Eye

Eyes help us see.

They catch light from the world. This light goes into the eye. It hits a special layer in the back. This layer sends signals to the brain.

Some animals have many small eyes. These are called compound eyes. Insects use them to see motion.

Other animals have one single lens. Humans and rabbits have these eyes. They help us see shapes.

Eyes can even see in the dark. Some eyes see heat from hot vents. The world is full of light to see!

Eyes are organs that help living things see. They catch light from the world. Then, they turn that light into signals. These signals travel to the brain.

There are many ways to build an eye. Some animals have compound eyes. These eyes are made of many small parts. Each part has its own lens. Insects use these eyes to see motion well. Some can even see in a full circle.

Other animals have simple eyes. These have one single lens. Humans have these eyes. They focus light onto a layer called the retina. The retina is a light-sensitive part in the back of the eye. Inside the retina, there are two types of cells. Rod cells help you see in low light. Cone cells help you see colors.

Some very simple eyes are just spots. We call these pit eyes. They can only tell if it is light or dark. Even the simplest eyes help animals stay safe. They help animals find their way in the world.

An eye is a special organ that helps living things see. It works by catching light from the world around it. The eye turns this light into tiny electrical signals. These signals travel through nerves to the brain. The brain then turns those signals into an image we can see.

This process is part of a larger visual system. It allows animals to know where things are and what they look like.

Many different eyes work in different ways. One type is called a compound eye. These are common on insects and crustaceans. A compound eye is made of many small parts called facets. Each facet has its own tiny lens and sensor. Some insects have up to 28,000 of these sensors. This can help them see in a full 360-degree circle.

These eyes are very good at noticing motion.

Other animals have non-compound eyes, which are also called simple eyes. These eyes use a single lens to focus light. Humans, birds, and many reptiles have this type of eye. The light hits a layer at the back called the retina. Inside the retina, there are two main types of cells. Rod cells help you see in low light. Cone cells help you see different colors.

This is how we see the world in detail.

Eyes have been around for a very long time. The very first proto-eyes likely evolved about 600 million years ago. This happened around the time of the Cambrian explosion. Since then, eyes have changed into many different shapes. Some extinct animals, like trilobites, had very unique eyes. Their lenses were made of clear calcite crystals.

Most other insects have eyes made of softer materials.

Even the simplest eyes help animals stay safe. Some animals have pit eyes, which are just small eye-spots. These spots can tell if it is light or dark. This helps them avoid bright sunlight or find their way. Other animals, like the mantis shrimp, have amazing color vision. They can see many more colors than humans can. Whether they are huge or tiny, eyes help life find its way.

An eye is a sensory organ used to perceive visual information. It works by detecting light and converting it into electro-chemical impulses in neurons. These impulses travel through neural pathways to the brain. In many higher organisms, the eye acts as a complex optical system. It collects light from the environment and regulates its intensity using a diaphragm. An adjustable assembly of lenses then focuses this light to form an image. This image is converted into electrical signals and sent via the optic nerve to the visual cortex.

There are ten fundamentally different eye layouts in nature. These are categorized into compound eyes and non-compound eyes. Compound eyes consist of multiple small visual units called facets. These are common in insects and crustaceans. Some compound eyes contain up to 28,000 sensors arranged hexagonally. This arrangement can provide a full 360° field of vision.

Because they use many small sensors, compound eyes are very sensitive to motion. They can produce a single pixelated image or multiple images per eye.

Non-compound eyes, often called simple eyes, use a single lens. These eyes focus light onto a light-sensitive layer called the retina. This type of eye is common in mammals, including humans. The retina contains specialized cells to process light. Cone cells allow for color vision, while rod cells help with low-light contrasts.

In many vertebrates, the eye is a spheroid shape filled with a transparent gel called vitreous humour. An iris often sits in front of the pupil to control light levels.

The simplest form of an eye is the pit eye, or eye-spot. These are small, sometimes comprising only 100 cells covering 100 μm. They may be set into a pit to reduce the angle of incoming light. This allows the organism to deduce the direction of the light source. Some insects also have ocelli, which are very simple eyes. Ocelli lack a lens to project a sharp image, so they mostly detect changes in light intensity. This helps animals, such as flying insects, sense which way is up.

Evolutionary history shows that eyes have changed significantly over time. The first proto-eyes evolved about 600 million years ago during the Cambrian explosion. The last common ancestor of animals already possessed the biochemical toolkit for vision. Advanced eyes have since evolved in 96% of animal species across six main phyla.

Extinct animals like trilobites had unique compound eyes made of clear calcite crystals. Most modern arthropods have much softer eyes compared to those trilobites.

Different environments have led to specialized visual adaptations. For example, some organisms living near deep-sea vents have compound eyes that see infra-red light. This helps them avoid being boiled by hot vents. The mantis shrimp possesses the world's most complex color vision system, known as hyperspectral color vision. In terrestrial animals, a refractive cornea helps focus light. However, animals like penguins that returned to the water lose this curved cornea. They rely more on lens-based vision because the refractive index of water is similar to their internal fluids.

Vision also serves functions beyond just making images. Retinal photosensitive ganglion cells can send signals to the suprachiasmatic nuclei. This process helps organisms manage circadian adjustment, or their internal biological clock. These cells also signal the pretectal area to control the pupillary light reflex.

This shows how the eye is connected to broader biological systems that regulate life cycles and light responses.