parudigital textbook

BIOLOGY

BIOLOGY Standard X Parvathy H Nair MSc Zoology Kerala University

HUMAN EYE Digital Textbook for Class X

NATIONAL ANTHEM OF IND IA Jana-gana-mana-adhinayaka jaya he Bharata-bhagya-vidhata Punjaba-Sindhu-Gujarata-Maratha Dravida-Utkala-Banga Vindhya-Himachala-Yamuna-Ganga Uuchchala-jaladhi-taranga Tava shubha name jage, tava shubha asisa mage, gahe tava jaya-gatha. Jana-gana-mangala-dayaka jaya he Bharata-bhagya-vidhata. Jaya he, Jaya he, Jaya he, jaya jaya jaya, jaya he.





Preface by Author You will be curious to know how we sense and respond to our environment through the windows of knowledge. You will certainly be surprised to learn how the vision is possible through the human eyes. The path of vision of human eyes has also been included in the book. How the wonder camera that is, the human eye helps in vision are also mentioned here, learning activities will be beneficial in your daily life. The student's role is pivotal in the process of construction of knowledge. This Science textbook is only a resource in the learning process. This will help you in the creation of knowledge, the textbook would definitely make your leaming activity in classrooms easy and joyful. Let all this be beneficial for you in the learning of science by joyfully participating in life related learning experiences that impart knowledge and happiness

OBJECTI VES of the book To understand the concept of human eye To distinguish different sense organs To know the importance of human eye To recognize the reasons for proper eye care To point out different strategies for eye care

Table of content Chapter 1 - The Human eye Chapter 2 - Structure of eye Chapter 3 - Process of vision Chapter 4 - Regulation of light in the eye Chapter 5 - Retina and photoreceptor Chapter 6 - Eye-Defects and Diseases Concept map Exercise

Certain icons are used in the text book convenience Let us assess Extended activity

Chapter 1 The Human Eye INTRODUCTION The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us. On closing the eyes, we can identify objects to some extent by their smell, taste, sound they make or by touch. It is, however, impossible to identify colours while closing the eyes. Thus, of all the sense organs, the human eye is the most significant one as it enables us to see the beautiful, colourful world around us. The human eye is like a camera.

Let us assess 1.Draw the structure of Human eye Extended Activity 2.Prepare a model of human eye using resuable materials

Chapter 2 Structure of Human eye The human eyes are the most complicated sense organs in the human body. From the muscles and tissues to nerves and blood vessels, every part of the human eye is responsible for a certain action. Furthermore, contrary to popular belief, the eye is not perfectly spherical; instead, it is two separate segments fused together. It is made up of several muscles and tissues that come together to form a roughly spherical structure. From an anatomical perspective, the human eye can be broadly classified into external structure and internal structure.

Sclera Choroid Retina It is the outer The middle layer The inner layer covering, a protective which contains a which has large number of tough white layer , photoreceptors made of connective blood vessels. tissue Iris Yellow spot Cornea The part of the choroid seen behind The part of the The projected the cornea. Presence retina transparent anterior of the pigment where plenty of part of the sclera melanin gives the photoreceptors are which refracts light rays to focus on the iris present. It is the a dark colour. point of maximum retina. Pupil visual clarity. Conjunctiva The layer which The aperture seen at Blind spot covers and protects the centre of the iris. the front part of The size of this The part of the sclera except the aperture increases and retina cornea decreases depending on from where the optic the intensity of light. nerve begins. Here there is no vision as photoreceptors are absent.

Ciliary muscle : Circular Lens :Elastic transparent muscles seen convex lens, around the lens. The connected to ciliary contraction and muscles by thread relaxation of these like ligaments. muscles alter the curvature of lens. Optic nerve : Transmits impulses from photoreceptors to the visual centre in the brain. The fluids in the eye Aqueous humor Vitreous humor The fluid filled in the aqueous A jelly like substance seen in the chamber between the lens and vitreous chamber between the the cornea. It is formed from retina and the lens. Helps in blood, and reabsorbed by blood. Nourishes the tissues of the eye maintaining the shape of the eye.

Let us assess 1) The human eye forms an image of the object at its, a) Cornea b) Iris c) Pupil d) Retina 2) What are the main parts of human eye? 3) What is the function of pupil and ciliary muscle?

