The Human Eye And The Colourful World

It is the minimum distance from which our eye can see the objects clearly (or able to produce a distinct image on the retina) without any strain in our eye. It is 25 cm for a human eye means that our eye can see up to 25cm without any strain on the eye. The least distance of distinct vision is less for young people and more for old people.

A bifocal lens consists of both convex lens as well as concave lens. It is used by the person suffering from myopia, hypermetropia, and presbyopia (not able to see very distant objects). Convex lens is used to correct hypermetropia (not able to see nearby objects clearly) and concave lens to correct myopia (not able to see far away objects clearly).The figure below shows the bifocal lens.

The figure below shows the labeled diagram.

A. Retina: Is a transparent, jelly-like mass located behind the lens. It acts as a ‘suspension’ for the lens so that the delicate lens is not damaged. It helps to maintain the shape of the posterior chamber of the eyeball.

B. Cornea: Helps to maintain the shape of the anterior chamber of the eyeball

C. Iris: A part of the retina that is directly opposite the pupil and contains only cone cells. It is responsible for good visual acuity (good resolution).

D. Optic nerve: The optic nerve is the second cranial nerve and is responsible for vision. Each nerve contains approx. one million fibers transmitting information from the rod and cone cells of the retina.

When white light is incident on a prism, it splits into seven colors (Violet, indigo, blue, green, orange, red). And among these colors, the Red light suffers the least deviation because it has the Maximum wavelength.

The figure below shows the diagram for the same.

Yes it is possible to obtain the white color from the recombination of the seven colors which are obtained by dispersion of light by prism. Figure below shows the set up for the same.

We take two glass prism and place the second one upside down w.r.t to the first one The first prism splits the white light into seven colors. When these colors fall on the inverted prism, it recombines all the colors into white light which is observed ultimately as a patch of white light on the screen.

(a) Since the student is not able to read distinctly the words written on the blackboard the student is suffering from Myopia or nearsightedness.

A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of its retina. This defect of vision occurs when the eyeball slightly elongates from front to back.

(b) This defect of vision can be remedied by using the appropriate concave lens, the concave lens is the diverging lens, therefore, it does not form the image on the retina thereby curing the defect.

Figure below shows the myopic eye without concave lens and with concave lens.

Accommodation is the ability or property of the eye lens to change its curvature or focal length so that images of objects at various distances can be formed on the same retina. Ciliary muscles help in changing the focal length.

Viewing Of Far Off Objects: When the ciliary muscles are completely relaxed, the eye lens is thin and its focal length is maximum (equal to distance between retina and eye lens).The rays which are coming from the distant object are parallel to each other and they are focused at the retina. Formation of image on retina is illustrated in the figure below.

Viewing nearby Objects: Ciliary muscles contract when we look at nearby objects. The eye lens bulges out and becomes thick. Focal length is reduced as a result. This focuses light from the nearby object on retina as shown in the figure below.

According to the question

Object Distance (u) = -25 cm

Image Distance (v) = -50 cm

By Lens Formula

⇒

⇒

⇒ F = 50 cm.

Since focal length is positive hence the lens is convex lens.

Power =

Focal length = 50 cm = 0.5 m

1 m = 100 cm.

⇒ Power =

Hence the Power is 2 Dioptre

Myopia: Those who suffer from myopia can see near objects clearly, but not the far away objects. This defect of vision occurs when the eyeball slightly elongates from front to back.

Below, we can see how images are focused on the retina of someone with myopia. It is also called as Short sightedness.

Myopia is corrected with a concave lens, which curves inward like the inside of a bowl. The lens changes the focus, so images fall on the retina as they should.

Figure below shows the defected and the corrected eye.

Hypermetropia: Hypermetropia is also known as hyperopia or farsightedness. Those who deal with hyperopia have difficulty seeing things that are near to them but have better vision for objects that are far away.

This defect of vision arises when the lens or cornea is out of place or insufficiently curved, leading to reduced vision quality.

Depending on by how much the focal length has been altered due to the effects of hyperopia, powered convex lenses can be used for the remedy of this defect. Figure below shows the defected and the corrected eye.

Defected Eye

Corrected Eye

The figure below shows the labeled diagram of the human eye. The main parts of eye are-
Cornea, Iris, Pupil, Ciliary muscles, Eye lens, retina and optical nerves which are labeled in the diagram below.

Image Formation: The light rays coming from object enter through cornea of eye, pass through the pupil of the eye and fall on the eye lens. The eye lens is a convex lens, so it produces real and inverted image of object.

The optical nerve conveys the image formed on retina to brain and we get sensation of vision.

Accommodation is the ability or property of the eye lens to change its focal length so that images of objects at various distances can be formed on the same retina. Ciliary muscles help in changing the accommodation.

