How do you see the board?
As you are sitting in the classroom, you may at first be focused on a paper directly in front of you. Then you may look to the front of the room to see a math problem worked out on the board. How can your eyes focus directly in front of you and then, a second later, at a distance? Your eyes are rather amazing!
How the Eye Works
The job of the eye is to focus light. The parts of the eye (Figure below) help it to carry out its job. Follow the path of light through the eye as you read about it below.
The human eye is a complex structure that senses light; the light passes through the cornea, pupil, and lens, and is focused on the retina. The vitreous humor is the clear gel that fills the space between the lens and the retina of the eye, and the hyaloid canal is a small transparent canal running through the vitreous humor from the optical disc to the lens.
Vision involves sensing and focusing light from people and objects. The steps involved are as follows:
- First, light passes through the cornea of the eye. The cornea is a clear, protective covering on the outside of the eye.
- Next, light passes through the pupil. The pupil is a black opening in the eye that lets light enter the eye.
- After passing into the eye through the pupil, light passes through the lens. The lens of the eye is a clear, curved structure. Along with the cornea, the lens helps focus light at the back of the eye. This is pictured below (Figure below).
- The lens must bend light from nearby objects more than it bends light from far-away objects. The lens changes shape to bend the light by just the right amount to bring objects into focus.
- The lens focuses light on the retina, which covers the back of the inside of the eye. The retina has light-sensing photoreceptor cells called rods and cones. Rods let us see in dim light. Cones let us detect light of different colors.
- When light hits rods and cones, it causes chemical changes. The chemical changes start nerve impulses. The nerve impulses travel to the brain through the optic nerve.
- The brain makes sense of the nerve impulses and tells you what you are seeing.
Light from objects at different distances is focused by the lens of the eye. Muscles in the eye control the shape of the lens so the light is focused on the back of the eye no matter how far the object is from the lens.
cone: Cone-shaped cell in the retina of the eye that detects light of different colors.
cornea: Clear, protective covering on the outside of the eye.
lens: Clear, curved structure that helps focus light on the retina at the back of the eye.
optic nerve: Nerve which carries impulses from the eye to the brain.
pupil: Black opening in the eye that lets light enter the eye.
rod: Rodlike cell in the retina of the eye that is sensitive to dim light.
retina: Part of the eye that contains light receptor cells.
- Light entering the eye is focused by the lens on the retina, which sends messages to the brain through the optic nerve.
- Muscles in the eye control the shape of the lens so the light is focused on the back of the eye no matter how far the object is from the lens.
Use the resource below to answer the questions that follow.
- What are the three layers of the eye? Which one acts to maintain the eyes shape?
- Where does light enter the eye? How is the amount of light entering the eye controlled? Why is it valuable to organisms to control how much light enters the eye?
- What structure contains the rods and cones in the eye? Where are they located?
- Outline the path of light through the eye.
- Describe the role of the lens of the eye.