Defects of Vision

The ability to see is called vision

In order to see clearly, the light from object must get focused on the retina.

If it doesn't, the object becomes blurred and we are not able to see

This inability to see is called Defects of Visiion

The common defects of the eye are

• Myopia
• Hypermetropia
• Presbyopia

Note - These defects are called refractive defects because they are caused by incorrect refraction of light rays by the eye lens

Let's look at them one by one

Myopia

It is also called shortsightedness

In Myopia,

• Person is able to see nearby objects clearly but is not able to see farway object
• Image of Faraway objects is formed in front of retina
• Far point of person is nearer than infinity

Causes of Myopia

It is caused because

• Excessive curvature of eye lens(eye lens become more curved)
• Eye ball becomes elongated

Correction of Myopia

It is corrected by using concave lens of suitable power

It helps to form image on the retina

To solve ray Diagram questions related to Myopia

We use lens equation: 1/f = 1/v - 1/u

To correct myopia, a concave lens is used. So, focal length is negative

Object distance (u) is taken as infinity (∞)

And image distance (v) is person's far point

Hypermetropia

It is called long sightedness

In Hypermetropia

• Person is able to see farway objects but are not able to see nearby objects
• Image of Nearby objects is formed behind the retina
• Near point of person is beyond 25 cm

Causes of Hypermetropia

It is caused because

• Eye ball becomes too small
• Focal Length of Eye lens is too long

Correction of Hypermetropia

It is corrected by using convex lens of suitable power

It helps to form image on the retina

To solve ray Diagram questions related to Hypermetropia

We use lens equation: 1/f = 1/v - 1/u

To correct hypermetropia, a convex lens is used. So, focal length is positive

Object distance (u) is taken as normal near point of the eye (25 cm). Since object is in front of lens, it will be -25 cm

And image distance (v) is person's near point

Presbyopia

It is the defect in which person is not able to see nearby objects clearly

It is formed in old persons whose cillary muscles become weak with age

Hence, they lose their power of accomodation.

Causes

It is caused due to

• Weakening of Eye muscles
• Decrease in Flexibility of Eye lens

Correction of Presbyopia

it is also corrected with the help of convex lens (just like Hypermetropia)

Note: - In both Presbyopia and Hypermetropia, person cannot see nearby objects clearly but can see far away objects clearly.
The difference is in their causes
Presbyopia is caused due to loss of Power of Accommodation
Hypermetropia is caused due to abnormal shape of the eyeball or abnormal shape of the eye lens

What are Bifocal Lens?

It is a lens whose upper part consists of concave lens and inside part for convex lens

It is used in case of those patients who are suffering from both myopia and hypermetropia