Mirror Formula

Light of Class 7

The distance of the position of an object on the principal axis from the pole of a spherical mirror is known as object distance. It is denoted by u.

The distance of the position of the image of an object on the principal axis from the pole of spherical mirror is known as image distance. It is denoted by v.

The relation between u, v and focal length (f) of a spherical mirror is given

mirror formula

Magnification (or linear magnification)

Linear magnification produced by a mirror is defined as the ratio of the size (or height) of the image to the size (or height) of the object. It is denoted by m

If h = size (or height) of the image produced by the mirror and h = size (or height) of the object.

Then, linear magnification, m = h'/h

Linear magnification has no unit.

Defects of Vision and their correct

Common defects of Vision

  • Long sightedness or Far sightedness or Hypermetropia

A human eye which can see for off objects or distant object clearly but can not see the near objects clearly is said to be suffered with a defect known as long sightedness or Hypermetropia.

Causes of ​Hypermetropia

This defect arises due to either (i) increase in the focal or (ii) the size of the eye ball become too small so that the light rays from the nearby point or object is nor brought to focus on the retina of the eye.

Correction of long-sightedness (or Hypermetropia)

This defect can be corrected by using a convex lens of suitable focal length.

Short-sightedness or near-sightedness or Myopia

  • A human eye is short sight or Myopic if it can see the near objects clearly but unable to see for off objects or distant object clearly.
  • This defect arises due to either
  1. elongation of the eye ball ball or
  2. the excessive curvature of the cornea.

Correction of short sightdness or Myopia

This defect can be corrected by using a concave lens of suitable focal length so  man suffering from this defect.

Presbyopia

A man eye which cannot see the near objects as well as far off (or distant) object clearly is said to suffer from a defect known as Presbyopia.

Cause

This defect arises due to the ageing of a person. The ciliary muscles are weakened ad the flexibility of the crystalline lens of the human eye decreases with age of the person.

Correction of Presbyopia

This defect can be corrected by using a bi – focal lens.

Astigmatism

A human eye which cannot focus on both horizontal and vertical lines simultaneously is suffering from a defect known as Astigmatism.

If such an eye sees an object then the horizontal part of the object will not be visible clearly but its vertical part will be clearly visible as shown in figure.

normal eye

Cause

This defect arises when the cornea of the eye has different curvatures in different directions in the horizontal and vertical planes.

Correction of Astigmatism

Astigmatism can be corrected by using glasses with cylindrical lens.

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