Chapter-11
The Human Eye and The Colourful World
Human eye : The sense organ that helps us to see.
• Located in eye sockets in skull.
• Diameter of eye ball - 2.3 cm
Parts of Human Eye
Cornea : It is the outermost, transparent part, It provides most of the refraction of light.
Lens : It is composed of a fibrous, jelly like material. Provides the focused real and inverted image of the object on the retina. This is convex lens that converges light at retina.
Iris : It is a dark musuclar diaphragm that controls the size of the pupil.
Pupil : It is the window of the eye, It is the central aperture in iris. It regulates and controls the amount of light entering the eye.
Retina : It is a delicate membrane having enormous number of light sensitive cells.
Far point : The maximum distance at which object can be seen clearly is far point of the eye. For a normal adult eye, its value is infinity.
Near point or Least distance of distinct vision
The minimum distance at which objects can be seen most distinctively without strain.
• For a normal adult eye, its value is 25 cm.
• Range of human vision - 25 cm to infinity.
Accomodation : The ability of the eye lens to adjust its focal length is called accomodation. Focal length can be changed with the help of ciliary muscles.
| Ciliary muscles ↓ |
Ciliary muscles ↓ |
| Relaxed ↓ |
Contract ↓ |
| Lens thin ↓ |
Lens thick ↓ |
| Focal length increases | Focal length decreases |
Myopia (Near sightedness)
• A myopia person can see nearby objects clearly but cannot see distant objects clearly.
• Image is formed in front of retina.
Causes of Myopia
• Excessive curvature of eye lens
• Elongation of eye ball
Correction
• Use of concave lens of appropriate power.
(a) In a myopia eye, image of distant object is formed in front of the retina (and not on the retina).
(b) The far point (F) of a myopia eye is less than infinity
(c) Correction of myopia The concave lens placed in front of the eye forms a virtual image of distant object at far point (F) if the myopia eye.
Hypermetropia (Far sightedness)
• Affected person can see far object clearly but cannot see nearby objects clearly.
• The near point of the eye moves away.
• Image is formed behind the retina.
•
• Focal length of the eye lens becomes too long.
• Eye ball becomes too small.
Correction
Use of convex lens of suitable power can correct the defect.
Presbyopia (Old age Hypermetropia)
It is the defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accomodation of the eye.
• The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 cm away.
Cause
• Gradual weakening of ciliary muscles.
• Diminishing flexibility of eye lens.
Correction
• Use of both concave and convex lens of suitable power.
• Sometimes a person may suffer from both myopia and Hypermetropia.
• Such people require bifocal lens for correction.
Advantage of the eyes in front of the face
• It gives a wider field of view.
• It enhances the ability to detect faint objects.
• It provides three dimensional view.
When white light is passes through a glass prism, it splits into its seven constituent colours to form a band of seven colours. This phenomenon is called dispersion.
Spectrum : The band of seven colours formed due to dispersion of white light is called specturm.
Acronym : It is a group of alphabets that represent sequential colours in spectrum.
V I B G Y O R
1
Angle of deviation ___________
wavelength
• Red is the least deviated colour as it has largest/ longest wavelength. • Violet is the most deviated colour as it has smalles wavelength in visible spectrum.
Q. Why spectrum is formed when white light is passed through a glass prism ?
Ans. Each colour has a definite wavelength and for each wavelength teh angle of deviation differs. Red is the least deviated and violet is the most deviated colour so different colours deviate at different angles to form spectrum.
Prism : It is a pyramidal piece of glass with two triangular bases and three rectangular lateral surfaces.
Angle of Prism : The angle between two adjoining lateral surfaces.
Refraction through a glass prism
Angle of deviation (d) : It is the angle between incident ray and emergent ray.
Issac Newton was the first person who proved that sunlight is made up of seven colorus :
-
(i) He passed sunlight through a glass prism to form a band of seven colours.
(ii) He tried to split the colours further by putting another prism ahead of the prism forming specturm but he failed to obtain more colours.
(iii) He formed a spectrum from and placed an identical but inverted prism in front of prism forming the spectrum. All the seven colours combined by the inverted prism and emerged as white light.
Q. What is referred as white light ?
Ans. Any light that forms a spectrum similar to that of sunlight is referred as white light.
Total Internal Reflection
When light enters obliquely from a denser medium to a rarer medium and the angle of incidence exceeds critical angle, the light reflects in the denser medium.
