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 -2.3cm
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 muscular 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 distinctively without strain.
•For a normal adult eye , its value is 25cm.
•Range of human vision -25cm to infinity.
Accommodation:The ability of the eye lens to adjust its focal length is called accommodation .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
The structure of human eye
Myopia (Near Sightedness)
•A myopic person can see near by 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 myopic eye, image of distant object is formed in front of the retina (and not on the retina)
(b)The far point (F) of a myopic eye is less than infinity
(c)Correction of myopia.The concave lens placed in front of the eye from a virtual image of distant object at far point (F) of the myopic eye.
Hypermetropia (far sightedness)
•Affected person can see far object clearly by cannot see nearby objects clearly.
•The near point of the eye moves away.
•Image is formed behind the retina.
Causes of hypermetropia
•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 accommodation of the eye.
•The near-point of the old person having presbyopia gradually recedes and becomes much more than 25cm away.
Causes
•Gradually 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.
Prism:It is a pyramidal piece of glass with two triangular bases and three rectangular lateral surfaces.
Refraction through a glass prism
Angel of deviation(d):It is the angle between incident ray and emergent ray.
When white light is passed through a glass prism, it spilts into its seven constituent colour 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 spectrum.
Acronym:It is a group of alphabets that represent sequential colours in spectrum.
Angle of deviation proportional
VIBGYOR
1
――――–―
Wave length
•Red is the least deviated colour as it has largest/Longest wavelength.
•Violet is the most deviated colour as it has smallest wave length in visible spectrum.
Q-What spectrum is formed when white light is passed through a glass prism?
Ans-Each colour has a definite wavelength and for each wavelength
The angle of deviation differs.Red is the least deviated and violet is the most deviated colour so different colours deviate at different angles to from spectrum.
Issac Newton was the first Person who proved that sunlight is made up of seven colours.
(I)He passed sunlight through a glass prism to from a band of seven colours.
(II)He tried to spilt the colour further
By putting another prism ahead of the prism forming spectrum but he failed to obtain more colours.
(III)He formed a spectrum from sunlight 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 angles:The angle of incidence for which the angle of refraction is 90°.
Rainbow:It is a natural spectrum appearing in the sky after rain showers.
•Rain 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 water droplets remain suspended in air after rain.These droplets behave as glass prism.when light enters the rain drop, it first reflects 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 flatting of sun's disc.
(v)Twinking 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 the medium are not constant .Due to changing refractive Index (RI) of medium , the light appers 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 top bottom .So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards the normal .As a result the star appears higher.
(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 refraction index of atmosphere.
The star appears bright when more light from star reacheds our eye 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 twinkling?
Ans-The planets are much closer to the earth and are thus seen as extended source .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 individual point sized sources will average out to zero and will nullify the twinkling effect.
Scattering effect:spreading of light in various directions by colloid particles.
Scattering proportional 1
—————
wave length
Tyndall effect:when light passes through a colloid its path becomes visible.This called Tyndall effect.
Eg:-
(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 tree in a forest.
The colour of scattered light depends on the size of scattering particles.
1.If particles are very fine , they scatter mainly the blue colour of light (shorter wavelength).
2.Medium sized particles scatter mainly the red colour (longer wavelength)
3.Even larger particles scatter all the colour of light that is why it appears white.
•wavelength of red light 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 clean day?
Ans-The upper layer of atmosphere contains very fine particles of water vapours and gases .These particles are more effective in scattering of light of shorter wavelength mainly blue than larger wavelength .So, the sky appears 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 particles.
Q-Why clouds appear white?
Ans-Clouds are formed by water vapours .water vapour condense to form water droplets due to larger size of droplets, all colours of light are scattered and clouds appear white.
Why colour of sun appear red during sunrise and sunset?
Ans-white 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 this most of the blue light (short 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.
Question 1:
What is meant by the power of accommodation of the eye?
Answer:
The power of accommodation of the eye refers to the ability of the eye lens to adjust its focal length to focus on both near and distant objects. This adjustment is made possible by the ciliary muscles, which change the curvature of the lens.
Question 2:
What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer:
The image distance in the eye remains the same when the object distance is increased. This is because the image is always formed on the retina, and the eye lens adjusts its focal length to ensure that the image remains in focus.
Question 3:
Why do stars appear to twinkle?
Answer:
Stars twinkle due to atmospheric refraction. When starlight passes through the Earth's atmosphere, it is refracted continuously due to the varying densities of air layers. This causes the light from the star to change its path slightly and makes the star's intensity fluctuate, giving it a twinkling effect.
Question 4:
Explain why the planets do not twinkle.
Answer:
Planets do not twinkle because they are closer to Earth and appear as extended sources of light (instead of point sources like stars). As a result, the effects of atmospheric refraction on different points of the planet cancel each other out, making the light from planets relatively steady.
Question 5:
Why does the Sun appear reddish early in the morning?
Answer:
The Sun appears reddish in the morning due to scattering of light. When the Sun is near the horizon, sunlight has to travel through a greater thickness of the Earth’s atmosphere. The shorter wavelengths (blue and violet) are scattered out, while the longer red wavelengths pass through, making the Sun appear red.
Question 6:
Why does the sky appear dark instead of blue to an astronaut?
Answer:
The sky appears dark to astronauts because there is no atmosphere in space to scatter sunlight. Scattering of light by the atmosphere causes the sky to appear blue on Earth. In space, with no atmosphere, there is no scattering, and hence the sky appears black or dark.
Additional Exercises
Question 7:
A 14-year-old student is unable to see clearly the questions written on the blackboard. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List two possible causes for this defect.
Answer:
The student is suffering from myopia or near-sightedness, a condition where he can see nearby objects clearly but has difficulty seeing distant objects.
Causes of myopia include:
- Elongation of the eyeball.
- Increased curvature of the eye lens.
Ray diagram for Myopia:
- A parallel ray coming from a distant object converges in front of the retina.
- Using a concave lens can correct this defect by diverging the light rays so they focus on the retina.
Question 8:
The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Answer:
The person needs a concave lens to correct myopia.
Given:
Far point, d=80cm=0.8m
Using the lens formula:
Power(P)=−f1(in metres)So, P=−0.81 = -1.25 D
Therefore, a concave lens of power -1.25 D is required to correct the myopic defect.
Question 9:
Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to enable him to read at 25 cm?
Answer:
Hypermetropia is corrected using a convex lens that converges the light rays so that they focus on the retina.
Given:
Near point, u=−25cm=−0.25m
Far point, v=−1m
Using the lens formula:
f1=v1−u1
f1=−11−−0.251Calculating this will give the focal length, and power P=f1.
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