Quick Answer: What Are the Defects of Vision?

Human Eye and Defects of Vision: Myopia, Hypermetropia, Presbyopia Explained

Quick answer: Myopia (near-sightedness) means the image forms in front of the retina and is corrected with a concave lens (negative power). Hypermetropia (far-sightedness) means the image forms behind the retina and is corrected with a convex lens (positive power). Presbyopia is an age-related loss of accommodation, corrected with bifocal lenses. The least distance of distinct vision for a normal eye is 25 cm, and all corrective lens powers are calculated using P = 1/f (f in metres, P in dioptres).

Structure of the Human Eye and Image Formation

The human eye is a remarkable optical instrument. Its essential parts for exam purposes:

• Cornea: thin, transparent bulging outer cover; provides most of the refraction (the eye’s primary converging surface).
• Eye lens: a fibrous, jelly-like convex lens (with the aqueous humour in front and vitreous humour behind) that fine-focuses light onto the retina.
• Iris: controls the size of the pupil, regulating light entering the eye.
• Ciliary muscles: change the curvature (and hence focal length) of the eye lens.
• Retina: the light-sensitive screen at the back of the eye, packed with rods and cones.

The lens system of the eye (cornea + eye lens) forms a real, inverted and diminished image of an object on the retina. The brain interprets this inverted image as erect. Reference coverage of the eye’s optics is available in the NCBI physiology texts and standard NCERT Class 10 Science treatment.

Accommodation: Power of the Eye Lens and Least Distance of Distinct Vision

What is accommodation? It is the ability of the eye to change its focal length by changing the curvature of the lens, using the ciliary muscles.

• Viewing a distant object: ciliary muscles relax, the lens becomes thin, focal length increases.
• Viewing a nearby object: ciliary muscles contract, the lens thickens, focal length decreases.
• Least distance of distinct vision (near point): 25 cm for a normal adult eye — the minimum distance at which an object can be seen clearly without strain.
• Far point: infinity for a normal eye.

The power of accommodation is greatest in youth and declines with age — which is exactly what leads to presbyopia.

Myopia (Near-Sightedness): Cause and Ray Diagram

What causes myopia? The image of a distant object forms in front of the retina instead of on it, because:

1. excessive curvature (converging power) of the eye lens, or
2. elongation of the eyeball.

Ray diagram (describe in exam): Parallel rays from a distant object converge and meet before reaching the retina, then diverge to strike the retina as a blurred spot. The far point of a myopic eye is closer than infinity — a person may see clearly only up to, say, 2 m. Myopia is also called short-sightedness or near-sightedness because the near point is unaffected.

Correction of Myopia with a Concave Lens and Power Calculation

A concave (diverging) lens of suitable power is placed before the eye. It first diverges the parallel rays; the eye lens then converges them exactly onto the retina.

Key rule for numericals: the concave lens must form the image of a distant object (at infinity) at the far point of the defective eye.

Formula logic: u = ∞, v = far point distance (negative, same side as object by Cartesian sign convention), so f = far-point distance. Then P = 1/f.

Worked example: A myopic person’s far point is 2 m. Power needed: f = −2 m, so P = 1/(−2) = −0.5 D. The corrective lens has power −0.5 dioptre — always negative for myopia.

Hypermetropia (Far-Sightedness): Cause and Ray Diagram

What causes hypermetropia? The image of a nearby object forms behind the retina, because:

1. the focal length of the eye lens is too long (low converging power), or
2. the eyeball is too short.

Ray diagram (describe in exam): Rays from a near object (at 25 cm) are insufficiently converged; they would meet at a point beyond the retina, so the retinal image is blurred. The near point of a hypermetropic eye is farther than 25 cm — hence the person can see far objects clearly but not near ones.

Correction of Hypermetropia with a Convex Lens and Power Calculation

A convex (converging) lens is used. It pre-converges the rays so that the eye lens can focus them on the retina.

Key rule: the convex lens must form the image of an object at 25 cm at the near point of the defective eye (a virtual image).

Worked example: The near point of a hypermetropic eye is 1 m. Where should the image of an object at 25 cm be formed? At the near point, 100 cm. Using the lens formula with u = −25 cm, v = −100 cm: 1/v − 1/u = 1/f gives 1/f = (−1/100) − (−1/25) = 3/100, so f = +33.3 cm = +0.333 m. Power P = 1/0.333 ≈ +3.0 D. Convex power — always positive for hypermetropia.

