Introduction to The Human Eye and the Colourful
Using the knowledge of light and some of its properties that students study in the previous chapters, students study how these concepts apply to the human eye in the class 10 Science chapter The Human Eye and the Colourful World. Students also study some of the optical phenomena. The nature and the formation of the rainbow, the splitting of white light, and the blue color of the sky are also examined in the chapter.
Structure of a Human Eye
Of all the sense organs, the human eye is the most significant one as it enables us to see the beautiful, colorful world around us. The eye is spherical in shape and has a diameter of 2.3 cm on average. The internal structure of the eye includes- the cornea, iris, pupil, lens, ciliary muscles, retina, nerve cells, optic nerve, yellow spot, aqueous and vitreous humor, and suspensory ligament.
The lens system of the human eye forms an image on the light-sensitive screen called the retina. The cornea is the thin membrane of the eye through which light enters. Most of the light refraction occurs at the outer surface of the cornea. A dark muscular diaphragm that controls the size of the pupil, known as Iris is present behind the cornea. The pupil is a structure in the eye that controls and regulates the amount of light entering the eye. The light-sensitive cells get triggered upon illumination and induce electrical signals. These signals are sent to the brain through the optic nerves. The brain analyses these signals, and finally, processes the information so that we sense objects as they are.
Defects of a Human Eye
There are few common eye disorders seen in all individuals and are caused by several factors. These conditions can be improved by the corrections. The defects include:
Myopia – This is also called short-sightedness. A person with this eye defect can only see nearby objects clearly compared to distant objects. This condition can be corrected using a concave lens.
EYE DEFECTS: MYOPIA
Introduction to Eye Defects
As you may have noticed, the human eyes gradually lose their power of accommodation. In such cases, the person cannot see distinctly and comfortably. There are three common eye defects, and they are
(i) myopia or near-sightedness,
(ii) Hypermetropia or far-sightedness, and
(iii) Presbyopia.
These defects are corrected by the use of suitable spherical lenses. In the article, we focus on myopia and its correction.
What is Myopia?
Myopia is also known as near-sightedness or short-sightedness. What happens here is that light rays do not focus on the surface of the retina but in front of it. This results in images being blurry when perceived. In such a case, distant images appear out of focus but objects nearby are seen clearly. This is why myopia is also called near or short-sightedness.
It can occur if the eyeball is elongated leading to image formation in front of the retina. It can also occur if the refracting power of the lens is affected due to the cornea or lens being too curved. This effect can be more pronounced with age. A combination of these effects will also lead to myopia.
Similar Articles
- Hyperopia
- Astigmatism
- Presbyopia
Hyperopia
What is Hyperopia?
Farsightedness is also known as hyperopia. It is a refractive error in which distant objects are clearly seen, but close objects appear blurry. People experience farsightedness differently, and some people, more commonly when they are young, do not notice any problems with their vision. In addition, for people with significant farsightedness, vision can be blurry for objects at any distance, near or far.
What causes Hyperopia?
The human eye relies on two critical parts to focus on an image: the cornea, which is the clear front surface of the eye, and the crystalline lens, a clear structure inside the eye that changes shape in order to focus on objects.
In an eye without refractive error, these focusing elements have a smooth curvature, much like the surface of a smooth rubber ball, and bend incoming light to make a sharply focused image on the retina.
If the cornea is not smoothly curved, like, in the case of people with farsightedness, the light does not properly bend or refract light properly, and this results in a refractive error. Farsightedness occurs when light entering the eye focuses behind or beyond the retina, instead of on it.
Both children and adults can be affected by farsightedness and it can even be hereditary. People whose parents are farsighted may likely also have the condition. An eye care professional (ECP) can diagnose farsightedness during a routine eye exam. Generally, at-school vision screenings are not effective in detecting farsightedness.
Hyperopia is generally classified into three types according to its clinical appearance –
- Simple hyperopia
- Pathological hyperopia
- Functional hyperopia
What are the symptoms of Hyperopia?
- Blurred vision at night or later in the day
- Trouble focusing on near objects
- Aching eyes, eye strain, and headaches
- Squinting
- Difficulty maintaining a clear focus
- Eye fatigue
- Irritability or nervousness after sustained concentration
Farsightedness and its Correction:
- Farsightedness or hyperopia is the inability of the eye to focus on nearby objects. The farsighted eye has no difficulty viewing distant objects. The lens can no longer presume the convex and curved shape lens that is needed to view the nearby objects. This vision problem can be corrected by using a converging lens.
- It will refract light before entering the eye and eventually decreases the distance of the image. By initializing the refraction process prior to light reaching the eye and the image of nearby objects is once again focused upon the retinal surface.
- Farsightedness is mostly seen among adults and occasionally in young people. If this vision problem happens among youth, the cause would rarely be related to the inability of the lens to assume a short focal length. The problem, in this case, is mostly related to an eyeball that is shortened.
- With the shortening of the eyeball, the retina lies closer than usual to the cornea and lens. This results in the formation of an image of nearby objects beyond the retina. To correct this problem, again the same converging lens is required for the adults.
