Eye Anatomy: How Do Eyes Focus?

The eye is a complex organ that manipulates light much like a camera does; it has lenses to change the focus and a sensor to recognize intensity and color.
How Does the Eye Focus? Starting with the Cornea
The first layer of the eye that light hits is the cornea, the surface of the eye. The cornea is a dome-shaped lens that starts the process of focusing light, contributing approximately two-thirds of the eye’s focusing power. But the cornea is like the lens of eyeglasses – it always refracts light the same amount, unlike the lens of a camera which can focus at different depths.
The shape of the cornea is maintained by the aqueous humour, a gel that lies between the cornea and the lens.
The Pupil and Iris Regulate Amount of Light
The iris is the colorful part of the eye. The pupil, the black spot in the middle of the iris, is actually just a hole in the iris, which can contract or relax to adjust the size of the pupil. In low light, the pupil expands to allow more light into the eye. In bright light, it contracts to protect the eye and increase contrast.
The Lens Focuses Light
Behind the pupil lies the crystalline lens, which is responsible for focusing light. The lens can change its focal length, like a camera. This is what allows you to focus close up on a book, or far away at the horizon, but not both at the same time. To switch from one to the other, the lens actually bends and changes shape because of contractions in the cilliary muscles.
A healthy lens is critical to good vision. As people age, the lens can become cloudy, causing a cataract, or stiff, causing presbyopia. When the lens becomes stiff, the cilliary muscles can no longer change the shape of the lens to focus on up close objects.
The Retina Detects Light
From the crystalline lens, light travels through another gel known as the vitreous humor, which maintains the shape of the eye, to the retina in the back of the eye. The retina contains light-receptor cells known as rods and cones. Rods are very sensitive and simply detect light, giving us our nearly-colorless night vision, while cones detect different colors. Cones are concentrated in the fovea, a pit in the center of the retina, providing very sharp central vision.
Oddities of the Retina: Flipped and Holey
The lens projects an image onto the retina, but it is rotated 180 degrees (upside down and backwards). If you flip upside down to watch a movie, what you see is actually what is being projected onto the retinas of your bemused friends. This is because the shape of the lens causes light to converge through a single point inside the lens, emerging out the back like light leaving a projector.
There is also a gap in your vision known as the blind spot, where blood vessels and nerves pass through the retina. So why don’t you see a flipped world with a hole in it? The brain corrects for both of these, providing you with a properly oriented image and filling in the blind spot with the surrounding color.
To recap: light is partially focused when it passes through the cornea, then travels through the aqueous humour to the crystalline lens, which lets the eye focus on different depths. The light converges in the lens and travels out the other side flipped, traveling through the vitreous humour to the retina on the inner back surface of the eye, where rods and cones detect light. Then your brain presents a coherent, correctly-oriented image.

The Most Common Eye Colors

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But now for our weekly blog, what really are the most common eye colors?.

Eye Color Guide – The Most Common Eye Colors
The human eye is both beautiful and unique. Much like a fingerprint, each individual’s eye color is specific only to them, with no others sharing the same shape, color and appearance. So what eye colors are the most common, and which are the rarest?

What is the most common eye color? Which eye color is the rarest?

Brown Eyes
Brown eyes are the most common eye color in the world with over 55% of the world’s population having brown eyes. Brown eye color is a dominant genetic trait, and is created by the presence of melanin in the eye. Over fifty percent of the world’s population has brown eyes, with nearly all individuals from Africa and Asia sharing brown eye color. Brown eyes are typically darker than other eye colors, and they may even appear black in certain individuals.

Hazel Eyes

Hazel eyes are similar to brown eyes, although they are typically lighter in color, and have more of a green-yellow tint. Hazel eyes have a higher concentration of melanin (pigment) around the eye’s border, which can result in a multi-colored appearance that varies between copper and green depending on the lighting. Most people estimate that around 5-8% of the world’s population has hazel colored eyes.

Blue Eyes

Blue eyes are genetically recessive, and therefore much less common worldwide. Blue eyes are formed by the absence of pigments in the eye, where the blue color is formed by the scattering of light as it’s reflected off the iris. While blue eyes are less common than brown eyes, they are frequently found from nationalities located near the Baltic sea in northern Europe. It’s estimated that approximately 8% of the world’s population has blue eyes.

Green Eyes

Green eye color is often confused with hazel eye color, yet is entirely separate and distinct. Green eye color is the rarest color found around the world, and it is estimated that only around 2% of the world’s population has green colored eyes. Green eye color is a result of a mild amount of pigmentation in the eye with a golden tint. When combined with the natural blue scattering of the eye, the colors mix to give a green appearance. Green eye color is most common in northern and central Europe, but can also be found in western Asian cultures on rare occasion.

Silver Eyes

Silver eye color is also quite rare, although many consider silver eyes to be a variation of blue eye color. Like blue eyes, silver eyes are the result of a very low amount of pigmentation in the eye, which reflects a gray-silver appearance. Silver eye color is most common in eastern European countries, and is one of the rarer eye colors worldwide.

Amber Eyes

Amber eyes show off a yellow-copper tone, which results from the yellow colored pigment lipochrome. Amber eyes are very rare worldwide, and are most common in Asia and South American countries. Amber eye color can range from golden yellow to a more copper tone.