When an object is flashed through the eye at a rate faster than ^th of a second, the eye perceives the object as if it is in motion. This concept is used in the cinematography.

Figure below shows the formation of the image on retina where eye acts as the convex lens.

Ciliary muscles help in changing the focal length or the curvature of the eye lens so that we can see the object clearly. This is called as the accommodation.

Red color has the maximum wavelength and amount of scattering is inversely proportional to the wavelength. Therefore the Red light scatters the least.

Wavelength of yellow is between 597 to 577 nm.

Wavelength of green is between 577 to 492 nm

Wavelength of blue is between 492 to 455 nm

Most of the refraction occurs at the light occurs at the first surface of the eye, and crystalline lens has the property to refract the light by bending it towards or away from the normal.

At sunrise and sunset sun lies below the horizon. The shorter violet and blue wavelengths are scattered away from our field of vision but longer wavelengths of light like red do not scatter and the sky looks red as shown in the figure below.

The speed of propagation of light which is an electromagnetic wave in a medium depends only on the refractive index of that medium. Thus the wavelength or the color of the light does not affect the speed of the light if they are in the same medium because the refractive index will be same.

A collection of suspended water droplets in the atmosphere serves as a refractor of light, the refraction occurs at the outer surface of drops. They also act as the small prism which scatters the white light and we see the seven colors.

On the inner surface, the internal refraction occurs. And the as shown below.

It is the ciliary muscles that change the focal length of the eye lens to see the distant object the focal length of the eye increases. The lens becomes thicker and relaxed. The figure below shows the lens at that time.

At noon the sun is overhead and the light travels a relatively shorter distance through the atmosphere to reach the earth. Due to this short journey of the light only blue light is scattered and most of the light is not scattered. Therefore the sun appears white.

Figure below shows the sun at noon.

The third color from the top is blue. In the orientation (ii) the third color from the top is yellow as shown in the figure below.

Hence if the prism is inverted than the third color from the top is blue which is the color of the sky. Therefore we get the orientation (i) as shown in the figure below

The person is having myopia. In this defect, a person cannot see objects beyond near point or far objects but can see the near objects clearly

This defect can be corrected by using convex lens.

Given distance of far point (D) = 2m

Let f be the focal length of eye lens.

⇒ f = -D = -2 cm.

Power of a lens =

⇒ Power =

Hence the power of the lens is -0.5 D

False

Accommodation is the property of the eye lens to change its focal length so that images of objects at various distances can be formed on the same retina. Ciliary muscles help in changing the accommodation.

False.

Nearsightedness, or myopia, actually means that seeing things at a distance is difficult for you. We may be able to read books with no problem but street signs that are far away appears blurry to you.

Nearsightedness is caused by light not falling directly on your eye’s retina but in front of it.

False

Presbyopia is an age-related condition in which the lens of the eye becomes less flexible. Seeing details like words in a book or an online article, or adjusting focus between far-away and nearby objects is difficult this condition is most common in people between the ages of 40 and 50.

While in far sightedness it’s hard to see things close up. People of any age, including babies, can be farsighted.

True.

Those who deal with long sightedness have difficulty seeing things that are near to them, but have better vision for objects that are far away. Therefore a Convex lens can be used for the remedy of this defect because it is converging lens, hence will form an image on the retina as shown in the figure below.

True.

If a person is suffering from astigmatism than he cannot simultaneously see both horizontal and vertical view of the object with same clarity.

Cylindrical lens has different curvature in horizontal and vertical direction which can enable the person to see the object clearly as shown in the figure below.

True

Those who suffer from Short-sightedness can see near objects clearly, but not the far away objects. This defect of vision occurs when the eye ball slightly elongates from front to back. The figure below shows it.

True.

When the person is not able to see the objects clearly or he is having difficulty in seeing the objects he may have defect of vision like myopia, hypermetropia, presbyopia, astigmatism.

True.

In myopia person is not able to see faraway objects due to elongation of the eyeball. The image is formed in front of the retina as shown in the figure below.

True

The white light coming from the sun is the spectrum of the seven colors violet, indigo, blue, green, yellow, orange, red. This can be proved by passing the white light through the prism As shown in the figure below.

True

The scattering\bending of the light is inversely proportional to the wavelength of light. Therefore violet color having the least wavelength bends the most while the red color having the highest wavelength bens the least.

False

Light of all colors travel with same speed in vacuum. But, in any transparent medium, such as glass or water the lights of different color travel with different speeds because the relative refractive index is different for the different colors of light. The red light travels the fastest and the violet light slowest of all the seven colors.

True.

Due to the difference in the speed the light of different colors bend at the different angles. In any transparent medium, The red light travels the fastest and the violet light slowest of all the seven colors. Therefore, the red light bends the least, and the violet light bends the most.

Thus dispersion of white light into seven colors occurs because the light of different colors travels at the different speed.