Hypermetropia – This is also called farsightedness. A person with this eye defect can only see distant objects clearly compared to near objects. This condition can be corrected using a convex lens.
EYE DEFACTS-Hypermetropia or Hyperopia
What is Hyperopia?
Some people see distant objects clearly, while some cannot even see nearby objects clearly. This is a result of the defects in the retina of the eye. People with hyperopia or hypermetropia are said to have far-sightedness or long-sightedness. What happens here is that light rays do not focus on the surface of the retina but on it. These results in blurry images when perceived. Blurred vision is caused by myopia as well, but the reasons differ in both cases. In hyperopia image formation of nearby objects appears out of focus but objects far away can be seen clearly.
Did you know that some children are born with this condition? This is because their eyeballs are short and therefore, the image is not formed in front of the retina. They outgrow this defect after a while and the eyeball lengthens with normal growth. An increase in the focusing power of the crystalline lens is another reason for hyperopia and results in the formation of the image behind the retina.
If you already know about myopia then you would have realized that hyperopia and myopia are opposites of each other.
Treatment for Hypermetropia
It is important to remember that Hyperopia is an eye defect and not an eye disease. Therefore, it can be corrected. Since the image is formed behind the retina, correcting this problem can counter the effects produced due to hyperopia.
The image above shows how the effects of hyperopia can be countered theoretically. It can be done in the following ways.
- By using corrective eyeglasses: Depending on how much the focal length has been altered due to the effects of hyperopia, powered convex lenses can correct this problem. It is also the safest option with minor inconveniences.
- Using contact lenses: Using this has the same effect as that of using eyeglasses but presents a more convenient and comfortable option. Using contact lenses isn’t safe for everyone, however.
- Refractive surgery: Since the refractive power of the eye is affected by hyperopia, altering this through surgery is another way to counter the defects. An example of refractive surgery is Lasik surgery which is quite common.
People with farsightedness have powers with a positive sign. For example +1.50 or +2.50 as their corrective powers. This signifies that the lens used is convex in nature. These people mostly use their eyeglasses or contacts only when they are reading or using the laptop and another close-up work.
Presbyopia – This is an age-related condition caused due to the weakening of ciliary muscles, hardening of the lens, and reduced lens flexibility. A person with this defect usually finds difficult to focus on nearby objects unable to read or write.
Cataract – This is an age-related condition caused due to the loss of transparency of the lens by erosion of lens proteins. It usually results in blurry vision, and cloudy lenses and can be corrected by replacing the old lens with an artificial lens.
Dispersion of white light by a glass prism
A prism splits the incident white light into a band of seven colors. The band of the colored components of a light beam is called its spectrum. Isaac Newton was the first to use a glass prism to obtain the sunlight spectrum. Different colors of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most. Thus the rays of each color emerge along different paths and thus become distinct.
Atmospheric Refraction
The refraction of light by the Earth’s atmosphere is known as atmospheric refraction. Atmospheric refraction is caused by the bending of light rays when they pass through the layers of the earth’s atmosphere, which are of different optical densities.
Twinkling of Stars
The twinkling effect of stars is due to the atmospheric refraction of starlight. The starlight undergoes continuous refraction as it passes through the atmosphere before it reaches earth. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers.
Scattering of Light
When a light beam goes through a medium, it hits the particles existing in them. Due to this phenomenon, some of the light rays get absorbed while a few get scattered in various directions. The intensity of the scattered light rays depends on the particles’ size and wavelength.
What is a Scattering of light?
Scattering of light
Light can be examined entirely from its source. When light passes from one medium to any other medium say air, a glass of water then a part of the light is absorbed by particles of the medium preceded by its subsequent radiation in a particular direction. This phenomenon is termed as a scattering of light. The intensity of scattered light depends on the size of the particles and the wavelength of the light.
Shorter wavelength and high frequency scatter more due to the waviness of the line and its intersection with a particle. The wavier the line, the more the chances of it intersecting with a particle. On the other hand, longer wavelengths have low frequency, they are straighter and the chances of colliding with the particle are less so the chances are less.
The bending of multicolored light can be seen in the afternoon due to the refraction and total internal reflection of light. The wavelength of the sunlight forms different colors in different directions. Rayleigh’s scattering theory is reasoned for the red color of the sun in the morning and the blue color of the sky.
Why is the color of the clear sky blue? And why are the clouds white?
Molecules with a larger size than the wavelength of light, experience the scattering effect differently, the phenomenon is known as Mie effect. Due to the largeness of particles, the light appears white. That is why the clouds, which are made of droplets of water are white. The blue colour is present in the major percentage among the lower wavelengths. With the wavelength of the light, the scattering efficiency of the small molecules in the atmosphere decreases. Sun radiates its light and its rays fall into the earth’s envelope thus, sunlight gets scattered in the atmosphere.
There are some examples that also show scattering, particles like dust, and smoke can also scatter radiation. In the same manner, we can explain the red colour appearance of the sun. For red light, the wavelength is more, and it is easy to go through the atmosphere as the scattering is less for the red light. When the light is on any other object, it gets scattered depending on its properties as different light has different intensity and each particle has different characteristics.