The Human Eye and Colourful World : Complete Set of Questions



This set of questions contains all the possible concepts
which could be asked in the examination

The Human Eye 

Q.1 Draw a neat labelled diagram of human eye and explain the function of each part

The main parts of the human eye are -

• Cornea- Front part of the eye bulging outwards

• Iris- It controls the amount of light entering in the eye, by varying the size of the pupil.

• Pupil- It is the central hole in the iris

• Ciliary muscles- These muscles hold the eye in position

• Eye lens- It is a crystalline lens made of transparent material. Its curvature and focal length can be changed by ciliary muscles.

• Aqueous humour- Liquid filling the space between cornea and eye lens.

• Retina- It is light sensitive screen on which the image is formed

• Vitreous humour- It is transparent liquid filling the space between eye lens and cormnea.

• Optic nerve- These carry electrical signal to the brain

• Blind spot- A raised point on the retina which is insensitive to light

• Optic nerve-Line passing through the centre of the eye lens and cornea.


Q.2 Why is blind spot so called?

The image formed at this point is not sent to the brain.


Q.3 Explain the working of the human eye:

The light coming from an object enters the eye through the cornea and the pupil

The crystalline lens focuses these light rays to form a real, inverted and highly diminished image on the retina.

When the light rays fall on the sensory cells (rods and cones), they get activated upon illumination and generate electrical signals.

These electrical signals are then sent to the brain by the optic nerve.

The brain interprets the signals and renders the erect image of the object.


Q.4 Define ‘power of accommodation of the eye’?

The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye. 


Q.5 How does an eye focus objects at varying distances?
To focus on distant objects the ciliary muscles relax making the eye lens thin.
As a result the focal length of the eye lens increases and we see the distant objects.
But to focus on nearby objects, the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects.
In short, it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.


Q.6 What is meant by near point and far point?

Near Point: Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly.
For a normal eye the least distance of distinct vision is about 25 centimetres.
However, it varies with age of the person. For example, for infants it is only 5 to 8 cm.

Far Point : Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.


Q.7 Define Range of Vision

The distance between the near point and the far point is called the range of vision


Q.8 What is meant by Persistence of vision?

The image of an object seen persists on the retina for 1/16 second even after the removal of the object. This continuance of sensation of eye for some timed is called persistence of vision.


Q.9 The image formed on the retina is inverted. Then, why do we see an erect image of our surroundings?

When the brain interprets the image, it again inverts the image formed on the retina, to make the objects visible in the same way as they are in our surroundings.


Q.10 How is the focal length of eye lens changed?
The focal length of eye lens is changed by changing of the curvature of the eye lens by ciliary muscles.

When the ciliary muscles relax, the lens becomes thin and its focal length increases.

When the ciliary muscles contract, the curvature of the lens increases and its focal length decreases. 

 

Defects of Vision and Their Correction 

Q.11 Name the three common refractive defects of vision of human eye?

Some of the defects of vision are

• Hypermetropia or long sightedness

• Myopia or short sightedness and

• Presbyopia


Q.12 What is Myopia? How is it caused? How can it be corrected? Explain with the help of a diagram.

Myopia : Myopia is also known as near-sightedness. A person with myopia can see near objects clearly while distant objects appear blurred.  In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself.

Causes 

This defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to either

(i)  Excessive curvature of the cornea, or

(ii) Elongation of the eyeball, that is, the distance between the retina and eye lens is increased.

Normal Eye - 

Myopic eye - 

Correction - 

This defect can be corrected by using a concave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.


Q.13 What is hypermetropia? How is it caused? How can it be corrected? Explain with the help of a diagram.

Hypermetropia : Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25 cm).

Normal Eye

Figure : Near point of a Hypermetropic eye

Causes of Hypermetropia

Hypermetropia is caused due to the following reasons:

• Increase in focal length of the eye lens i.e., the focal length of the eye lens is too long

• The eyeball has become too small.

Hypermetropic Eye

Correction - 

This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.


Q.14 What is presbyopia? How is it caused? How can it be corrected?

