The tiny rings of muscle that make your vision sharp
cameras and human eyes both focus light using a lens. This structure bends the incoming wavelengths so that they hit the right spot on a photographic plate, or on the back of the eye. A camera lens is made from solid glass, and focuses on near and distant objects by physically moving closer or further away. A biological lens is actually squishy, and it focuses by physically changing shape. In the eye, this process is known as ‘accommodation’, and is controlled by a ring of smooth muscle called the ciliary muscle. This is attached to the lens by fibers known as suspensory ligaments. When the muscle is relaxed, the ligaments pull tight, stretching the lens until it is flat and thin. This is perfect for looking at objects in the distance. When the ciliary muscle contracts, the ligaments loosen, allowing the lens to become fat and round. This is better for looking at objects that are nearby. The colored part of the eye (called the iris) controls the size of the pupil and ensures the right amount of light gets through the lens.
The lens is responsible for focusing the light on the back of the eye.
A ring of muscle surrounding the lens can pull it tight, or let it relax.
When the muscle relaxes, the ligaments are pulled tight.
Seeing in three dimensions human eye
Each eye sees a slightly different image, allowing the brain to perceive depth
Our eyes are only able to produce two-dimensional images, but with some clever internal processing, the brain is able to build these flat pictures into a three-dimensional view. Our eyes are positioned about five centimeters (two inches) apart, so, each sees the world from a slightly different angle. The brain then compares the two pictures, using the differences to create the illusion of depth.
Due to the positioning of our eyes, when objects are closer than about 5.5m (18ft) away, each eye sees a slightly different angle.
The incoming signals from both eyes are compared in the brain, and the subtle differences are used to create a three-dimensional image.
Try it for yourself
By holding your hand in front of your face and closing one eye at a time, it is easy to see the different 2D views perceived by each eye