Extended activity Identify the parts labelled as A, B, C, D & E from the figure

Chapter 3 Process of Vision Almost 80% of our sensory responses from the environment is said to be perceived through the eyes. All the different parts of the eyes work together to help you to see. Light enters the eye through the cornea(the clear, dome-shaped surface that covers the front of the eye). From the cornea, the light passes through the pupil. The iris, or the coloured part of your eye, controls the amount of light passing through. From there, it then hits the lens. (the clear structure inside the eye that focuses light rays onto the retina) . Then the light passes through the vitreous humor, the clear, jelly-like substance that fills the center of the eye. It helps to keep the eye round in shape.

Then the light passes through the vitreous humor, the clear, jelly-like substance that fills the center of the eye. It helps to keep the eye round in shape. Finally, the light reaches the retina,the light-sensitive nerve layer that lines the back of the eye. Here the image is inverted The optic nerve is then responsible for carrying the signals to the visual cortex of the brain. The visual cortex turns the signals into images (for example, our vision

Complete the flow chart Light Aqueous humor Impulse Sense of sight Let us assess 1. Which part of the human eye conveys the electrical signals generated by the light sensitive cells of the retina to the brain? 2. Name the part of our eyes that helps us to focus near and distant objects in quick succession. Extended activity Prepare a model to show how the human eye works.

Chapter 4 Regulation of Light in the Eye Light which passes through the cornea reaches the lens through an aperture called the pupil. Do you know how bright light is prevented from entering the eye? The size of the pupil is regulated with the help of muscles in the iris. Thus the amount of light falling on the lens is regulated according to the intensity of light Pupil dilates in dim Pupil constricts in bright light light Radial Muscles and Circular Muscles The size of the pupil is regulated by the antagonistic activity of the radial and circular muscle. When the circular muscles contract in intense light, the size of the pupil decreases. When yheradial muscles contract in dim light, the size of the pupil increases.

We can see nearby and distant objects clearly. This is due the property of the lens to adjust focal length according to the distance between the eye and the object. You are aware that in an artificial lens the focal length is constant. In order to get a clear image, we have to adjust the position of lens and screen. But it is the pecularity of the eyes to adjust the focal length of the lens While viewing distant objects While viewing While viewing distant nearby objects object While viewing near Ciliary muscles relax objects Ligaments stretch Ciliary muscles contract Curvature of lens decreases Ligaments relax Focal length increases Curvature of lens increases Focal length decreases

The capacity of the eye to change the curvature of lens depending on the distance between the eye and the object by adjusting the focal length is called the power of accommodation of the eye. Let us assess 1. Define power of accommodation of eye. 2.Explain antagonistic activity of radial and circular muscles. 3. How bright light is prevented from entering the eye? Extended activity Write a note on the changes that occur in your eyes ehile seeing nearby object and distant object.

Chapter 5 Retina and Photoreceptor The retina of our eye has a large number of light- sensitive cells (photoreceptors). Photoreceptors in the retina are stimulated by light which falls on it. As a result impulse are formed in the eye. There are two kinds of light- sensitive cells (photoreceptors) on the retina: rods and cones. (1) Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light. We can see things to some extent in a dark room due to the presence of rod cells in the retina of our eyes. Nocturnal animals (animals which sleep during the day and come out at night) like the owl have a large number of rod cells in their retina which help them see properly during the night when there is not much light. Rod cells of the retina, however, do not provide information about the colour of the object.

(2) Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light (or normal light). The cone cells of our retina also respond to colours. Cone cells cause the sensation of colour of objects in our eyes. The cone-shaped cells of the retina make us see colours and also make us distinguish between various colours. When it is getting dark at night, it becomes impossible to see colours of the car on road. Rod cells contain visual pigment rhodopsin. This pigment is formed from a protein named opsin and retinal, which os a derivative of vitamin A. Cone cells contain a pigment called photopsin. This is also called idopsin. This pigment is composed of opsin and retinal. Three types of cone cells which help us to detect primary colours of light.

When light rays fall on the pigments present in photoreceptors, the pigments dissociate in to retinal and opsin. This chemical change leads to the formation of impulses. These impulsesare transmitted to cerebrum through the optic nerves and this enable vision Photopsin Retinal +opsin Rhodopsin Retinal +opsin

Let us assess 1. Rods and cones a) have different physical shapes b) have different distributions across the retina (more cones in foveal region) c) have different distributions across the retina (more rods in foveal region) d) a and b e) a and c Extended activity What are rods and cones in the retina of the eye? Why is our night vision relatively poor compared to the night vision of an owl?