Presbyopia : The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia.

Causes - 

It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. 

Presbyopia – The image of a near object focuses behind the retina

Correction-

Eyeglasses with bifocal lenses are the most common correction for presbyopia. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

Correction for presbyopia : A plus lens focuses the image on the retina

Bifocal glasses - 


Q.15 What do we mean when we say that a person is suffering from cataract?

Cataract is a condition of partial or complete loss of vision. During old age, the crystalline lens of some people becomes milky and cloudy. This causes partial or complete loss of vision. It is possible to restore vision through a cataract surgery.


Q.16 Why do we have two eyes for vision and not just one?

There are several advantages of our having two eyes instead of one. It gives a wider field of view. A human being has a horizontal field of vision of about 150 degrees with one eye and of about 180 degrees with two eyes. The ability to detect faint objects is, of course, enhanced with two detectors instead of one. As our eyes are separated by a few centimetres, each eye sees a slightly different image.


Q.17 How does a convex lens correct hypermetropia?

A farsighted eye or hypermetropic eye is assisted by the use of a converging lens also called as convex lens. This converging lens will refract light before it enters the eye and subsequently decreases the image distance. By beginning the refraction process prior to light reaching the eye, the image of nearby objects is once again focused upon the retinal surface.


Q.18 How does a concave lens correct myopia?
In case of myopia or nearsighted eye, we use concave lens also called as diverging lens because a diverging lens will serve to diverge light before it reaches the eye. This light will then be converged by the cornea and lens to produce an image on the retina.


Q.19 Define power of a lens:
The power of a lens is defined as the reciprocal of its focal length in metres.

 

Refraction of Light Through a Prism 

Q.20 What is angle of the prism?

The angle between two plane refracting surfaces of a prism is called the angle of the prism or refracting angle.


Q.21 What is a prism?

Prism is a transparent optical element which refracts light. An optical object to be defined as prism must have at least two faces with an angle between them. Triangular prism is the most common type of prism. It has a triangular base and rectangular sides.


Q.22 What is meant by refraction?

It is that phenomenon due to which a ray of light on passing from one medium to another changes its direction i.e., it bends towards or away from the normal. The ray of light changes its direction or phenomenon of refraction takes place because of difference in speed in different media.


Q.23 What do we mean by angle of incidence?

The angle formed between the incident ray and the normal at the point of incidence is known as the angle of incidence. 

‘i’ is the angle of incidence in the above pic.


Q.24 What do we mean by angle of refraction?

The angle between the normal and the refracted ray is known as the angle of refraction.

Angle ‘r’ is the angle of refraction in the above diagram


Q.25 What do you mean by angle of Deviation in prism?

The emergent ray bends at an angle to the direction of incident ray. This angle is called the angle of deviation in the prism and is represented by Greek letter δ or qd.

d is the angle of deviation in the above diagram.

 

Dispersion of White Light By a Glass Prism 

Q.26  What is meant by dispersion of light? What are the causes for dispersion?

It is the phenomena of splitting of white light into its constituent seven colours on passing through a Glass prism.

Cause of Dispersion : 

Every colour has its own characteristic wave length or frequency. Different colours move with same speed in air/vacuum. But their speeds in refracting media like glass are different. Therefore, refractive index of the medium for different colours is different. As a result, different colours undergo different deviations on passing through the prism. Hence, different colours emerge from the prism along different directions.


Q.27 What is a spectrum?

The band of colours obtained due to the dispersion of white light is referred to as a spectrum.


Q.28 How did Newton come to know that sunlight is made up of seven colours?
Newton first used a glass prism to obtain the spectrum of sunlight. 

He tried to split the colours further by placing another prism. However he could not get further colours.

So he placed a second identical prism in an inverted position with respect to the first prism.

He found a beam of white light emerging from the other side of the second prism. This observation gave Newton the idea that the sunlight is made up of seven colours.


Q.29 What do you mean by decomposition of white light?
Recombination of the seven colours of the dispersed white light to get white light is known as decomposition of light.


