PhysicsThe human eye and the colourful worldClass 10Class 12

Human eye & defects of vision: interactive simulation

The human eye functions like a camera: light enters through the transparent cornea, passes through the pupil, and is focused by the crystalline lens onto the light-sensitive retina.

Useful for: CBSE Boards, NEET, ICSE Class 10

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Accommodation allows the ciliary muscles to alter the curvature of the crystalline lens so both distant and near objects focus sharply. When the eyeball is elongated or the lens is too curved, Myopia occurs (rays focus in front of the retina). When the eyeball is shortened or the lens is too flat, Hypermetropia occurs (rays focus behind the retina).

  1. 1

    Click "2. Myopia" and look at where the light rays meet.

    What you should see

    Light rays from infinity converge and focus IN FRONT OF the retina because the eyeball is elongated. The image on the retina is a blurry circle.

  2. 2

    Now toggle "Spectacle Lens Correction" ON and select Concave (-3.0 D).

    What you should see

    The concave lens slightly diverges the parallel incoming rays before they enter the eye, pushing the focal point back onto the retina for 20/20 sharp vision!

  3. 3

    Click "3. Hypermetropia" and notice the focal point.

    What you should see

    Rays from a nearby reading book (25 cm) focus BEHIND the retina. Turning on a Convex (+2.75 D) lens provides extra convergence to focus rays onto the retina.

  4. 4

    Move the "Ciliary Muscle Contraction" slider from relaxed to thick.

    What you should see

    Thicker lens = shorter focal length = stronger convergence. This is the Power of Accommodation (up to ~4 Diopters in young eyes).

Formulas behind it

Lens Formula

1f=1v−1u\frac{1}{f} = \frac{1}{v} - \frac{1}{u}

Power of a Lens

P=1f (in metres)(Unit: Dioptre, D)P = \frac{1}{f\text{ (in metres)}} \quad (\text{Unit: Dioptre, D})

For Myopia (Far point = d)

f=−d,P=−1d(Negative / Concave)f = -d, \quad P = -\frac{1}{d} \quad (\text{Negative / Concave})

For Hypermetropia (Near point = d, D = 25 cm)

f=d×Dd−D,P>0(Positive / Convex)f = \frac{d \times D}{d - D}, \quad P > 0 \quad (\text{Positive / Convex})

Least Distance of Distinct Vision

D=25 cm(Normal near point)D = 25\text{ cm} \quad (\text{Normal near point})

Common mistakes

  • Confusing which lens corrects which defect: Myopia (Short-sightedness) is corrected with a CONCAVE lens; Hypermetropia (Long-sightedness) is corrected with a CONVEX lens.
  • Thinking myopic people cannot see near objects. Myopic people CAN see near objects clearly; they cannot see distant objects distinctly.
  • Forgetting that concave lens focal length and power are always NEGATIVE (-D), while convex lens power is always POSITIVE (+D).
  • Confusing Hypermetropia with Presbyopia. Presbyopia is caused by aging (loss of ciliary muscle power and lens flexibility), whereas hypermetropia is usually due to eyeball shape or cornea curvature.
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FAQ

Why does light refract most at the cornea rather than the eye lens?

The greatest difference in refractive index occurs at the air-cornea interface (1.00 to 1.376), providing roughly 70–80% of the eye’s total focusing power. The crystalline lens provides fine-tuning and accommodation.

What causes Myopia (Near-sightedness)?

Myopia is caused by (1) excessive curvature of the eye lens (too thick/convergent), or (2) elongation of the eyeball from front to back.

What causes Hypermetropia (Far-sightedness)?

Hypermetropia is caused by (1) focal length of the eye lens being too long (insufficiently converging), or (2) the eyeball becoming too short/flattened.

What is Astigmatism and how is it corrected?

Astigmatism occurs when the cornea or lens has different curvatures in different meridians (like a rugby ball instead of a football). It is corrected using Cylindrical lenses.

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