Topic: Eye - organ of sight

Author: Elżbieta Szedzianis

Target group

7th grade students of an eight‑year elementary school.

Core curriculum

10. Sensory organs. Student:

1) recognizes the elements of the eye (on a model, drawing, according to the description, etc.) and describes their functions in creating an image, conducts an observation that proves the existence of the optic disc nerve;

2) presents the reasons why eye defects happen and the methods of correcting them (myopia, hyperopia, astigmatism).

Lesson aim

The students describe the connection between the construction and functions of an eye.

Key Success Criteria

• you will describe the build of the eye;

• you will prove on four examples that the build of the eye is connected with its functions;

• you will explain how visual sensations are created;

• you will explain what myopia, hyperopia and astigmatism are.

Key competences

• communicating in the mother language;

• communicating in a foreign language;

• Mathematical competence and basic competences in science and technology; 

• digital competence;

• learning to learn;

• Social and civic competences.

Methods/forms of work:

Work with text, observation.

Individual and group work.

Teaching measures:

• abstract;

• interactive whiteboard or traditional blackboard;

• tablets/computers;

• jigsaw puzzle - one for each group.

Lesson plan overview (Process)

Introduction

The teacher gives the topic of the lesson, key success criteria and presents the proposed course of the class.

Realization

1. The teacher divides students into groups. Each group receives a jigsaw puzzle - two fragments of an abstract that were cut into sentences: “Sclera, choroidea, retina” and “How are images created”. In groups, the students draw sentences and read them out one after another, and then arrange them so that they form a description of how an eye is built and how visual sensations are created. After assembling the jigsaw puzzle, they draw a schematic drawing that represents the build of the eye.

2. The teacher shows an interactive illustration “The build of the eye” and asks the students to compare their
   schematic drawings with the illustration. On a scale 1 to 5, the students assess how closely their drawings resemble the interactive illustration. They correct potential errors.

3. The students on an interactive illustration (projected onto the interactive whiteboard) draw the path that light rays travel inside the eye. They explain how visual sensations are created.

4. The students describe the phenomenon of how the pupil diameter adapts as an unconditioned reflex. They explain why recognizing objects (e.g. recognizing a friend) is the effect of an unconditioned reflex.

5. The students conduct observations that are supposed to determine the existence of the blind spot on the retina (“Observation 1”). They write down their observations and explain the results.

6. The students read the fragment titled “Eye illnesses and defects” and analyze the illustration
   in the photo gallery “Eye defects and ways of correcting them”.

Summary

7. Students complete interactive exercises.

8. The teacher asks the students to finish the sentence: “During this lesson, I have learned...”.

Homework

Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Eye - the organ of sight

An eyeball has a spherical shape. Its outer layer is composed of three membranes. Sclera surrounds the eyeball and protects it from injuries. It also gives it its shape. At the back part of the eyeball it is thick and opaque, whereas in the front of the eyeball, where we call it cornea, it is thin and transmits sun rays.

Under the sclera there is the choroid, which has blood vessels, it surrounds the eye and provides it with oxygen and products of metabolism. In the front part of the eyeball, the choroid converts into the iris, which has the shape of a ring. The opening in that ring is the pupil. In the iris there are smooth muscles, thanks to which it can change the diameter of the pupil and regulate the amount of light transmitted into our eye. Adapting to the intensity of light is reflex. The colour of the iris depends on the amount of the pigment – melanin.

The most internal membrane of the eyeball is the retina. It has 2 different types of sensory cells that are sensitive to light – cone cells and rod cells. Rod cells are sensitive to the intensity of the light – thanks to them our brain differentiates between different shades of gray, but not colours. Cone cells ensure chromatic vision and are active only when the light is bright. This is the reason why we don’t see colours well in semidarkness. Each cone cell is responsible for catching light of different colour (wave lenght). On the retina there is 125 million rod cells and 6 million cone cells, whereas rod cells are found on the outside part of the retina and cone cells are found at the rear part of the eyeball. Its highest concentration (250 thousand per mmIndeks górny 2²) is found in the point called macula lutea. It is the area of the sharpest vision. On the retina there is also a place that has no sensory cells. It is the blind spot, from which departs the optic nerve that transfers neural impulses to the encephalon.

After passing through the cornea, the light reaches the pupil behind which is the lens. It is transparent and its shape can change. Sun rays go through the lens and then through the vitreous humour (vitreous body) that lies directly behind it and that consists in 98% of water. Next, they reach the surface of the retina. On the way, they become refracted by the cornea and by the lens, thanks to which the sharp image is created on the retina.