This is called internal reflection.
Conditions necessary for Internal Reflection
(i) Light should enter obliquely from a denser to a rarer medium.
(ii) The angle of incidence should exceed critical angle, the light reflects in the denser medium.
Critical angle : The angle of incidence for which the angle of refraction is 90o.
Rainbow : It is a natural spectrum appearing in the sky after rain showers.
• Rainbow is observed in the direction opposite to the sun.
• Three phenomenon which are involved in rainbow formation are :
(a) Dispersion
(b) Refraction
(c) Internal reflection
Some wate droplets remain suspended in air after rain. These droplets behave as glass prism. When light enters the rain drop, it first refracts and disperses. Then it reflects internally and again refracts as it come out of the drop and the seven colours reach the eye of observer in form of rainbow.
Atmospheric Refraction : The refraction by different layers of atmosphere is called atmospheric refraction.
| (i) Apparent flickering of objects placed behind a hot object or fire. |
| (ii) Stars near the horizon appear slightly higher than their actual position. |
| (iii) Advanced sunrise and delayed sunset. |
| (iv) Apparent flattering of sun's disc. |
| (v) Twinkling of stars. |
(i) An object placed behind the fire or a hot surface appears to flicker when seen through the air.
The air above hot surface becomes hot and rises. The space is occupied by cool air. The refractive index of hot air is less than that of cool air. So, the physical condition of hte medium are not constant. Due to changing Refractive Index (RI) of medium, the light appears to come from different directions.
It results in fluctuation in apparent position of object.
(ii) Stars when seen near the horizon appear slightly higher than their actual position due to atmospheric refraction.
The refractive index of earth's atmosphere in general increases from top to bottom. So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards to normal. As a result the star appears higher.
(iii) Advanced sunrise
The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset.
When the sun is below horizon, the rays have to pass from rarer to denser medium. So rays bend towards the normal. As a result the sun appears heigher than its actual position.
(iv) Twinkling of stars
Stars are very far from us, so they behave as point source of light. Since the physical conditions of the earth's atmosphere are not constant the light from stars appears to come from different directions. This results in fluctuation of apparent position of star.
The amount of light coming from stars also vary due to changing Refractive Index of atmosphere.
The star appears bright when more light from star reaches our eyes and the same star appears dull when less amount of light reaches our eyes.
Both these effects are responsible for twinkling of stars.
Q. Why do planets not twinkle ?
Ans. The planets are much closer to the earth and are thus seen as extended source. If we consider a planet as collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all individual point sized sources wil average out to zero and will nullify the twinkling effect.
Scattering effect : Spreading of light in various direction by colloid paritcles.
1
Scattering ___________
wavelength
Tyndall effect : When light passes through a colloid its path becomes visible. This is called Tyndall effect.
E.g.,
(i) Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.
(ii) Path of light becomes visible when light passes through dense canopy of trees in a forest.
The colour of scattered light depends on the size of scattering praticles
(i) If paritcles are very fine, they scatter mainly the blue colour light (shorter wavelength).
(ii) Medium sized paritcles scatter mainly the red colour (longer wavelength).
(iii) Even larger paritcles scatter all the colours of light that is why it appears white.
• Wavelength of red length is about 1.8 times to that of blue light.
Q. Why danger signs are made of red colour ?
Ans. Red is the least scattered colour. It is least scattered by fog and smoke and can be seen in the same colour over a long distance. So, danger signs are made in red colour.
Q. Why the colour of sky appears blue on a clear day ?
Ans. The upper layer of atmosphere contains very fine praticles of water vapours and gases. These particels are more effective in scattering of light of shorter wavelength mainly blue than larger wavelength. So, the sky appeara blue.
Q. How does the sky appear to an astronaut in the space or to a passenger of jet plane flying at high altitude ?
Ans. The sky would appear dark to an astronaut in the space as scattering is not very prominent at such high altitude due to absence of paritcles.
Q. Why clouds appear white ?
Ans. Clouds are formed by water vapours. Water vapours condense to form water droplets due to larger size of droplets, all colours of light are scattered and clouds appear white.
Q. Why colour of sun appear red during sunrise and sunset ?
Ans. While sunset and sunrise, the colour of the sun and its surrounding appear red. During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in atmosphere. Due to this most of the blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why sun appear red in colour.
