The Human Eye and the Colourful World: We see the world around us with our eyes. The light after reflection from the objects around us enters our eyes, helping us see them. Did you know, our eyes can focus on \(50\) different objects at the same time every second and our eyes can pick out approximately \(10\) million different colours. Let us read further to understand how our eyes function and many phenomena that are associated with light.

The Human Eye

The human eye is the most valuable sense organ; without our eyes, we wouldn’t be able to see the colours that brighten up our world and would solely be dependent on our other senses like smell, touch, taste etc.

Cornea: Light from our surroundings enters our eyes through a thin membrane called the cornea. It is the transparent budge present in front of our eyeball. The eyeball is a sphere having an average diameter of \(2.3\;{\rm{cm}}\), and a significant amount of light rays that enter our eyes are refracted at the cornea’s outer layer.

Iris: It is a dark, muscular diaphragm present behind the cornea. It controls the size of the pupil.

Pupil: It is the opening that controls and regulates the amount of light entering our eyes.

Crystalline lens: It provides adjustment to the focal length necessary to focus objects kept around us. The lens of our eyes is made up of fibrous, jelly-like material. The crystalline lens forms a real and inverted image of the objects on the retina.

Ciliary Muscles: The curvature of the eye lens can be adjusted to some level with the help of the ciliary muscles. The change in curvature of the eye lens helps alter its focal length. When muscles are relaxed, the lens becomes thin, its focal length increases, enabling us to see distant objects. When muscles contract, the lens becomes thick, and its focal length decreases, enabling us to see nearby objects clearly.

Retina: Our eye is basically like a camera, and the lens system forms an image on the light-sensitive screen called retina present at the back of our eyes. It is a delicate membrane that contains several light-sensitive cells. As light falls on these light-sensitive cells, the cells get illuminated, generating an electric signal.

Optic nerve: The electric signals generated by the cells are sent to the brain by the optic nerves. The brain interprets these signals as sight, enabling us to understand the world as we see it.

Power of Accommodation

Power of accommodation of the eye lens is defined as the ability of the eye lens to adjust its focal length. This property helps us to see clearly, both near and far lying objects.

Least distance of distinct vision: The minimum distance up to which our eyes can see clearly without straining is the least distance of distinct vision. For a normal human eye, this distance is \(25\;{\rm{cm}}\) from the eyes. This is referred to as the near point of the eye.

The far point of the eye: The maximum distance up to which we can see the objects is known as the eye’s far point. For the normal human eye, this distance is infinity. A normal human eye can see clearly between the distance of \(25\;{\rm{cm}}\) and infinity.

Defects of Vision

Sometimes due to the loss of power of accommodation of the eye lens or due to the refractive defects of the eye lens, we may not be able to see the world around us clearly. Some common vision defects are:

1. Myopia: A person with myopia can clearly see nearby objects but can’t see far away objects. This defect of vision is also known as near-sightedness. Thus, the far point of a person having this defect is closer than infinity. The image of an object in a myopic eye is formed in front of the retina but not on it. This defect may be caused due to elongation of the eyeball or excessive curvature of the eye lens.

This defect can be corrected using a concave lens of suitable power.

2. Hypermetropia:  A person suffering from hypermetropia can see distant objects clearly but cannot see close-lying objects. This defect of vision is also known as far-sightedness. Thus, the near point of a person having this defect is beyond \(25\;{\rm{cm}}\). The image of an object in a hypermetropic eye is behind the retina but not on it. This defect may be caused due to increase in focal length of the eye lens or a shortened eyeball.

This defect can be corrected using a convex lens of suitable power.

3. Presbyopia: A person suffering from presbyopia may suffer from both myopia and hypermetropia. With ageing, a person may lose the power of accommodation of their eyes. This defect arises due to the weakening of the ciliary muscles or reducing the flexibility of the eyes lens.
This defect can be corrected using bi-focal lenses. The upper part consists of a concave lens, while the lower part is made from a convex lens. It can also be corrected using contact lenses or surgeries.

4. Cataract: The crystalline lens of old people might become foggy or milk, causing a partial or complete loss of vision. This defect of vision is called a cataract, and it can be corrected using cataract surgery.

Refraction Through a Glass Prism

A glass prism consists of two triangular surfaces and three rectangular lateral surfaces inclined to each other at a certain angle. The angle between the two lateral faces of a prism is referred to as the angle of prism. Consider a monochromatic ray of light entering a glass prism as shown below:

As it travels from one medium into another, the light changes its direction of motion due to refraction. The ray of light, while going from air to glass, bends towards the normal, but as the ray of light moves from the glass into the air, it bends away from the normal. Due to the peculiar shape of a prism, the emergent ray is bent at an angle with the direction of the incident ray. The angle between the emergent ray and incident ray is called the angle of deviation.

Dispersion of White Light Through a Glass Prism

A ray of white light incident on the surface of a prism splits into a band of seven colours, as shown in the figure below:

The sequence of colours can be represented by an acronym VIBGYOR, i.e. Violet, Indigo, Blue, Green, Yellow, Orange and Red. This band of the component colours of a light beam is called a spectrum. The phenomenon of a beam of light splitting into its component colours is known as dispersion of light. Dispersion of light occurs as a ray of white light (which contains all colours) passes through a glass prism because of different colours of light bend at different angles with respect to the incident ray. The light of red colour bends the least while that of violet colour bends the most. Each ray emerges through the prism along different paths and can be seen distinctly.

