NCERT Textbook

(a) Myopia



Myopia is also known as near-sightedness. A person with myopia …………………. This defect may arise due to excessive curvature of the eye lens, or (ii) elongation of the eyeball.

(b) Hypermetropia



Hypermetropia is also known as far-sightedness… …………………….. This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.

Correction



Myopia and hypermetropia are due to problems associated with cornea. It doesn’t have much to do with eye lens.

NCERT Textbook

This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bi-focal lenses.

Correction

Presbyopia is often confused as the existence of myopia and hypermetropia together but a close examination of the cause of hypermetropia and myopia would assure you that they cannot occur together. Presbiopia is a problem associated with ciliary muscle and eye lens.

NCERT Textbook

……..This is called Ohm’s law. Inother words –



V α I



or V/I = constant= R



or V=IR

Correction

V = IR is an equation which is derived from the mathematical definition of resistance in DC circuits R = V / I . This is true in every conductor ohmic or non-ohmic. Ohm’s law actually says that the resistance is independent of the applied voltage and hence R is a constant.



So V / I is a constant



And V α I.

NCERT Textbook

Correction

The power of cornea is around 43 Dioptres whereas the power of eye lens is 15 Dioptres. The diagrams shown below as in the NCERT textbook show that the maximum deviation is occurring at eye lens which is grossly misleading.

Misconceptions about light

1. A plane mirror does not form an inverted image

A plane mirror can form an inverted image when the plane of the object is perpendicular to the plane of the mirror.

For example, consider a line object with end points A and B in front of a plane mirror as shown.

If we just concentrate at the end points, as we usually do in drawing ray diagrams, we see that A should be as far behind the mirror as it is in front of it and so for B. So it is obvious that the image is inverted.

2. Plane mirrors never form real images



Plane mirror may form a real image when converging rays from an optical device fall on it.