The Human Eye And The Colourful World (Class X Standard) : Full Explanation

We will begin with the structure of the eye.What does an eye look like?We all have gone through 'Heredity And Evolution' and have come to know that eyes develop bit by bit in every generation,which is a very popular adaptation.The eyes have enough variation in each organism to have different evolutionary origin.Here,we are going to deal with the human eye.

The human eye is one of the most valuable and sensitive sense organs.It opens up the amazing world of colour before our eyes.

The human eye is like a camera.Its lens system forms an image on a light sensitive screen called the retina.Light enters the eye through a thin membrane called cornea.It forms the transparent bulge on the front surface of the eyeball (as you can see in the figure below).

The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina.We find a structure called iris before the cornea.Iris is a dark,muscular diaphragm that controls the size of the pupil.The pupil regulates and controls the amount of light entering the eye.The eye lens (which is a convex lens,remember it is bulging outside) forms an inverted,real image on the retina.The retina is a delicate membrane having enormous number of light sensitive cells.The light sensitive cells get activated upon illumination and generate electrical signals which are sent to the brain via the optic nerves.The brain interprets these signals and finally,process the information so that we perceive objects as they are.

The structure of the human eye

Why do you have difficulty in locating objects in a dark or dim-lit room once you step in the dark room from a well-lit room?

It is very interesting to look into why this transition causes difficulty of the eye to locate objects.You will also find that after some time,you can locate objects in the dim-lit room.

The reason is : The pupil of an eye acts like a variable aperture whose size can be varied with the help of the iris.When the light is very bright,the iris contracts the pupil to allow less light to enter into the eye.However,in dim light,the iris expands the pupil to allow more light to enter into the eye.Thus,the pupil opens completely through the relaxation of the iris.

Power of accommodation of the eye

The eye lens is composed of a fibrous,jelly-like material.Its curvature can be modified to some extent by the ciliary muscles.The change in the curvature of the eye lens can thus change its focal length.When the muscles are relaxed,the lens becomes thin.Thus,its focal length increases.This enables us to see distant objects clearly.When the objects are closer,the ciliary muscles contract,the eye lens becomes thick and the focal length reduces,which enables us to see nearby objects clearly.

The ability of the eye lens to adjust its focal length is known as accommodation.However,the focal length of the eye cannot be adjusted beyond a certain limit.That is why,when we try to see objects that are located too far (you can't see the Eiffel tower sitting in Massachusetts!) and you will see blurred images and feel a strain in your eye when you hold objects closer than 25 cm from the eye.

The minimum distance at which an object can be viewed clearly and distinctly is termed as near point.For a young adult with normal vision,it is about 25 cm.

The farthest point upto which the eye can see objects clearly is termed as far point of the eye.Therefore,a normal eye can see objects clearly that are between 25 cm and infinity.

Sometimes,the crystalline lens of people at old age become cloudy and milky.This condition is called cataract.It is possible to restore vision through a cataract surgery.

A Myopic eye

Myopia : Causes And Correction

Causes of Myopia

Myopia can be seen in people because of two main reasons : - 

• Excess curvature of the eye lens.
• Elongation of the eye ball.

Correction of Myopia

Myopia or nearsightedness is the defect of the eye where the far point comes closer than infinity,i.e. the problem of the eye is to see objects located far-off clearly.This defect can be corrected using concave lens which diverges and shifts the image to the retina,thus allowing the person to see the objects clearly.

A Hypermetropic eye

Hypermetropia : Causes And Correction

Causes of Hypermetropia

Hypermetropia,also known as Hyperopia or farsightedness is caused in people due to : - 

• The eyeball becoming small in size.
• Increasing of the focal length of the eye lens.

Correction of Hypermetropia

Hypermetropia is corrected using convex lens of suitable power which converges and shifts the image to the retina from beyond.

 A Presbyopic eye

In human eye,along with increase in age,the near point of the eye recedes and the far point reduces resulting in both Myopia and Hypermetropia at the same time.This defect is known as Presyopia.

Causes of Presbyopia 

• Weakening of the ciliary muscles.
• Reducing ability of the eye lens to change its curvature.

Presbyopia can be corrected with the help of bi-focal lenses.A very common example of bi-focal lenses is a lens whose upper part is concave and lower part is convex lens.

