The Human Eye and The Colourful World.
The Human Eye :
9. Optic Nerve:
• Consider a ray of light PE incident on refracting face AB at point E.
• The phenomenon of splitting of white light into its seven constituent/component colours, when it passes through a glass prism is called Dispersion .
• When white light is passed via prism dispersion of light happens, the dispersed light that was produced will revert to white light when another prism is held inverted. It is known as the recombination of spectrum of light.
• A rainbow is a natural spectrum produced by the dispersion of sunlight by raindrops in the atmosphere/sky after rain.
• Light emitted by stars passes through the atmosphere of the earth before reaching our eyes.
• The sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
Examples of Tyndall effect are
(ii) Tyndall effect is seen, when sun light comes down through the clouds.
• The molecules of air and other fine particles in the upper layer of atmosphere have smaller size and are more effective in scattering light of shorter wavelength at the blue end than light of longer wavelengthat the another end
• Danger signs/signal light are made in red colour as red is least scattered by fog or smoke and it can be seen in the same colour over a long distance.
• The human eye is one of the most valuable/significant sensitive light sensing organ which helps/enable us to see the beautiful / wonderful colorful world & object around us.
• It is the natural optical Instrument.
• It has lens (convex) in its structure.
• It works on the refraction of light.
• It is spherical in Shape like a ball of diameter about 2.3 cm.
The main/essential parts The human are:
1. Cornea:
• Light enters the eye through a thin, transparent membrane is called cornea.
• It is front projected part of eye bulging outwards.
• Light first enters our eye through this.
• It refracts most of the incident light into the eye.
• It protect eye from foreign agents like dust.
2. Aqueous humour:
• Between the cornea & eye lens, their is a space filled with transparent Liquid (water like consistency) called Aqueous humour.
• It helps the refracted light to be focused on retina
• It also provides nutrition to eye as well as maintain pressure balance.
3. Iris:
• It is a dark muscular diaphragm blw conea & lens which/that contol the size of pupil and controls the light level inside the eye or amount of light entering the eye.
• It is just behind the cornea.
4. Pupil:
• A small hole b/w/middle/centre of the iris called pupil.
• It controls & regulates the amount of light entering inside the eye.
• The pupil of the eye acts like a variable aperture whose size can be varied/adjust with the help of Iris according to/depending upon the intensity/amount of the light received/entering by the eye.
• In dim light the iris expands the pupil to allow more light to enter the eye.
Thus, the pupil open up completely due to relaxation of iris lens.
• In bright light, the iris contracts the pupil to allow less light to enter the eye.
Thus, in bright light, the pupil becomes very small due to Contraction of iris.
5. Crystalline lens:
• It is also called eye lens.
• It is a soft, flexible, fibrous jelly-like material made of proteins.
• Basically, the eye lens is a convex lens.
• It convergs/focus the light of the object on retina.
• It produced / forms real & Inverted image on retina.
6. Ciliary Muscles:
•They hold the lens in position and help in modifying/to change the curvature/focal length of the lens.
• It change the thickness of eye lens while focusing the light on retina.
• They adjust & controls the focal length of the eye lens according to the distance of an object.
• When the muscles an relaxed, the lens becomes thin.
Thus its focal length increases.
This enable us to see distant object clearly.
• When you Looking at object closer to the eye, ciliary muscles contracts.
This makes the lens thick.
Thus its focal length decreases.
This enable us to the See the nearby objects clearly.
7. Vitreous Humour :
• Space between eye lens and retina is filled with a liquid (Honey like consistency) called Vitreous humour.
• It maintains the spherical shape of eye.
8. Retina:
• The Photo receptive/ light-sensitive screen on which the image is formed in the eyes called retina.
• It is at the back part of the eye.
• The retina is a delicate membrane which consists of/having enormous number of/numerous light-sensitive/vision cells called rods and cones.
Or
It consists of a thin membrane which is rich in nerve fibres, containing two kinds of vision cells (rods and cones).