Why blue-eyed men prefer blue-eyed women – but not vice versa

Most people learn in high school (if they’re paying attention) that human eye color is a genetic trait which follows fairly simple rules. The primary gene that controls eye color exists in several forms, called alleles. The protein produced by one of the alleles causes the eyes to be brown, while a variant allele producing a slightly different protein does not. Since the human genome contains two copies of each gene, if even one of these copies is the allele for brown eyes, brown will be the resulting color, regardless of the other allele. Such an allele is said to be “dominant”. Another eye color (e. g. blue) will result only if both copies of the gene are non-brown alleles. Such alleles are said to be “recessive”.

Children receive one copy of each paired chromosome from each parent. It follows that if one parent has two copies of a dominant gene, every one of their children will receive at least one copy, regardless of what the other parent has. All children of this mother and father will have brown eyes, if even one parent has two brown eye alleles, and even if the other parent has two blue eye alleles.

If both parents have one brown and one blue allele, then for any particular child, there’s a 1 in 4 chance of receiving two brown alleles, a 1 in 4 chance of receiving two blue alleles (the only case that will result in blue eyes), and a 2 in 4 chance of receiving one brown and one blue allele (hence brown eyes). If one parent has blue eyes, and the other has both a brown and a blue allele, then the odds are 50/50 for each of their children to have either brown or blue eyes. So if one or both parents have brown eyes, it’s possible for them to have blue-eyed children. But when both parents have blue eyes, so all of their alleles are for blue eyes, all of their children will have blue eyes. In that case, if any child has brown eyes, it must be the case that one parent – most likely the male – is not the biological parent. Oops.

So a blue-eyed man has an interesting advantage over men with brown eyes – a very dependable way of knowing that he is not the father of a particular child, provided he mates with a blue-eyed woman. Further, a blue-eyed man who regards blue-eyed women as more attractive than women of other eye colors is more likely to mate with blue-eyed women. And so such a blue-eyed man has a selective advantage over other blue-eyed men who have no such preference (or a preference for brown-eyed women).

This would be advantageous, at least in prehistoric times, if in addition such a man was less inclined to provide for a child without blue eyes – even if there was no conscious recognition that the child could not be his own. Some recent research has indicated that blue-eyed men sometimes actually, if unconsciously, do have a tendency to regard blue-eyed women as more “attractive”, and hence (presumably) are more likely to choose them as mates:

Eighty-eight male and female students were asked to rate facial attractiveness of models on a computer. The pictures were close-ups of young adult faces, unfamiliar to the participants. The eye color of each model was manipulated, so that for each model’s face two versions were shown, one with the natural eye color (blue/brown) and another with the other color (brown/blue). The participants’ own eye color was noted.

Both blue-eyed and brown-eyed women showed no difference in their preferences for male models of either eye color. Similarly, brown-eyed men showed no preference for either blue-eyed or brown-eyed female models. However, blue-eyed men rated blue-eyed female models as more attractive than brown-eyed models.

Since a mother almost always can be sure a given child is hers (except for rare events like accidental switching of infants), a mechanism that provides a way to recognize that a child isn’t her own provides little additional advantage. And so, blue-eyed women do not have an evolutionary advantage from a tendency to regard blue-eyed men as more attractive than others. So they do not, in fact, have that tendency.
If you’re looking around for an example of specific, and unexpected, behavior for which evolutionary psychology offers the simplest explanation, this may be a good choice.

What Determines Our Eye Color

If the eyes are the windows to the soul, then what colour are your windows? Baby blue? Big and brown? Green or hazel? Haven’t you ever wondered about how your eyes came to be the colour they are?

What determines eye color? Did your parents pick them out for you? Not exactly, but they did play a big part in determining what color eyes you have. How? They passed along their genes to you!

what determines eye color

There are two main factors that help determine your eye color: the amount and pattern of dark brown pigment (called melanin) in the part of your eye called the iris and the way in which the iris scatters light that passes through the eye. The more important factor is the pigment, which is determined by your genes.

Eye Color DNA Interchange

Deep inside the nucleus of your body’s cells are 46 chromosomes, divided into 23 pairs. When you were conceived, you inherited one chromosome from each parent to make each pair of your chromosomes.

Chromosomes are comprised of pieces of DNA called genes. These genes, which also come in pairs, determine many of your characteristics you will develop. Scientists believe that as many as 16 different genes play a role in determining eye color. The two main genes believed to be responsible are OCA2 and HERC2, both of which are part of chromosome15.

Genes are, in turn, made up of alleles that ultimately determine whether any particular characteristic will appear. For each trait you can inherit, there are two alleles. If the two alleles are the same, they are homozygous. If they are different, they are heterozygous. For each trait, one allele (dominant) is expressed (the trait it represents appears), while the other allele (recessive) is unexpressed (the trait it represents does not appear). Recessive alleles are only expressed if there is no dominant allele present.

For example, the alleles for eye color can be separated into blue, green, and brown. Green alleles are dominant over blue alleles, and brown alleles are dominant over both blue and green alleles. If you received a blue allele and a brown allele, your eye color would be brown because brown is the dominant allele. If you have blue eyes, that means you received blue alleles from both parents.

What determines your eye color are your genes, which dictate how much (and where) melanin is produced in your iris. The more melanin produced, the darker the eye color will be. Because melanin production does not begin at birth, babies’ eyes appear blue. True eye color will be determined over time. It’s usually not until age three that a child’s true permanent eye color reveals itself.

Have you ever noticed how some people’s eyes seem to change colour depending upon the lighting? That occurs because the iris has two layers. Sometimes there is pigment in both layers. In people with blue or green eyes, however, the front layer will have very little or no melanin. Depending on the amount and diffraction of light, their eyes may appear to change colors.

Some people have two different eye colors. This results from a condition called heterochromia. It’s very rare, but usually harmless. It occurs due to differences in the early stages of iris development.