The Human Eye

Image Formation in the Human Eye: The process of vision involves light refraction, focusing, and signal transmission to the brain.

Steps of Image Formation

1. Light enters through the cornea and is refracted (bent).
2. It passes through the aqueous humour, pupil, and crystalline lens, which adjusts focus.
3. The vitreous humour further directs the light toward the retina.
4. Retina's rod and cone cells detect light intensity and colour.
5. Optic nerve transmits electrical signals to the brain.
6. The brain processes the image, correcting its inversion to provide a clear visual experience.
7. This process is similar to a convex lens, where light converges to form an inverted image on the retina.

Role of the Iris and Pupil in Light Control:

• The iris adjusts the pupil size based on the intensity of light.
• Bright light: The pupil contracts to allow less light into the eye.
• Dim light: The pupil expands to allow more light to enter.
• This mechanism protects the retina from excessive light and ensures clear vision in different lighting conditions.

Accommodation of the Eye:

The ciliary muscles adjust the lens curvature to change its focal length, helping to focus on objects at different distances.

The change in the shape of the lens while seeing distant and nearby objects

Focusing on Distant and Near Objects

1. For distant objects:

Ciliary muscles relax → Lens becomes thin → Focal length increases → Objects appear clear.

2. For nearby objects:

Ciliary muscles contract → Lens becomes thicker → Focal length decreases → Objects appear clear.

This adjustment in focal length is known as accommodation.

Vision Range and Limits: Near Point and Far Point

1. Near Point (Least Distance of Distinct Vision): The minimum distance at which an object is seen clearly without strain.  For a normal eye: 25 cm.
2. Far Point: The maximum distance at which objects can be seen clearly.  For a normal eye: Infinity.

Effect of Age on Near Point:

• With age, the eye’s ability to accommodate decreases.
• For older individuals (~60 years), the near point increases to about 200 cm, requiring corrective lenses.

Refractive Index of Eye Components: Different parts of the eye have varying refractive indices, allowing light to bend and focus correctly on the retina.

These differences ensure proper refraction and image formation.

Persistence of Vision:

• The image on the retina does not disappear instantly after the object is removed.
• The image remains for about 1/16th of a second after the object is no longer in sight.  This continuation of visual sensation is called persistence of vision.

Application in Motion Pictures: Movies and animations use 24 frames per second to create a smooth visual effect.  This ensures that images blend smoothly without flickering, giving the illusion of continuous motion.

• The human eye works like a camera, forming a real and inverted image on the retina, which is light-sensitive.
• The cornea allows light to enter the eye and performs most of the refraction, while the lens fine‑tunes the focus.
• The iris controls the pupil, regulating the amount of light entering the eye—contracting in bright light and widening in dim light.
• The power of accommodation is the ability of the eye lens to change its focal length by altering its curvature using the ciliary muscles.
• For a normal eye, the near point is 25 cm and the far point is at infinity.