Rods =These cells sesitive to intensity of light •
• We can see in the dark due to the presence of rod cells in the retina for eyes.
• Owl have a large number of rod cells in their retina which help them to see properly during the night.
Cons = These cells sensitive to colour of light.
• The retina converts light that enters into your eye into electrical signals.
• These signals are sent to the brain via the optic nerves.
• It connect with brain and send/transmit/carry the electrical signal/impulse from retina to brain.
• The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.
10. BLIND SPOT:
• The point where optic nerve enters the eye ball is called blind spot.
• It has no nerve endings.
• It is the area on the retina which does not have light sensitive cells (rods and cones).
• So if the image is formed on this part, then no signal is sent to the brain.
As a result object is not visible to the eye.
Power of Accomodation:
• The ability of an eye to focus the distant/far objects as well as the nearby objects on the retina by changing/adjusting the focal length is called accomodation.
Near point of the eye:
• The nearest point/minimum distance up to which the eye can see clearly/distinctly without any strain is called the near point of eye.
• For a young adult with normal vision, the near point of the eye is about 25 cm.
• This distance is known as least distance of distinct vision.
Far point of the eye:
• The farthest point/maximum distance upto which the eye can see objects clearly/distinctly is called the far point of the eye.
• It is infinity for a normal eye.
Note: A normal eye can see/focus objects clearly that are between 25 cm and infinity.
Cataract:
• Sometimes, the crystalline lens of people at old age becomes milky and cloudy.
• This condition is called cataract.
• This causes partial or complete loss of vision.
• It is possible to restore vision through a cataract surgery.
DEFECTS OF VISION AND THEIR CORRECTION
There are three common defects of vision.
These are:
(i) Myopia (Near-sightedness)
• It is also known as near-sightedness.
• It is kind of defect of vision in the human eye by virtue of/due to which a person can see nearby objects clearly but cannot see the distant/far off objects clearly/distinctly.
• The far point of an eye suffering from myopia is less than infinity.
• Such a person may see clearly upto a distance of a few metres.
• In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
Causes: This defect may arise due to
(i) Excessive curvature of the eye lens/Decrease in focal length of the eye lens.
(ii) Elongation of the eyeball/Increase in the length of the eye ball (as if distance of retina from the eye lens has increased/retina is at a larger distance from the eye lens).
Correction : This defect can be corrected by using a concave lens of suitable power.
• A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
ii) Hypermetropia (Far-sightedness)
• Hypermetropia is also known as far-sightedness.
• It is kind of defect of vision in the human eye by virtue of/due to which a person can see distant/far off objects clearly but cannot see the nearby objects clearly/distinctly.
Or
A human eye which can see far off objects or distant objects clearly but cannot see the near objects clearly is said to be suffered with a defect known as long sightedness or far sightedness or Hypermetropia.
• The near point, for the person, is farther away/little far from the normal near point of eye (25 cm).
• In a hypermetropic eye, the image of a nearby object is formed/focused at a point behind the retina and not at the retina itself.
Causes: This defect arises/causes due to
(i) the focal length of the eye lens is too long/Increase in focal length of the eye lens (when the eye is fully relaxed).
(ii) The eyeball has become too small/Decrease in length of the eye ball (as if distance of retina from the eye lens has decreased).
Correction: This defect can be corrected by using a convex lens of appropriate/suitable focal length/power.
iii) Presbyopia (Old Age Hypermetropia)
• A human eye which cannot see the near objects clearly due to loss of power of accommodation of the eye is said to suffer from a defect known as presbyopia.
• This is a kind of hypermetropia which is experienced in old age and therefore, also called old age hypermetropia.
• The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 centimetres away.
Causes: It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens i.e decrease in flexibility of the crystalline lens of the human eye.
Correction: This defect is corrected in the same way as hypermetropia i.e To correct presbyopia, an old person has to use spectacles with/having a convex lens of suitable focal length, or power
Note: The cause of hypermetropia is decrease in length of eye ball or increase in focal length of eye lens.
