The optical instruments such as cameras, microscopes, telescopes, film projectors and spectacles, etc., work on the refraction of light through various types of artificial lenses (man-made lenses) made of transparent glass. The human eye works on the refraction of light through a natural convex lens made of transparent living material and enables us to see things around us. Our eye is the most important optical instrument gifted to us by the God. Without eye, all other optical instruments would have no value at all.
In this article you will find various important information about human eye.
The Human Eye and the Colourful World: The Human Eye
The main parts of the human eye are: Cornea, Iris, Pupil, Ciliary muscles, Eye lens, Retina and Optic nerve (see Figure). The eye-ball is approximately spherical in shape having a diameter of about 2.5 cm. We will now describe the construction and working of the eye.
Construction of the Eye
- The front part of the eye is called cornea. It is made of a transparent substance and it is bulging outwards.
- The outer surface of cornea is convex in shape. The light coming from objects enters the eye through cornea. Just behind the cornea is the iris (or coloured diaphragm).
- Iris is a flat, coloured, ring-shaped membrane behind the cornea of the eye. There is a hole in the middle of the iris which is called pupil of the eye. Thus, pupil is a hole in the middle of the iris. The pupil appears black because no light is reflected from it.
- The eye-lens is a convex lens made of a transparent, soft and flexible material like a jelly made of proteins. Being flexible, the eye-lens can change its shape (it can become thin or thick) to focus light on to the retina.
- The eye-lens is held in position by suspensory ligaments. One end of suspensory ligaments is attached to the eye-lens and their other end is attached to ciliary muscles (see Figure).
- Ciliary muscles change the thickness of eye-lens while focusing. In other words, the focal length of eye-lens (and hence its converging power) can be changed by changing its shape by the action of ciliary muscles. In this respect an eye differs from a camera. The focal length of the convex lens used in a camera is fixed and cannot be changed but the focal length of the convex lens present inside the eye can be changed by the action of ciliary muscles.
- The screen on which the image is formed in the eye is called retina.
- The retina is behind the eye-lens and at the back part of the eye. The retina of an eye is just like the film in a camera. The retina is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’ which respond to the ‘intensity of light’ and ‘colour of objects’ respectively, by generating electrical signals.
- At the junction of optic nerve and retina in the eye, there are no light sensitive cells (no rods or cones) due to which no vision is possible at that spot. This is called blind spot. Thus, blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye (see figure). When the image of an object falls on the blind spot, it cannot be seen by the eye.
- It should be noted that there is an eye-lid in front of the eye which is just like the shutter in a camera. When eye-lid is open, light can enter the eye but when eye-lid is closed, no light enters the eye.
- The space between cornea and eye-lens is filled with a watery liquid called ‘aqueous humour’. And the space between eye-lens and retina is filled with a transparent jelly like substance called ‘vitreous humour’ which supports the back of the eye.
Working of the Eye
The light rays coming from the object kept in front of us enter through the cornea of the eye, pass through the pupil of the eye and fall on the eye-lens. The eye-lens is a convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. (Actually, the outer surface of cornea also acts as a convex lens due to which cornea converges most of the light rays entering the eye. Only the final convergence of light rays is done by the eye-lens to focus the image of an object exactly on the retina). The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. Actually, the retina has a large number of light-sensitive cells. When the image falls on the retina then these light-sensitive cells get activated and generate electrical signals. The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision. Although the image formed on the retina is inverted, our mind interprets the image as that of an erect object. As far as physics is concerned, the eye consists of a convex lens (called eye-lens) and a screen (called retina). The eye-lens forms a real image of the objects on the retina of the eye and we are able to see the objects.
The human eye is like a camera. In the eye, a convex lens (called eye-lens) forms a real and inverted image of an object on the light-sensitive screen called retina whereas in a camera, the convex lens (called camera-lens) forms a real and inverted image of an object on the light sensitive photographic film.
Frequently Asked Questions (FAQs)
1. What are the main components of the human eye?
- The human eye consists of several key components, including the cornea, pupil, iris, lens, retina, optic nerve, and various supporting structures.
2. How does light enter the eye and reach the retina?
- Light enters the eye through the cornea, passes through the pupil (whose size is controlled by the iris), and then goes through the lens, which focuses the light onto the retina at the back of the eye.
3. What is the function of the cornea in the eye?
- The cornea is the transparent front surface of the eye, responsible for refracting (bending) light as it enters the eye. It plays a crucial role in the eye’s focusing process.
4. What is the role of the lens in vision?
- The lens further refracts and focuses light onto the retina, allowing the eye to adjust focus for objects at different distances through a process called accommodation.
5. What is the retina, and what is its role in vision?
- The retina is a light-sensitive layer at the back of the eye. It contains photoreceptor cells (rods and cones) that convert light into electrical signals, which are then transmitted to the brain through the optic nerve for image processing.
6. How do rods and cones differ in their function?
- Rods are highly sensitive to low levels of light and are responsible for black-and-white vision in dim conditions. Cones are responsible for color vision and detailed visual perception in bright light.
7. What happens to the electrical signals generated by photoreceptor cells in the retina?
- Electrical signals generated by photoreceptor cells are transmitted through bipolar cells and ganglion cells in the retina. The ganglion cells form the optic nerve, which carries visual information to the brain.
8. How does the brain process visual information received from the eyes?
- The brain processes visual information by interpreting the electrical signals received from the optic nerve. It assembles these signals into a coherent image and interprets details such as color, shape, and motion.
9. What is the role of the ciliary muscles in the eye?
- Ciliary muscles control the shape of the lens, allowing it to change thickness and curvature. This adjustment, known as accommodation, enables the eye to focus on objects at different distances.
10. Can you explain how the pupillary reflex works?
- The pupillary reflex is a rapid response to changes in light intensity. When exposed to bright light, the iris contracts, reducing the size of the pupil to limit the amount of light entering the eye. In dim light, the iris dilates to allow more light in.