Q.30 What is a rainbow?

Rainbow : It is a concentric coloured circular arc in the sky when the sun rays fall on rain drops during or after a shower. To watch a rainbow, an observer must stand with his back facing towards the sun.


Q.31 Explain the formation of rainbow.

The water droplets in the atmosphere act like small prisms.

• When white light from the sun enters a spherical raindrop, the light is refracted and dispersed. The different colours of light are bent through different angles.

• When different colours of light fall on the back inner surface of the water drop, it(water drop) reflects them(different colours of light) internally(total internal reflection).

• The water drop finally refracts the different colours of light again when it comes out of raindrop.

• Due to the dispersion of light and internal reflection, different colours reach the observer’s eye, and the observer can see a rainbow.          


Q.32  What is vibgyor?
Vibgyor is an acronym for the seven colours of a rainbow, in order -violet, indigo, blue, green, yellow, orange and red.


Q.33 What are the phenomena involved in the formation of rainbow?

1. Dispersion of light

2. Internal reflection.


Q.33 Explain the dispersion of white light by a prism.
The light of larger wavelength deviates through small angle, whereas the light of smaller wavelength deviates through larger angle. Thus, when a polychromatic light like white light is incident on the first face of a prism and enters it, the waves of different wavelengths (colours) deviate through different angles and in the process get dispersed. While passing out from the opposite face of the prism they do not suffer any dispersion, but suffer refraction and the colours are separated further. Thus, a beam of white light incident on a prism splits into its constituent colours to form a spectrum.

Violet suffers the maximum deviation and red the least.

 

 

Atmospheric Refraction - 

Q.34 Why do stars twinkle but planets do not twinkle

The twinkling of a star is due to atmospheric refraction of starlight. Since the stars are very distant, they approximate point-sized sources of light. 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 – the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.

 The planets are much closer to the earth, and are thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect. 


Q.35 Why do the objects seen through air above fire appear flickering?

The air above the fire is hotter than surrounding air.

• The hotter air is lighter(less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air above it.

• Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the objects, as seen through the hot air, fluctuates. 


Q.36 Name some phenomena associated with atmospheric refraction.

1. Twinkling of stars          

2. Wavering of objects seen through a turbulent stream of hot air.

3. Advance sunrise and delayed sunset.


Q.37 Why does sun appears to rise 2 minutes early than actual sunrise, and why does sunset appears to take place 2 minutes after the actual sunset?

In the early morning, before the sun actually crosses the horizon, the sunrays reach us due to atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun. So, the sun appears to rise 2 minutes earlier than it actually rises.

Similarly in the evening, when the sun crosses the horizon, the sunrays still reach us for 2 minutes due to atmospheric refraction. So the sun appears to set 2 minutes late than the actual sunset. 


Q.38 Why does the star appear slightly higher than its actual position?
The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. So, the star appears slightly above than its actual position when viewed near horizon.

 

Scattering of Light  

Q.39 What is meant by scattering of light?

Scattering is the phenomenon by which a beam of light is redirected in many different directions when it interacts with a particle of matter. For example- When sunlight strikes the molecules in our atmosphere, the light is redirected in many directions. The blue light is scattered more than the red light causing the sky to be blue.


Q.40 State some phenomena associated with the scattering of light.

1. The blue colour of the sky

2. Colour of water in deep sea

3. Reddening of the sun at the sunrise and sunset


Q.41 Why does the Sun appear reddish at sunrise and sunset?

In the early morning and evenings, the sun is situated near the horizon. Light from the Sun passes through thicker layers of air and has to travel larger distance through the earth’s atmosphere before reaching our eyes. Due to the presence of molecules of air and other fine particles in the atmosphere, scattering of light takes place. The blue light due to shorter wavelength get scattered in midway and is not visible in the sky. Only red light having larger wavelength is able to reach our eye due to less scattering. Hence, the sun appears reddish during sunrise and sunset. 