Light stimulates receptor cells on the retina. Each one of them has photosensitive pigment which, under the influence of the light energy, decomposes and causes the creation of a neural impulse. The impulse from the receptor cells travels to the optic nerve and then, through the optic nerve, to the optic cortex in the brain. There, neural impulses are translated and interpreted (recognized), thanks to which we learn how our surroundings look like.

The most sensitive part of the retina is the macula lutea which consists mainly of cone cells. Image which is created there is sharp, clear and colorful. In complete darkness the cone cells become less sensitive, and the rod cells, situated at some distance from the macula lutea, become stimulated. The image created thanks to the rod cells is less sharp and gray.

Eyes adjust (adapt) to the amounts of light. After leaving a dark place into the light, and from a light room into the dark room, our eyes may be blinded for a short time, but then they begin to send information to our brain. Strong light can damage our eyesight, which is why we should not look directly into the sun without protective lenses. Strong light is a stimulus that causes us to squint and causes our pupil to narrow.

Tracking objects that are approaching or departing is possible thanks to the shape of the lens (curvature). It allows you to adjust the visual acuity. When we look at the image from up close, the lens becomes more convex, whereas when you are looking at the object from a distance, the lens becomes flatter. This phenomenon is called accommodation of the eye.

Seeing on both eyes allows us to judge the distance. Eyeballs are directed towards the front, they are at distance of about 6 cm from each other. Because of that, they receive 2 slightly different images. Optic nerves transfer them to the brain and there, in the optic centre, both information are analyzed and superimposed on each other. This is how the spatial (three dimensional) image is created.

Eyes deliver about 70% of information from the surroundings. Every day many external factors influence them, which is why they require special care and protection. Hygiene of the organ of sight should include those actions, which allow the eyes to function correctly. These are, among other things:

• protecting the eyes from too intense light by using sunglasses with UV filter;

• protecting your eyes from having a contact with chemical substances in water (e.g. in a swimming pool) by using swimming glasses;

• remembering about correct moistening of the eyeball, by drinking around 2.5 l of liquids every day and maintaining adequate humidity in rooms, especially during heating season;

• caring for correct lighting of your work place and positioning the computer screen at a distance not shorter than 50 cm from the eyes, and, when you work longer using a computer – using glasses with anti‑glare lenses;

• limiting time we spend in front of the TV or computer;

• regular eye checks.

Wrong shape of the lens or the eyeball is the cause of myopia or hypermetropia, and wrong curvature of the elements that refract the light leads to astigmatism. Eyesight defects can be corrected using adequate lenses.

The cause of myopia can be the incorrect, too bulgy shape of the lens or too elongated eyeball. As a result, the light is focused before the retina, which causes the person with such defect to see clearly only the objects that are at a short distance from her eyes. In order to correct this defect, we use dispersive lenses, marked with the minus sign. In the case of hypermetropia, the eyeball is shortened or the lens is too flat, which causes the light to focus behind the retina. In order to correct this defect, we use focusing lenses, marked with the plus sign. The ability of the eye to see is expressed in diopters.

The irregular curvature of the cornea or the lens focuses the light in different points of the retina or outside the retina, which causes the image to be blurry. This type of defect is called astigmatism. In order to correct it, we use cylindrical lenses.

Working with a computer requires focusing your eyes on a screen that is at fixed distance, which, in consequence, may lead to weakening of the muscles that are responsible for changing the shape of the lens and problems with eyes’ ability to accommodate. That is why it is recommended to make short breaks while working, during which we should look at something far away. Looking at a computer screen lowers the frequency of blinking, which leads to bad moistening of the eyeball surface. In this situation, you feel your eyes burning, or you have the impression you have sand in your eyes. If this discomfort continues, it can lead to cornea infections.

• Organ of sight consists of the eyeball and the protective and moving apparatus.

• Eyeball is covered by three membranes: sclera, choroid and retina.

• On the retina there are photoreceptor cells - cone cells and rod cells.

• Macula lutea is the place of sharpest vision.

• Light passes through the cornea, lens, vitreous humour (vitreous body) and reaches the retina.

• The image created on the retina is smaller and inverted.

• The phenomenon of accommodation allows us to see sharply objects that are close and far.

• Problems with sight can be caused by eye diseases, genetic conditions or result from wrong construction and shape of the elements of the eye.