Prayer Question No.1. Which way the does poet implore his Lord to lead him to? Ans. The poet implores his Lord to lead him to the way of truth. He also prays God to remove all his doubts and darkness of ignorance. Question NO.2. Name the blessings that the poet prays for? Ans. The poet prays for the path of truth. He prays for the nectar of knowledge. He also prays that his heart is surcharged with passion, zest, and hope. Above all, he implores his Lord to save him from sloth and infirmity. Question No.3. What is it that the poet wants to sing? Ans. The poet wants to sing those songs which will infuse life even into the dead and will inspire people to come under the same umbrage of love The poet wants to sing beautiful songs of love and brotherhood. He wants to motivate people to remove bitterness against each other. Question No.4. “Subject me not to the trials, shame not my human form”, explain? Ans. The poet shows his humble submission before his Lord....
Human Eye
Human eye is one of the most valuable and sensitive organs. It resembles a camera. The human eye works on the refraction of the light through a natural convex lens. Main parts of human eye :
Structure of Human Eye
· Iris
· Pupil
· Cillary muscles
· Eye lens
· Retina
· Optic nerve
1) Cornea: The front part of the eye is called as cornea. It is made of a transparent substance .It is a thin membrane of eye, through which light enters the eye first and gets refracted.
2 ) Iris: Iris is suited just behind the cornea. Iris is a flat, coloured, ring shaped membrane. It control the size of pupil.
3) Pupil: There is a hole in the middle of the iris which is called the pupil of the eye. The pupil appears black because no light is reflected from it. Pupil regulates and controls the amount of light entering the eye.
4) Cillary muscles : It holds the eye lens and lens by focusing. The focal length of the convex lens present inside the eye can be changed by the action of the Cillary muscles.
5) Eye lens: Eye lens is a double convex lens made of a transparent , soft and flexible material like a jelly made of proteins . Being flexible the eye lens can change its shape (it can become thin or thick) to focus light on the retina.
6) Retina : The screen on which the image is formed in the eye is called retina. The retina is behind the eye lens. It is a delicate membrane having a large number of light, sensitive cells called ‘rods and cones’ which response to the intensity of light and colour of object respectively by generating electrical signals.
7) Optic nerve: It is a nerve connecting eye and brain. It sends the electrical signals to brain.
Working of Human Eye
The light rays coming from the object kept in front of us enter through the cornea of the eye, pass through the pupil of the eye and fall on the eye lens so it converges the light rays and produces a real and inverted image of the object on the retina is converged to the brain by the optic nerve and gives rises to the sensations of vision.
NOTE : Rod and Cone cells: The retina of our eye has a large number of light sensitive cells. These are two kinds of light sensitive cells on the retina : rods and cones.
Rods are the rod – shaped cells present in the retina of an eye which are sensitive to dim light. Rods are the most important for vision in dim light. Natural animals which sleep during the day and come out at night like the owl have a large number of rod cells in their retina which help them to see properly during the night where there is not much light.
Cones are the cone shaped cells which are sensitive to bright to bright light for normal light. The cone cells of our retina also respond to colour.
Power of Accomodation
An eye can focus the images of the distant object as well as the nearby object on its retina by changing the focal length (or converging power of its lens. The focal length of the eye lens is changed by the action of the Cillary muscles). The Cillary muscles can change the thickness of the soft and flexible eye lens and hence its focal length which in turn, changes the converging power of the eye lens.
The ability of an eye to focus the distant object as well as the nearby objects in the retina by changing the focal length for converging power of its lens is called accommodation. An normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focused on the retina.
Far point : the farthest point from the eye at which an object can be seen clearly is known as the “ far point” of the eye. The far point of a normal human eye is at infinity.
Near point: The minimum distance at which an object must be placed so that a normal eye may see it clearly without any strain, is called the least distance of distinct vision. The least distance of distant vision for a normal eye may see it clearly without any strain, is called the least distance of distant vision for a normal eye is about 25 cm .
Note: colour combination:
Red + green= yellow
Blue + green = Peacock blue
Red + Blue= Magenta
Defect of vision
Sometimes the eye of a person cannot focus on the retina properly. In this case the person is said to have a defect of vision . There are three common defects of vision.
Myopia: Myopia ( or short – sightedness ) is that defect of vision due to which a person cannot see the distant objects clearly through he can see the nearby objects clearly. The far point of an eye suffering from myopia is less than infinity.
Causes of myopia
The defect of eye called myopia (for short – sightedness) is caused
1) Due to high converging power of eye lens.
2) Due to eye ball being too long.
Correction of myopia
Myopia (short –sightedness or near –sightedness ) is corrected by using spectacles containing concave lenses
Diagram
Hypermetropia
Hypermetropia or ( long – sightedness) is that defect of vision due to which a person cannot see the nearby objects clearly though he can see the distant objects clearly. The near point of a hypermetropia is centimetres away from retina.
Causes of Hypermetropia
1 Due to low converging power of eye lens.
2 Due to eye – ball being too short.
Correction of hypermetropia
Hypermetropia ( long – sightedness or far – sightedness) is corrected by using spectacles combining convex lenses.
Diagram
Presbyopia
Presbyopia is that defect of vision due to which an old person cannot see nearby objects clearly due to the loss of power of accommodation of the eye.( The near point in this case become less than 25 cm ) .
