refers to the closest distance at which an individual can clearly focus on an object with the help of their spectacles. This distance is determined by the power of the lens in the spectacles and the individual's ability to accommodate or adjust their focus.

2. How is the near-point of the eye-plus-spectacle measured?

The near-point of the eye-plus-spectacle is typically measured using a device called a near-point ruler. This ruler has a series of small letters or symbols that gradually decrease in size. The individual is asked to read the smallest print they can see clearly while wearing their spectacles, and the distance from their eyes to the ruler is measured.

3. Can the near-point of the eye-plus-spectacle change over time?

Yes, the near-point of the eye-plus-spectacle can change over time, especially as a person ages. This is because the lens of the eye becomes less flexible and the ability to accommodate decreases, making it harder to focus on close objects. As a result, the near-point may shift further away and individuals may require stronger spectacles to see objects up close.

4. What factors can affect the near-point of the eye-plus-spectacle?

The near-point of the eye-plus-spectacle can be affected by various factors such as age, health conditions, and the type of spectacles being worn. Other factors that can impact near-point include lighting conditions, fatigue, and the individual's visual habits and needs (e.g. frequent reading or computer use).

5. Is the near-point of the eye-plus-spectacle the same for everyone?

No, the near-point of the eye-plus-spectacle can vary from person to person depending on their individual visual abilities and needs. Some individuals may have a near-point that is closer or further away than others, and this can also change over time. It is important for each person to have their own spectacles and near-point measurement to ensure clear and comfortable vision.

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