Dispersion of light was discovered by Isaac Newton. He used a glass prism to obtain the spectrum of sunlight. He further tried to split the colours by placing another prism adjacent to the first prism, but he was not successful. He then placed another identical glass prism inverted with respect to the first prism. The prisms were placed so that the spectrum obtained from the first prism fell on the second prism, and a beam of white light emerged from the second prism.

It was then realized that sunlight comprises seven colours, and any light with a spectrum similar to the spectrum of sunlight is known as white light.

Rainbow

A rainbow is a natural spectrum obtained by the dispersion of sunlight by tiny droplets of water. It often appears in the sky after rainfall, and it is always formed in a direction away from the Sun. Tiny droplets of water behave like small glass prisms as they refract and disperse the light falling on them. The incident sunlight gets refracted and dispersed as it strikes the droplets. After entering the drop, it undergoes total internal reflection. It gets refracted and dispersed again as it exits the drop. This dispersion and subsequent internal reflection lead to the formation of a rainbow.

Twinkling of Stars

Stars do not actually twinkle; they appear to twinkle due to the refraction of starlight. The light coming from stars undergoes refraction several times before reaching the earth’s surface. Due to the gradual change in the refractive index, the starlight bends towards the normal each time it goes from one layer into another. The apparent position of a star appears slightly higher from its actual position when viewed from the horizon. Since the physical conditions in the earth’s atmosphere vary continuously, the amount of light coming from the distant and almost point-sized stars varies slightly, and its apparent position fluctuates. Due to this, the light coming from a star varies; it sometimes appears brighter and sometimes fainter, producing the twinkling effect.

Planets are, on the other hand, much closer to earth and are considered as extended sources. We can assume a planet to be a collection of sized point sources of light. Thus, total variation produced by all individual sources will average to zero and nullify their twinkling effect. Planets do not twinkle as stars do.

Delayed Sunset and Early Sunrise

Due to the atmospheric refraction, we can see the Sun \(2\) minutes before the actual sunrise and \(2\) minutes after the actual sunset. The actual sunrise or sunset implies the condition of actual crossing on the horizon by the Sun. In fact, the apparent flattening of the Sun’s disc at the time of sunrise or sunset is also due to atmospheric refraction.

Tyndall Effect

We all know that the path of a beam of light passing through a true solution is not visible. Still, when a colloidal solution replaces the true solution, the path of light becomes visible because the colloidal particles being larger in size scatter the light, thereby making it visible.

Earth’s atmosphere is a heterogeneous mixture of minute particles like tiny water droplets and molecules of dust and air. The light from the Sun is visible to us after it is scattered by these particles. This scattering of light by the colloidal particles suspended in our atmosphere is known as the Tyndall effect. Due to this effect, the path of light entering a small hole becomes visible in a smoke-filled room. It can also be observed when sunlight passes through trees in a dense forest.

The colour of the scattered light depends upon the size of the scattering particles. We know that light consists of several colours; thus, if the size of scattering particles is small, blue light is scattered strongly, while particles of larger size scatter the light of a longer wavelength. The scattered light may appear white if the size of the scattering particles is large enough.

The bluish appearance of the sky throughout the day or the reddish appearance of the sky at sunrise and sunset is due to the scattering of light.

In general, the finer particles of the earth’s atmosphere scatter the light of shorter wavelength readily; hence the sky appears blue. But during sunrise or sunset, the light from the Sun has to travel a longer distance in the atmosphere, blue light gets scattered more, and only the light of longer wavelength reaches us, and the sky, therefore, appears red. If there were no atmosphere on earth, the sky would have appeared dark in the absence of scattering of light.

Summary

The human eye is the most valuable sense organ; without our eyes, we wouldn’t be able to see the colours that brighten up our world. The light strikes the outer cornea of our eye and enters the eye through the pupil that is controlled by the iris. The light falls on the crystalline lens, and an image is formed on the retina. The focal length of the eye lens is controlled by the ciliary muscles. The most common defects of vision are myopia, hypermetropia, presbyopia and cataract.
The phenomenon of a beam of light splitting into its component colours is known as dispersion of light. Dispersion of light occurs as a ray of white light passes through a glass prism because of different colours of light bend at different angles with respect to the incident ray.

A rainbow is a natural spectrum obtained by the dispersion of sunlight by tiny droplets of water. It often appears in the sky after rainfall, and it is always formed in the direction away from the Sun. Tiny water droplets behave like small glass prisms as they refract and disperse the light falling on them.
Scattering of light by colloidal particles suspended in our atmosphere is known as the Tyndall effect. Due to this effect, the path of light entering a small hole becomes visible in a smoke-filled room. It can also be observed when sunlight passes through trees in a dense forest. The bluish appearance of the sky throughout the day or the reddish appearance of the sky at sunrise and sunset is due to the scattering of light.

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Frequently Asked Questions

Q.1. A person cannot see properly. He complains that his vision has become foggy or milky. Identify the defect of vision.
Ans: The defect of vision is the cataract.

Q.2. What is the role of ciliary muscles?
Ans: Ciliary muscles control the focal length of our eye lens. These help our eyes to focus on nearby and far away objects so that we can see clearly.

Q.3. Which type of lenses are used for the correction of hypermetropia?
Ans: Convex lenses

Q.4. What would be the colour of the sky if the earth had no atmosphere?
Ans: Sky, in the absence of an atmosphere, would appear to be dark (black).

Q.5. Name the phenomenon responsible for the twinkling of stars.
Ans: Atmospheric refraction

We hope this detailed article on the human eye and the colourful world and helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!