Presbyopia and Bifocal Lenses

How is presbyopia different? Presbyopia is not a structural eyeball defect — it is the age-related weakening of ciliary muscles and loss of lens flexibility, typically after age 40. The power of accommodation falls, so near objects cannot be seen clearly.

Correction: bifocal lenses — the upper portion is concave (for distant vision, needed if myopia coexists) and the lower portion is convex (for reading). Modern alternatives include progressive/graduated lenses with a smooth power transition. Presbyopia frequently coexists with myopia or hypermetropia, which is precisely why bifocals carry both signs in one lens.

Comparison Table: Myopia vs Hypermetropia vs Presbyopia

Solved Numericals: Lens Power for Vision Defects (Exam-Style)

Problem 1 (Myopia): The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required?

Solution: f = −80 cm = −0.8 m. P = 1/f = 1/(−0.8) = −1.25 D (concave lens).

Problem 2 (Hypermetropia): The near point of a hypermetropic eye is 50 cm. What lens power is needed to read at 25 cm?

Solution: u = −25 cm, v = −50 cm. 1/f = (−1/50) − (−1/25) = 1/50, so f = +50 cm = +0.5 m. P = 1/0.5 = +2.0 D (convex lens).

Problem 3 (Myopia, distant vision): A person cannot see objects beyond 1.5 m distinctly. Find the lens power.

Solution: f = −1.5 m. P = 1/(−1.5) = −0.67 D (concave).

Problem 4 (Combined concept): A myopic person uses −2.0 D glasses. What is the far point?

Solution: f = 1/P = −0.5 m. The far point is 50 cm from the eye.

Other Defects: Astigmatism and Cataract (One-Liner Facts)

• Astigmatism: blurred vision due to unequal curvature of the cornea in different planes; corrected with cylindrical lenses.
• Cataract: clouding of the eye lens in old age (or by disease); the lens becomes milky and opaque. Cured surgically — the opaque lens is removed and replaced with an artificial (intraocular) lens.

NEET & Board Exam Tips: Common Mistakes and Memory Tricks

• Sign discipline: Myopia power is always negative; hypermetropia power is always positive. Half the numericals lost in exams fail on sign convention.
• Unit of power is the dioptre (D): always convert f from cm to m before applying P = 1/f.
• Memory trick: Myopia = image moves “too early” (in front) → push back with concave; Hypermetropia = image “too late” (behind) → pull forward with convex.
• Do not confuse hypermetropia with presbyopia: hypermetropia is a structural defect (can occur at any age); presbyopia is age-related muscular weakening. Both use convex power for near vision, but their causes differ — a favourite NEET assertion–reason trap.
• For diagram questions, always label retina, eye lens, and the point of focus — CBSE marking schemes award stepwise marks.

Frequently Asked Questions

Q: What lens corrects myopia and why?

A concave (diverging) lens, because the myopic eye focuses the image in front of the retina; the concave lens diverges the rays so the image shifts back exactly onto the retina.

Q: What is the least distance of distinct vision for a normal eye?

25 cm — the minimum distance at which the eye can see objects clearly without strain. All hypermetropia numericals take the object at this distance.

Q: Can a person have both myopia and hypermetropia?

Not in the same eye — the eyeball cannot be both too long and too short. However, presbyopia can coexist with either defect, and such persons use bifocal lenses (upper part concave, lower part convex).

Q: How do you calculate the power of a corrective lens?

Use P = 1/f, with f in metres and P in dioptres. For vision defects, set the object at 25 cm (or infinity for distant vision correction) and the image at the defective eye’s near point or far point, then apply the lens formula.

Q: Is presbyopia a disease?

No. It is a natural, age-related weakening of the ciliary muscles and loss of lens flexibility, usually appearing after about 40 years of age. It is managed, not cured, with convex (or bifocal) lenses.

Related reading

• Significant Figures: How Honest Is Your Number?
• Dimensional Analysis: The X-Ray Machine for Formulas

Related explainers

Human Eye Defects for NEET & SSC: Myopia, Hypermetropia, Presbyopia Explained with Ray Diagrams and Numericals

Significant Figures: How Honest Is Your Number?

Dimensional Analysis: The X-Ray Machine for Formulas