The Eye Summarized Biology Notes

 Learning Objectives

● identify the structure of the eye, limited to cornea, iris, pupil, lens, retina, optic nerve and blind spot, and state the function of each part
● explain the pupil reflex in terms of light intensity and antagonistic action of circular and radial muscles in the iris
● explain accommodation to view near and distant objects in terms of the contraction and relaxation of the ciliary muscles, tension in the suspensory ligaments, shape of the lens and refraction of light
● distinguish between rods and cones in terms of function and distribution of structure in the retina of a human

The Eye

Your eyes inform you about the colour, shape, size and position of objects. How is this possible? You see objects when the light that they reflect forms a picture, or image, on the retina at the back of your eye.

The pupil reflex

The size of the pupil changes automatically when there is a change in the intensity of the light. This automatic change is known as the pupil reflex. If you shine a bright light into the eye, the pupil will become smaller. If you move into a dark room, the pupil will become larger. The iris controls how much light enters the eye. In dim light, the iris gets thinner, so that the pupil increases in size; this lets more light into the eye. In bright light, the iris becomes wider and makes the pupil smaller; this stops too much light from entering the eye and damagirng the retina. Figure below shows how the iris changes the size of the pupil.

The iris Contains two sets of antagonistic muscles that allow the iris to adjust the size of the pupil. Circular muscles lie in circles around the pupil; when they contract, they make the pupil constrict, or get smaller. Radial muscles run outwards from the edge of the pupil; when they contract, they make the pupil dilate, or get larger. The pupil retlex involves an electrical impulse being transmitted from the retina to the brain, and another message from the brain being transmitted to the muscles of the iris.

 Structure of the eye

The human eye is made of many ditterent parts or structures. Each structure has its own job to do, described as its function. Figure below shows the structure of the human eye.

Identify each part on Figure above as you read about its function.
• The cornea bends the light rays that enter the eye through the pupil, which is the hole in the iris,
• The pupil, or black dot at the centre of the eye, is a hole through which Iight can enter the eye.
• The iris, or coloured part of the eye, surrounds the pupil so that light cannot pass through. It is made up of the circular and radial muscles, which contract and relax to alter the size of the pupil. The iris controls the amount of light entering the eye.
• The lens is a transparent, bi-convex, flexible disc behind the iris, attached by the suspensory ligaments to the ciliary muscles It focusesight onto the retina.
• The retina is a layer of light-sensitive receptor cells. These are called rods - sensitive to dim light, and black and white - and cones that are sensitive to colour. A small area called the fovea in the middle Of the retina has many more cones than rods. Light is focused onto the retina and the optic nerve carries the impulses from the retina to the brain. The blind spot is where the optic nerve leaves the eye.

• The optic nerve is a bundle of sensory neurones at the back of the eye. It carries impulses from
the eye to the brain.
• The blind spot is a small area in the retina that is insensitive to light. It is also referred to as the optic disk within the retina. There are no photoreceptors (that is, rods or cones) in the blind spot, and, therefore, there is no image detection in this area.

 Rods and Cones

The rods and cones are light-sensitive cellsfound on the retina. Rods and cones both Contain light-sensitive pigment. When light falls on the pigment, the pigment breaks down. It is this breakdown of pigment that causes the light energy to be changed into the electrical energy of nerve impulses, which are sent to the brain. Rods work in dim light and cannot distinguish colour; they occur all over the retina except at the fovea. Rods do not give as sharp an image as cones.

Cones are stimulated by bright light and can distinguish colour; different cones detect a red, green and blue light. Cones occur mainly at the centre of the retina, particularly in the fovea (yellow spot) that is directly in line with the centre of the lens. When light retlects from objects into the eye, the rods and cones in the retina are stimulated and messages are sent along the optic nerve to the brain. The brain builds up an image of the object by interpreting the various impulses from the rods and cones. If you have to look at something in detail, you usually hold it directly in front of your eye. You only have to watch someone reading to see this happening. Images normally fall on the fovea because the eye muscles are continuously moving the eye to place images on it. Images formed on the fovea are seen in much sharper detail than images falling on any other part of the retina. Where the optic nerve leaves the retina, there are no light sensitive receptor cells. This area is called the blind Spot. When the  image from an object falls directly on this point, you do not see the object. Figure below shows the retina

 The blind spot

There are no receptor cells where the optic nerve leaves the retina. This area is called the blind spot. When light talls on this part of the retina, no messages will be sent to the brain and you cannot see the object.

Practical investigation: How to find your blind spot

Spend about 5 minutes on this activity.
1. Hold your at arms length, straight in front of you. Close your right eye. Look at the dot on the right with your left eye.

2. Bring the book slowly towards you, keeping your right eye closed and your left eye open, looking at the dot. At first you are able to see the cross. As you move the book towards you, the cross disappears. Write down why you think
the cross disappears.

How the eye focuses light

 Light and lenses

You need to know something about lignt and lenses to understand how the eye works. Light travels in straight lines, called light rays. Light rays come from the sun or from other sources of light, such as candles, gas lamps or electric lighting. These light rays fall on the things around you and are reflected in all directions. When the reflected light rays enter the eye, the eye has to bend (refract) the light rays inwards, so that they come together and form a sharp image on the retina. Light bends when it passes from one transparent medium to another, such as from air into a liquid or a solid. An example of this is when light passes through the lens of the eye. A lens is any transparent object that has atleast one curved surface, as shown in Figures below.

There are two sorts of lenses:

Figure below) shows a converging lens that has a convex shape. It is thicker in the middle and bends light inwards.

Figure below) shows a diverging lens that has a concave shape. It is thinner in the
middle and bends light outwards.

Light rays entering the eye must be bent first by the cornea and lens, so that the rays converge onto the retina at the back of the eye. The image that is focused onto the retina is upside down. The optic nerve carries the impulses to the brain that interprets the impulses asa picture, so that you see the image the right way up. Figure below shows how the cornea and lens focus light onto the retina.

 Focusing on near and far objects - accommodation

Your eyes are able to adjust to focus on near and far objects by changing the shape of the lens. This is called accommodation. Accommodation describes the change in the shape of the lens to focus light coming from different distances onto the retina. Light rays from different sources have to be bent by different amounts to form a clear image on the retina.
● Light rays coming from a nearby object are going away from.one another, or are diverging; they will need to be bent inwards at quite a sharp angle.
● Light rays coming from an object in the distance will be almost parallel to one another; they will not need so much bending.

How the lens changes shape

The lens is connected to ciliary muscles by suspensory ligaments. Contraction and relaxation of the ciliary muscles cause changes in the tension of the suspensory ligaments and therefore changes in the shape of the lens.
● When the suspensory ligaments are pulled tight. they pull on the lens and make the lens thinner.
● When the suspensory ligaments are not pulled tight, the lens is fatter.

 Accommodation for near vision

Figure below shows how your eye accommodates to focus on near objects.

When you look at a nearby object, your lens has to bend the light rays more:
● the ciliary muscles.contract
● the suspensory ligaments slacken
● the lens becomes fatter
● light rays are bent (refracted) more
● the image of a nearby object is focused on the retina, as shown in figure above.

Focusingon a nearby object figure

Accommodation for far vision

Figure below shows how your eye accommodates to focus on objects that are far away.

When you look at a distant object, your lens has to bend the light rays less:
● the ciliary muscles relax
● there is more pull on the suspensory ligaments, so they become tighter
● the lens becomes thinner
● light rays are not bent (refracted) as much
● the image of the distant object is focused on the retina, as shown in Figure below.

 The End