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