But the cause of presbyopia is only increase in focal length of eye lens.
The eye ball, in presbyopia, has normal length.
Sometimes same person may suffer from both the defects of vision, i.e., myopia and hypermetropia.
• Such type of defects can be corrected by using spectacles having bifocal lenses which consist of both concave and convex lenses.
• The upper portion of the spectacles consists of concave lens for the person to see distant objects and the lower portion consists of convex lens which help him to see nearby objects.
Prism
• Prism is a homogenous transparent refracting object/medium/material (made up of glass) bounded/enclosed by two lateral surfaces , inclined to each other at certain/some/fixed angle.
• It has two triangular bases and three rectangular lateral surfaces.
• These surfaces are inclined to each other.
• The two plane surfaces PQBD and PRCD are called as refracting faces.
• The section DBC of the prism is known as principal section.
• The line along which the two refracting faces meet is called refracting edge of the prism.
• The angle between the two refracting/lateral faces is called the angle of prism and is denoted by A.
Refraction of light through glass prism
• The incident ray PE is going/enters from air (rarer medium) into glass (denser medium), so it bends towards the normal NN' at point E and refracted along EF inside the glass prism.
• The refracted ray EF again suffers refraction at F, since the ray EF is going from glass (denser medium) into air (rarer medium), so it bends away from the normal MM' at point F and emerges out along FS in the form of emergent ray.
• If the incident ray PE is produced forward and emergent ray FS is produced backward, then the two rays meet at G and The angle ∠D is formed known as angle of deviation.
Angle of Deviation
• The angle between incident ray and emergent ray is called angle of deviation.
• In refraction through a glass slab, the emergent ray is parallel to the incident ray but in refraction through a glass prism, the emergent ray is not parallel to the incident ray.
• The emergent ray of light in a glass prism is not parallel to the incident ray of light because the opposite faces of the glass prism (where refraction takes place) are not parallel to one another.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM:
• The band of seven colours formed/obtained on a white screen, when a beam of white light is passed through a glass prism, is called spectrum of white light.
• The colours can be remembered by the word VIBGYOR.
• The order/sequence of the colours from the lower end of the spectrum is violet, indigo, blue, green, yellow, orange and red.
Note: White light is a mixture of seven colours
Why Dispersion occurs?
• Light waves/rays of all different colours travel with the same speed or have same velocity in a vacuum/air, but they travel at different speeds and bends through different angle in some/other refracting medium, which leads to dispersion of light.
Note: Dispersion occurs due to the different degrees of refraction experienced by different colours of light.
• The red colour has the maximum speed in glass prism, so the red colour is deviated/bends the least. Due to this the red colour forms the upper part of the spectrum.
• On the other hand, the violet colour has the minimum speed in glass prism, so the violet colour is deviated/bends the maximum/most. Due to this violet colour appears at the bottom of the spectrum.
Recombination of Spectrum of Light
• Sir, Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight.
• He tried to split the colours of the spectrum of white light further by using another similar prism.
• However, he could not get any more colours.
• He then placed a second identical prism in an inverted position with respect to the first prism.
• This allowed all the colours of the spectrum to pass through the second 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.
• Any light that gives a spectrum similar to that of sunlight is often referred to as white light.
• The first prism is known as dispersing prism as dispersion takes place at the first face of prism (A) and second prism (B) recombines the dispersed colours therefore it is known as recombination prism.
• Both the prism A and B together act as a glass slab with parallel sides, and emergent ray is parallel to the incident ray.
Rainbow
• Some water droplets remain suspended in air act/behave like/as small prisms.
• When the sunlight incident/enters/falls on the rain/water drop, it refracts and disperses(splits). Then it reflects internally and again refracts as it come out of the drop and the seven different colours reach the eye of observer in form of rainbow.
• A rainbow is always formed in a direction opposite to that of the Sun.