An Experiment To Understand Why The Sky Generally Appears Blue But Appears Reddish During Sunrise Or Sunset

• We have to place a strong source of light at the focus of a converging lens (Marked L1 in the given figure.)This lens provides a parallel beam of light.
• We have to allow the light to pass through a transparent glass tank (Marked T in the given figure.) containing clear water.
• Now,we have to allow the beam of light to pass through a circular hole (Marked C in the given figure.) and subsequently through another converging lens (Marked L2 in the given figure.) so that a sharp image is obtained on the screen placed next to it (Marked M in the figure.)
• We have to then dissolve 200 g of Sodium Thiosulphate (Hypo) in 2 L of water and add 1 to 2 ml of concentrated sulphuric acid into it.

Schematic Representation of The Experiment

Observations And Conclusions

We will be observing that from the three sides except the side of the glass tank facing the circular hole that the colour of the light observed is blue.But when we observe from the fourth side of the glass tank facing the circular hole,we will first observe an orange red colour which subsequently changes to bright crimson red. This experiment helps us to understand why the sky generally appears blue but appears reddish during the sunrise or sunset.We will have a clear discussion regarding this in the next question.

• Question 1 : Why does the sky appear reddish during sunrise or sunset?

During the sunrise or sunset,the sunlight has to pass through thicker layers of air and cover a larger distance.Here,the atmospheric particles are larger in size and as the size of the particles is directly proportional to the wavelength of the colour of light scattered,most of the blue light of relatively shorter wavelength is scattered away and the colours of longer wavelengths reaches our eyes.This gives the sky the reddish appearance during sunrise or sunset.

• Question 2 : Why does the sky appear white at the noon?

At noon,the sun has to travel relatively shorter distances and so,it goes through comparatively less amount of refraction because of which less blue is scattered and the sky appears white.

Differing colour of the sky due to differing position of the Sun

• Question 3 : Why is the sky visible to us two minutes before sunrise and two minutes after sunset?

The Sun is visible to us two minutes before sunrise and two minutes after the sunset due to atmospheric refraction.By actual sunrise,we mean the actual crossing of the sun but we are able to visualise the Sun even before it actually crosses the horizon and we are able to see it even after it has crossed the horizon during sunset.The figure demonstrates the actual and apparent positions of the Sun clearly.

The apparent flattening of the Sun during sunrise or sunset is also due to atmospheric refraction.

Advance Sunrise and Delayed Sunset

Question 4 : Why does dispersion take place? 

The speeds of all the components of light in the air or vacuum is the same but in different media,such as glass,the speed of every separate component of white light varies.This leads to differing refractive indices and white light splits up into its seven constituent colours.

This phenomenon of splitting up of white light into its constituent colours is termed as dispersion of white light.

Dispersion of white light through a glass prism

Question 5 : What will happen if two prisms are placed next to each other in a manner such that the second prism is inverted in relation to the first prism?

This will lead to the passing of white light from the second prism or will result in the recombination of white light.This is due to the fact that the bending of light produced by the second prism is equal and opposite in comparison to the bending of light produced by the first prism.This was first explained by Sir Issac Newton.

Twinkling of the stars

• Question 6 : Why do the stars appear higher than they actually are located?

The stars approximate point sized sources of light placed far away from the eyes.Since the light from the stars has to pass through media of differing refractive indices and the light subsequently keeps bending towards the normal,the stars appear higher than they actually are.

• Question 7 :Why do the stars twinkle?

Due to varying refractive indices in different layers of the atmosphere,the amount of light entering our eyes from the stars differ.Sometimes,they appear relatively brighter whereas sometimes they appear relatively fainter,which is the twinkling effect.

• Question 8 :Why do not the planets twinkle? 

Since the planets are located much closer to the earth than the stars,they act as extended sources.They act as a collection of a large number of point sized objects,the total variation in the amount of light entering our eyes from all the point sized sources will average out to zero and will nullify the twinkling effect.

• Question 9 :Why does a rainbow form?

A rainbow forms due to refraction,followed by total internal reflection and refraction again of sunlight through the tiny water droplets present in the atmosphere.

• Question 10 : What are the necessary conditions for the formation of a rainbow?

(a) The observer's back needs to be turned towards the Sun.

(b) Presence of tiny water droplets in the atmosphere is required.

Image Credit : www.ekshiksha.org.in/Miscellaneous sources