Focal Length and Nature of Lens for a Myopic Person

Focal length and the nature of the lens are important factors to consider when correcting the vision of a myopic person. Myopia, also known as nearsightedness, is a condition where a person can see nearby objects clearly but has difficulty seeing distant objects. It occurs when the eyeball is too long or the cornea is too curved, causing the light to focus in front of the retina instead of directly on it. To correct this, a lens with a specific focal length and nature is used.

1. Definition of Focal Length:
Focal length is the distance between the lens and the point where the light converges or diverges after passing through the lens. In the case of myopia, a concave lens is used to diverge the incoming light before it enters the eye, allowing it to focus properly on the retina.

2. Negative Power of -1.25 Dioptre:
The power of a lens is measured in diopters. A negative power indicates a concave lens, while a positive power indicates a convex lens. The higher the absolute value of the power, the greater the divergence or convergence of light. In this case, a myopic person with a power of -1.25 diopters requires a concave lens.

3. Determining the Focal Length:
The focal length of a lens can be calculated using the formula:
Focal Length (in meters) = 1 / Power (in diopters)

Using this formula, we can determine the focal length for a myopic person with a power of -1.25 diopters:
Focal Length = 1 / (-1.25) = -0.8 meters

4. Conversion to Centimeters:
To convert the focal length from meters to centimeters, we multiply by 100:
Focal Length (cm) = -0.8 m * 100 = -80 cm

5. Nature of the Lens:
A negative focal length indicates a concave lens, which is used to correct myopia. Therefore, the correct answer is option 'D', which states that the focal length is 80 cm and the lens is concave.

In conclusion, a myopic person with a power of -1.25 diopters requires a concave lens with a focal length of 80 cm to correct their vision. The concave lens diverges the incoming light, allowing it to focus properly on the retina and improve distance vision.