• Three phenomenon which are involved in rainbow formation are : 1) Dispersion 2) Refraction 3) TIR
Twinkling of stars:
• The atmosphere of the earth is not uniform but consists of many layers of different densities.
• The layers close to the surface of the earth are optically denser.
• As we go higher and higher, the density of layers and refractive index decreases progressively.
• As the light from a star enters the upper-most layer of the atmosphere, it bends towards the normal as it enters the next layer.
• This process continues till the light enters our eyes.
• So due to refraction of light, the apparent position of the star is di๏ฌerent from the actual position of the star.
• The star appears slightly higher (above) than its actual position when viewed near the horizon.
• Moreover, the di๏ฌerent layers of the atmosphere are mobile and the temperature and the density of layers of atmosphere changes continuously.
• The physical conditions of the earth’s atmosphere are not stationary. Hence, the apparent position of the star changes continuously.
• The change in the apparent/actual position of the star continuously leads to the twinkling of a star.
Why don’t the planets twinkle?
• The planets are much closer to us as compared to the stars.
• Since the planets are closer to us, they appear much bigger and the light to come from more than one point.
• The light coming from several points undergoes refraction also. But this refracted light from several points cancels the twinkling effect.
• Hence, planets do not twinkle, due to their relatively large size and close distance from Earth
Advanced Sunrise and Delayed Sunset:
• The actual sunrise takes place when the sun is above the origin.
• The time difference between actual sunset and the apparent sunset is about 2 minutes. Thus, we gain about 4 minutes of additional daylight each day.
• The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon.
Scattering of Light
• When sunlight enters the atmosphere of the earth, the atoms and molecules of different gases present in the atmosphere absorb this light.
• Then these atoms and molecules of the gases re-emit light in all directions.
• This process is known as scattering of light.
• It can even be observed when a strong beam of light is passed through colloidal solution; the path of beam becomes visible.
The colour of the scattered light depends on the size of the scattering particles:
• Shorter the wavelength greater will be the Effects of scattering of light.
• Very fine particles scatter mainly blue colour of light (shorter wavelength).
• Medium size particle scatter mainly red color (longer wavelength)
• Larger particle is scattere white light i.e all the colours of light.
Tyndall Effect
• The phenomenon of scattering of light by the colloidal particles (like dust, smoke and tiny water droplets suspended in air) is known as Tyndall effect.
• When a beam of light passes through a colloid its path becomes visible due to scattering of light by the particles.
(i) When a beam of sunlight enters a dusty room through a window, then path becomes visible to us.
Or
A beam of sunlight becomes visible as it passes through dust particles in the air of a room.
(iii) When sunlight passes through a canopy of a dense forest, tiny water droplets in the mist scatter light.
Why the colour of sky appears blue on a clear day ?
• Thus when sunlight passes through atmosphere the fine particles in air scattered the blue colour more strongly than that red or any other colour.
• The scattered blue light enters our eyes which makes the color of the sky blue.
• If the earth had no atmosphere, there would not have been any scattering and the sky would have looked dark.
•That is why it appears dark to the passengers flying at very high altitudes, as scattering is not prominent at such heights.
Why danger signs are made of red colour ?
• When light falls on the signal, all colours are scattered much more than that of red colour.
• So the red colour suffering least scattering remains confined around the signal, which in turn illuminates the signal significantly.
Why colour of sun appear red during sunrise and sunset ?
• During sunset and sunrise the sun is near to horizon, and therefore, the sunlight has to travel larger distance in atmosphere.
• Due to this, most of the blue light (Shorter wavelength) is scattered away by the particles.
• The light of longer wavelength (red colour) reaches our eye.
• This is why sun appear red in colour.
• When the sun is overhead (as at noon), then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us.
• Due to this only a little of the blue colour of the white light is scattered (most of the blue light remains in it).
• Since the light coming from the overhead sun has almost all its component colours in the right proportion, therefore, the sun in the sky overhead appears white to us.
Comments
Post a Comment