Question

In a Std. X class, out of 40 students ,10 students use spectacles,2 students have positive power and 8 students have negative power of lenses in their spectacles. Answer the following questions:
->What does the negative power indicate?
->What does the positive power indicate?
->Generally, which type of spectacles do most of the students use?
->What defect of eyesight do most of the students suffer from?
->Give two possible reasons for the above defect

Answer 1

This question uses the concept of power of lens. Power of lens is defined as the ‘ability of lenses to ‘converge or diverge’ a beam of ray of light. It is a measure of deviation of light rays produced by a lens. More the power, more is the deviation.

Complete step-by-step solution:
->Negative power indicates that the students are using a Concave lens in their spectacles.
1. Concave lenses have negative focal length with virtual focus and hence, it has negative power.
2. Concave lens is also known as a ‘diverging lens’ as it diverges parallel incident light rays and forms a virtual image by extending traces of light rays passing through the lens to a focal point behind the lens.
->Positive power indicates that the students are using convex lenses in their spectacles.
1. Convex lenses have positive focal length with real focus and hence, it has positive power.
2. Concave lens is also known as a ‘converging lens’ as it concentrates the light rays that are incident on it and the bending of light rays is maximal at the top and bottom of the lens.
->Generally, students use ‘Concave lenses in their spectacles. Concave lens is thinner at the middle and thicker at the edges. These minus lenses move the focus of light in a near-sighted eye from a point in-front of the retina backward-so it falls directly onto the surface of the retina. This shift corrects the blurry distance vision caused by myopia and restores clear vision.
->Most of the students suffer from ‘Myopia’. Myopia is also known as ‘near-sightedness’. Near-sightedness is the refractive disorder of the eye where a person can see near objects clearly but distant objects are blurry.
->The two possible reasons for myopia are stated as follows:
1. When the eyeball is too long.
2. When the cornea-the protective outer layer of eye is too curved or abnormally shaped.
When the above conditions occur in the eye, then the light that enters the eyes won’t focus correctly, thus making the objects that are distant look blurry. Instead of light focusing on the retina itself, light focuses in front of the retina.

Note:- Power of lens is also defined as the reciprocal of focal length (in metres) of the lens. S.I unit of power of lens is Dioptre (D).
${\text{Power of lens}}\left( P \right){\text{ = }}\dfrac{1}{{{\text{Focal length}}\left( f \right)}}$.