Chapter 6 Eye - Defects and Diseases Colour Blindness You are aware that the retina contains cone cells which can detect red, green and blue colours. Colour blindness is caused due to the defect of cone cells which detect red and green. Some persons cannot distinguish green and red colours due to the defect of cone cells. This condition is called colour blindness.

Glaucoma Aqueous humour is the fluid which nourishes the tissues in the eyes. If the reabsorption of aqueous humour does not occur; it causes an increase in the pressure inside the eyes. This causes damage to the retina and the photoreceptor cells and ultimately leads to blindness. This defect can be rectified by laser surgery Healthy eyes Glaucoma Cataract It is a condition in which the lens of the eyes become opaque resulting in blindness. This can be rectified by replacing the lens with an artificial one, through surgery.

Conjunctivitis This is an infection of the conjunctiva. The causative organisms may be bacteria, virus etc. This disease is transmitted through contact and can be prevented by maintaining personal hygiene. Myopia Myopia or Near-sightedness occurs when the eye loses its ability to focus on far-off objects as the lenses do not possess a long focal length. Objects that are near are clearly visible to patients of this defect. A diverging lens can be used to fix the problem of near sightedness. As mentioned above, the image was blurry as the light rays were not diverging at the right point and creating a focused image.

The rays get focused before it reaches the retina. The convergence of the light rays takes place due to the cornea producing an image on the retina. Hypermetropia Hypermetropia is the condition of the eyes where the image of a nearby object is formed behind the retina. Here, the light is focused behind the retina instead of focusing on the retina. The person suffering from hypermetropia will have difficulty in focusing on nearby objects but can clearly see distant objects.

Presbyopia Presbyopia is caused as part of the natural ageing process of the eye. The ciliary muscles lose their ideal functioning power over time and do not work effectively. Even the crystalline lens loses its flexibility over time (it starts hardening), thereby affecting it focusing power, corrected using converging lens. Night blindness You have studied that the retinal, a part of the visual pigment, is derived from Vitamin A. The deficiency of Vitamin A results in the low production of retinal. This in turn prevents the resynthesis of rhodopsin. In this condition, objects cannot be seen clearly in dim light and this disease is called night blindness.

Normal vision Night blindness Xerophthalmia If there is a prolonged deficiency of Vitamin A, the conjunctiva and cornea become dry and opaque. This causes xerophthalmia and leads ultimately to blindness.

Let us assess 1. Explain the following defects in vision and correction provided to it a) Myopia b) Far-sightedness 2. What causes glaucoma? Extended activity 1. A student cannot see clearly a chart hanging on a wall placed at a distance 3 m from his eye. Name the defect of vision he is suffering from. List its two possible causes. (a) An eye donation camp is being organised by social workers in your locality. How and why would you help in this cause?

Concept Map Iris Cornea Vitreous Aqueous humor humor Pupil Outer Inner Yellow parts spot part Blind Lens Human Ciliary spot Eye muscle Sclera Layers Defects Myopia Choroid Retina Xeroph Hyper thalmia metropia Presbyo pia

Concept Map

Exercise 1. Name the following part of human eye: A thin membrane through which light enters the eye 2. Mention the name of a structure formed in human eye that controls the size of the pupil. 3. What is the function of pupil in human eye? 4. What is the nature of the image formed at the retina of human eye? 5. What is hypermetropia or far-sightedness? 6. Name the type of a lens used to correct hypermetropic eye. 7. State two reasons due to which myopia may be caused 8. What is glaucoma?

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https://www.webmd.com/eye- health/picture-of-the- eves https://www.nei.nih.gov/learn-about- eye- health/healthy-vision/how-eyes-work • https://www.optimax.co.uk/blog/struc ture-human- eye/ https://www.khanacademy.org/scien ce/in-structure- of-human-eye https://www.livescience.com/3919- human-eye-works.html • https://www.thoughtco.com/how-the- human-eye- works- https://www.thoughtco.com/how-the- human-eye-works-4155646 https://unsplash.com/s/photos/human-eve • https://www.eyeglassguide.com/my-eyes/how-the- eye-works.aspx • https://www.healthline.com/health/human-eye-fps