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:

1. What does the negative power indicate?

2. What does the positive power indicate?

3. Generally which type of spectacles do most of the students use?

4. What defect of eyesight do most of the students suffer from?

5. Give two possible reasons for the above defect.

Answer:

(1) The negative power indicates that the spectacles used by the students have concave lens.

(2) The positive power indicates that the spectacles used by the students have convex lens.

(3) Diverging or concave lens spectacles are generally used by the students.

(4) Most of the students suffer from the eye defect known as myopia (near sightedness).

(5) The two possible reasons for the eye defect (myopia) are

1.  Converging power of the eye lens becomes high as ciliary muscles do not relax sufficiently.


2.  Length of the eye ball increases as the distance between the eye lens and the retina increases.