A person wears glasses of power `- 2.5 D`. Is the person short sighted or long sighted ? What is the far point of the person without glasses ?

Answer

To solve the problem, we will follow these steps:

### Step 1: Determine if the person is short-sighted or long-sighted
- The power of the glasses is given as `-2.5 D`.
- A negative power indicates that the lenses are diverging lenses, which are typically used to correct short-sightedness (myopia).

**Conclusion**: The person is short-sighted.

### Step 2: Calculate the focal length of the lens
- The formula for the power (P) of a lens is given by:
\[ P = \frac{1}{f} \]
where \( f \) is the focal length in meters.
- Rearranging the formula gives:
\[ f = \frac{1}{P} \]
- Substituting the power:
\[ f = \frac{1}{-2.5} = -0.4 \text{ m} \]
- Converting to centimeters:
\[ f = -0.4 \times 100 = -40 \text{ cm} \]

### Step 3: Determine the far point of the person without glasses
- For a short-sighted person, the far point is the distance at which they can see clearly without corrective lenses. This distance is equal to the absolute value of the focal length of the lens.
- Since we calculated the focal length to be `-40 cm`, the far point is:
\[ \text{Far point} = -40 \text{ cm} \]

**Final Conclusion**: The far point of the person without glasses is `-40 cm`.

### Summary of the Solution:
1. The person is short-sighted because the power of the lens is negative.
2. The far point of the person without glasses is `-40 cm`.