A person with myopia can see nearby objects clearly but cannot see distant objects distinctly
Key Characteristics
- Far point is nearer than infinity
- Can see clearly only up to a few metres
- Image of distant object forms in front of retina
🔴 Causes of Myopia
(i) Excessive curvature of the eye lens
(ii) Elongation of the eyeball
✅ Correction Method
Use a CONCAVE LENS of suitable power
The concave lens diverges light rays before they enter the eye, bringing the image back onto the retina
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly
Key Characteristics
- Near point is farther than normal (> 25 cm)
- Must keep reading material beyond 25 cm
- Image of nearby object forms behind the retina
🟠 Causes of Hypermetropia
(i) Focal length of eye lens is too long
(ii) Eyeball has become too small
✅ Correction Method
Use a CONVEX LENS of appropriate power
The convex lens provides additional converging power to form the image on the retina
The power of accommodation decreases with ageing
Key Characteristics
- Near point gradually recedes with age
- Difficulty seeing nearby objects comfortably
- Common in people above 40 years
🟣 Causes of Presbyopia
(i) Gradual weakening of ciliary muscles
(ii) Diminishing flexibility of the eye lens
✅ Correction Methods
For presbyopia alone: Use CONVEX LENS (reading glasses)
For both myopia and hypermetropia: Use BI-FOCAL LENSES
- Upper portion: Concave lens (distant vision)
- Lower portion: Convex lens (near vision)
Modern Solutions: Contact lenses or surgical interventions can also correct refractive defects
Understanding the differences between common vision problems
| Defect | Vision Problem | Image Formation | Corrective Lens |
|---|---|---|---|
| Myopia | Can't see far objects | In front of retina | Concave (Diverging) |
| Hypermetropia | Can't see near objects | Behind retina | Convex (Converging) |
| Presbyopia | Age-related near vision loss | Behind retina (for near) | Convex or Bi-focal |
Remember
💡 Concave lens = Diverges light = Corrects Myopia (far point problem)
💡 Convex lens = Converges light = Corrects Hypermetropia & Presbyopia (near point problem)
Let's solve a real problem about vision correction
Question:
A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
Solution:
Given: Far point of myopic eye = 1.2 m
Required: Type of corrective lens
Analysis:
- The person has myopia (near-sightedness)
- Far point is at 1.2 m instead of infinity
- Image of distant objects forms in front of retina
Answer:
A CONCAVE LENS (diverging lens) should be used. This lens will diverge the light rays from distant objects so that they appear to come from 1.2 m, allowing the myopic eye to focus them properly on the retina.
Quick Tip: For myopia correction, the focal length of the concave lens should equal the negative of the far point distance. In this case, f = -1.2 m