Cause
1 Due to Cillary muscles becoming weak
2 Due to the eye lens becoming rigid
Correction
Presbyopia is corrected by using spectacles containing cylindrical lens.
There are few modes of defect of vision
Cataract
It is a defect of vision in which usually comes in oblage is cataract. The medical condition in which the lens of the lens of eye of a person becomes progressively resulting in blurred vision is called cataract.
Cause
Cataract develop when the eye lens of a person becomes cloudy due to the formation of a membrane over it.
Correction
The opaque lens is removed from the ye of the person by surgical operation and a new artificial lens is inserted in the place.
Colour blindness
It is a genetic disorder caused due to the absence of cone cells in the retina. In this defect, vision is normal, but eye cannot between colours.
Q Why do we have 2 eyes for vision and not just one?
Ans There are many advantages of having two eyes instead of one. Some of them are given below:
1. Having two eyes gives a wider field of view
2. Having two eye enables us to judge distance more accurately.
Q What is night blindness?
Ans. Some persons have the difficulty to see the objects in dim light during night. This defect of eye is called night blindness.
Cause
1 Lack of vitamin a in the food
2 Improper functioning of rods – shaped cells. The rods shaped cells respond to intensity variation OF LIGHT.
Q What is total internal reflection ?
Ans When light passes from an optically denser medium to a rarer medium beyond certain angle of incidence it undergoes reflection instead of refraction. This is called total internal reflection and that certain minimum angle of incidence is called critical angle condition for total internal reflection.
1 Light should pass from denser to rarer medium.
2 Angle of incidence should be greater than critical angle.
Atmospheric refraction : The refraction of light by the Earth’s atmosphere is called as atmospheric refraction.
Twinkle stars
The twinkling of stars is due to the atmospheric refraction of starlight. The starlight on entering the Earth’s atmosphere undergoes refraction continuously before it reaches the Earth. The continuously changing atmosphere refracts the light from the stars by different amounts from one moment to the next. When the atmosphere refracts more star light toward us, the star appears to be bright and when the atmosphere refracts less star light, then the star appears to be dim. In this way, the star light reaching our eyes increases and decreases continuously due to atmospheric refraction and the star appears to twinkle at night.
· Advance sunlight and delayed sunset
Sun is visible to us about 2 min after the actual sunset because of atmospheric refraction. By actual sunset because of atmospheric refraction. By actual crossing of the horizon by the sun. The time difference between the actual sunset and the apparent sunset is about 2 min.
Rayleigh law of scattering
According to ray light, the amount of scattering is inversely proportional to the fourth power of the wavelength . Therefore the light of shorter wave length is scattered much more than the light of longer wavelength. There is no change in the wavelength of light rays during scattering.
Note: thE MOST BEAUTIFUL PHENOMENA OF NATURE SUCH AS BLUE COLOUR OF SKY. White colour of clouds, Red of sunrise and sunset can be explained in terms of scattering of light.
Blue colour of sky
Blue colour have a shorter wavelength than red. So according to Ray light scattering law, Blue colour of sunlight scattered much more strongly by the larger no. Of molecules present in the Earth’s atmosphere. Hence the sky appears blue.
Mirage
The apparent inverted images formed due to not surface in deserts is called mirage. This is caused by total reflection of light in atmosphere.
TYNDALL EFFECT
The phenomenon of scattering of light by the colloidal particles known as Tyndall effect. This effect can be observed when;
1 A fine beam of sunlight enters a room containing suspended particles of dust, the path of the beam of light visible .It is due to the scattering of light.
2 Sunlight passes through a canopy of dense forest, mist contains tiny droplets of water which acts as particles of colloid dispersed in air.
Dispersion of light
In the year 1665 Newton found that if a beam of white light is passed through a triangular glass prism the white light splits to form a band of seven colours. The splitting of white light into its constituents colours is called dispersion.
The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, it is called spectrum of white light. The seven colours of the spectrum are red, orange, yellow, green, blue, indigo and violet.
Note: When the light rays change its path due to some particles it is termed as Scattering of light.
Causes of Dispersion : The dispersion of white light occurs because colour of white light travels at different speed through a glass prism. When white light consisting of seven colour falls on a glass prism, each colour in it is refracted by different angles, with the result that seven colours are spread out to form a spectrum.
Ques. How rainbow is formed ?
Ans. A Rainbow is produced by the dispersion of white (sun) light by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum. A Rainbow is always formed in a direction opposite to that of the Sun.
Ques. Why does not glass prism produce a dispersion of light?
Ans. A glass slab acts as a combination of two identical glass prism is in an inverted position with respect to the first. The first prism splits the white light into the seven colours. The colour components fall on the second prism, where it recombines them to form white light.
Ques. Why does the sun appear reddish early in the morning?
Ans. At sunset or sunrise, the sun rays pass through a maximum length of atmosphere. Much of the blue light and shorter wave length has been taken out by scattering. Only the red colour of light reaches the observer. That is why when sunset and sunrise appear red.
Ques. Explain why the planets do not twinkle?
Ans. 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 individual point -sized sources will average out to zero thereby nullifying twinkling effects of planets.
Ques. Why does the sky appear dark instead of blue to an astronaut?
Ans. The sky appears dark instead of blue to an astronaut because at such a huge height there is no atmosphere to scatter the light. Hence the sky appears completely dark.