Q.42 The sky appears dark to passengers flying at very high altitudes. Give reason

As the astronaut moves far away from the earth’s atmosphere and flies at a high altitude in space, the medium becomes rarer. There is the vacuum in the space and hence no particle is available for scattering of light. In the absence of scattering, none of the colours from the visible spectrum reach the viewer’s eye and the sky appear dark to the astronaut.


Q.43 What is Tyndal Effect? Explain with an example.

The phenomenon of scattering of light by the colloidal particles like particles of atmosphere is known as Tyndall effect. When a beam of sunlight enters a dusty room through a window then its path becomes visible to us. This is because the tiny dust particles present in the air of room scatter the beam of light all around the room. And when thus scattered light enters our eyes, we can see the beam of light. Thus, an example of Tyndall effect is the way a beam of sunlight becomes visible as it passes through dust particles in the air of a room.


Q.44 How does the colour of the scattered light depend on the size of the scattering particles?
Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear as white.


Q.45 What causes scattering of light in atmosphere?

Smoke, tiny water droplets, suspended particles of dust and molecules of air cause scattering of light in the atmosphere.


Q.46 Why can we see Tyndall effect when sunlight passes through a canopy of a dense forest?

When sunlight passes through a canopy of a dense forest, tiny water droplets in the mist scatter light. So, we can see Tyndall effect.


Q.47 Why are ‘danger’ signal lights red in colour?

The red light is least scattered by fog or smoke. Therefore ‘danger’ signals are red in colour so that they can be seen in the same colour at a distance.


Q.48 Why does not the sun appear reddish during noon?
At noon the sun is overhead. So, the light travels shorter distance and appears white as only a little of blue and violet colours are scattered.


Q.49 Why do clouds appear to be white in colour?

Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths the component colours of white light(i.e red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light.


Q.50 We are able to see the movie picture in a cinema hall.

The ability of the human eye to continue to see the image of an object for a very short duration even after the removal of the object is called persistence of vision. It is due to the persistence of vision that we are able to see movie pictures in a cinema hall.

 

Value Based Questions :

Q.1 In a class-room, there were four or five students who were not able to read the material written on black board. The other students of class helped them to sit at the front seat, so that they can also read the material written on blackboard.

(a) What is the name of the refractive defects of vision of human eye the 4 students are suffering from?

(b) What are its causes and can it is corrected.

(c) What moral value of the students are highlighted

(a) Myopia : Myopia is also known as near-sightedness. A person with myopia can see near objects clearly while distant objects appear blurred.  In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself.

(b) Causes : This defect arises because the power of the eye is too great due to the decrease in focal length of the crystalline lens. This may arise due to either

(i) Excessive curvature of the cornea, or

(ii) Elongation of the eyeball , that is, the distance between the retina and eye lens is increased. 

Correction -

This defect can be corrected by using a concave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.

(c) The other students display their helpful attitude, kindness and also display humanitarian feelings. 

On the rainy day, Ram reached his grandfather’s place in village. On the way to his grandfather’s house he saw a beautiful rainbow in this sky. In night, he saw lots of twinkling stars in the clear sky. He was very excited to see these beautiful natural phenomenons, which he was not able to see in the city, where he lived with his father. He takes a few photos and a couple of videos using his mobile of the rainbow so that he could show his friends how a real rainbow looks like.  


Q.2 How is a rainbow formed?

What do you think would be the reason behind the lesser frequency of appearance of rainbows in cities?

What value of Ram is highlighted here?

(i) The water droplets in the atmosphere act like small prisms.

• When white light from the sun enters a spherical raindrop, the light is refracted and dispersed. The different colours of light are bent through different angles.

•When different colours of light fall on the back inner surface of the water drop, it(water drop) reflects them(different colours of light) internally(total internal reflection).

• The water drop finally refracts the different colours of light again when it comes out of raindrop.

• Due to the dispersion of light and internal reflection, different colours reach the observer’s eye, and the observer can see a rainbow.          

(ii) The reason behind the lesser frequency of appearance of rainbows might be because of pollution and deforestation has reduced the occurrence of rainfall in cities.

(iii) Helpful mentality and his commitment towards studies.



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