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how does the human eye work

The fascinating optical instrument on the planet is the human eye. It is a remarkable creation of nature that forms detailed and colored images. People who can see clearly with their eyes are said to possess a normal vision. However, at the same time, some have trouble seeing the objects.

Functioning of the human eye

A light-sensing image is formed when the lens and cornea work together. The light-sensing retina has the densest concentration of receptors to form an image. These receptors are present in the fovea, along with a blind spot. The power of the lens can be adjusted to provide a clear image of the retina. The center of the image must fall on the fovea to get a sharp visual field. The variable opening of the pupil controls the amount of light to enter the eye. This adaptation can detect light intensities from lowest to 1010 times without damaging the eyes.

A variety of functions, such as sense direction, movement, sophisticated colors, and distance, are all performed by our eyes. The process of visual nerve impulses begins when the retina connects the brain. Then, the message from the eye to the brain is conveyed by the optic nerve.

Importance of refractive index

Rather than on the lens, the most significant change in the refractive index, and bending of rays, occurs at the cornea. Due to the change in speed of light, the cornea provides about two-thirds of the power of the eye. The lens provides the remaining one-third of the power to make a complete image on the retina. Therefore, even though the light passes through several layers, like aqueous humor, cornea, and different layers on the thin lens, both cornea and lens can be treated as a single thin lens.

The below table shows refractive indices relevant to the human eye-

Material                        Index of Refraction
Water                            1.33
Air                                1.0
Cornea                          1.38
Aqueous humor            1.34
Lens                              1.41 average (variable)
Vitreous humor             1.34

The above image shows the image formed on the retina when the rays of light enter and exit the lens yet converge at the cornea. An inverted and clear image is formed on the retina when the object is traced.

The above image shows a relaxed and accommodated vision for distant and close objects. (a) To produce an image on the retina, light rays from the same point on a distant object must be nearly parallel while entering the eye and converge. (b) Light rays from a nearby object can diverge more but still enter the eye. If they were parallel, a more robust lens would have been needed to converge the image.

The power of lens is given by-

P = 1/f

Rewriting this equation-


di must equal the lens-to-retina distance to obtain clear vision. This sort of normal vision is possible for objects kept at a distance do = 25 cm to infinity.

Home experiment for the pupil

In this experiment, you have to look at the pupil of someone’s eye. Allow a beam of normal light to pass through their eyes. Note an approximate diameter of the pupil. Now, turn off the lights. Note down the diameter in this case as well. Whatever you have observed, you can note it down and make an observation yourself.

Size of an image on the retina

What is the image size on the retina of a 1.20 × 10−2 cm diameter human hair, held at arm's length (60.0 cm) away? Take the lens-to-retina distance to be 2.00 cm.

We can use a simple strategy to solve the above question. We want to find the height of the image hi when we know the object's height is ho = 1.20 × 10−2 cm. We also know that the object is 60.0 cm away so that d­o=60.0 cm. For clear vision, the image distance must equal the lens-to-retina distance, di = 2.00 cm. We can use the following equation-

hiho = - dido = m


We know,


Rearranging them, we get,


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