Johannes Kepler was a student and associate of Tycho, but he was a contrast to the latter. While Tycho was rich, robust and noble, passionate and an inquisitive mechanical and experimental genius.
Kepler was in penury, sickly and a poor experimenter. Tycho was an average in mathematics, his disciple was an expert in the subject. Kepler was a man of indomitable persevarance and overcame all his deficiencies and misfortunes to attain a high rank among the scientific men.
Key Words
* Least distance of distinct vision
* Angle of vision
* Accommodation of eye lens
* Myopia
* Hypermetropia
* Presbyopia
* Power of lens
* Prism
* Angle of prism or refracting angle of prism
* Angle of minimum deviation
* Dispersion
* Scattering
* Refractive index of the material of the prism
* Intensity of light
* Eye lens
* Far point
* Near point
Key Words - Explanation
Least distance of distinct vision: To see an object comfortably and distinctly, one must hold it at a distance about 25 cm from his eyes. This distance is called least distance of distinct vision.
Angle of vision: The maximum angle, at which one is able to see the whole object is called angle of vision. The angle of vision for a healthy human being is about 60°.
Accommodation of the eye: The process of adjusting focal length of the eye lens by the ciliary muscles is called 'accommodation'.
Myopia: The defect, in which people cannot see objects beyond far point is called 'Myopia'.
Hypermetropia: Hypermetropia is also known as 'far sightedness'. A person with hypermetropia can see distant objects clearly but cannot see objects at near distances.
Presbyopia: It is a vision defect when the ability of accommodation of the eye usually decreases with ageing.
Power of lens: The degree of convergence or divergence of light rays that can be achieved by a lens is expressed in terms of its power. The reciprocal of focal length is called power of lens.
Power of lens P = 
(in cm)
The unit of power is Dioptre. It is denoted by the letter 'D'.
Prism: A prism is a transparent medium separated from the surrounding medium by atleast two plane surfaces which are inclined at a certain angle in such a way that, light incident on one of the plane surfaces emerges from the other plane surface.
Angle of the prism or refracting angle of prism: The angle between the two refracting surfaces of the prism is called angle of the prism (or) refracting angle of the prism.
Dispersion: The splitting of white light into different colours (VIBGYOR) is called dispersion.
Scattering: The process of re-emission of absorbed light in all directions with different intensities by atoms or molecules is called scattering of light.
Refractive index of the material of the prism: The refractive index of a prism is
Where 'A' is the angle of the prism and D is angle of minimum deviation.
Angle of minimum deviation: The angle of deviation for which the angle of incidence (i1) is equal to the angle of emergence (i2) is known as the angle of minimum deviation.
Intensity of light: The intensity of light is the energy of light passing through unit area of plane, taken normal to the direction of propagation of light in one second.
Eye lens: Eye lens is hard in the middle and gradually becomes soft towards the outer edge which is responsible for image formation.
Far point: The point of maximum distance at which eye lens can form an image on the retina is called 'far point'.
Near point: The point of minimum distance at which eye lens can form an image on the retina is called near point.
Synopsis
* To see an object comfortably and distinctly, one must hold it at a distance about 25 cm. This is called the least distance of distinct vision.
* The rays coming from the extreme ends of an object form an angle at the eye. If the angle is below 60°, we can see the whole object. If this angle is above 60°, then we can see only a part of the object.
* The values of least distance of distinct vision and angle of vision change with person and age.
* The human eye is one of the most important sense organs. It enables us to see the object and colours around us.
* Human eye contains eye ball, cornea, aqueous humour lens, iris, pupil, ciliary muscle, and retina.
* The 'iris' enables pupil to act as 'variable aperture' for entry of light into eye.
* The eye lens is hard in the middle and becomes soft towards the outer edge.
* The light that enters the eye forms an image on the retina.
* The distance between the lens and the retina is about 2.5 cm.
* The ciliary muscle to which eye lens is attached helps the eye lens to change its focal length by changing the radii of curvature of the eye lens.
* When the eye is focused on a distant object, the ciliary muscles are relaxed so that the focal length of eye lens has maximum value.
* When the eye is focused on a closer object, the ciliary muscles are strained and focal length of eye-lens decreases.
* The ciliary muscle adjust the focal length of eye-lens in such a way that the image is formed on retina and we see the objects clearly.
* This process of adjusting focal length is called 'accommodation'.
* The eye-lens forms a real and inverted image of an object on the retina.
* The retina is a delicate membrane, which contains about 125 million receptors called 'rods' and 'cones' which receive the light signal (rods identify the colour, cones identify the intensity of light).
* These signals are transmitted to the brain through about 1 million optic-nerve fibres.
* The brain interprets these signals and finally processes the information so that we can perceive the object in terms of its shape, size and colour.
* There are mainly three common defects of vision.
* They are a) Myopia b) Hypermetropia C) Presbyopia.
* The defect, in which people cannot see objects beyond far point is called "Myopia".
* "Myopia" can be corrected by using 'bi-concave lens' of suitable focal length.
* 'Hyper metropia' is also known as 'far sightedness'.
* A person with hyper metropia can see distant objects clearly but cannot see objects at near distances.
* Hypermetropia can be corrected by using 'bi-convex lens' of suitable focal length.
* Presbyopia is vision defect when the ability of accommodation of the eye usually decreases with ageing.
* The degree of convergence or divergence of light rays that can be achieved by a lens is expressed in terms of its power.
* The reciprocal of focal length is called power of lens. Power P = 
* The unit of power is Dioptre. It is denoted by the letter D.
* A prism is a transparent medium separated from the surrounding medium by atleast two plane surfaces which are inclined at a certain angle in such a way that, light incident on one of the plane surfaces emerges from the other plane surface.
* The angle of deviation, at which the angle of incidence is equal to the angle of emergence is called angle of minimum deviation (D).
* Refractive index of a prism
Where A is the angle of the prism and D is the angle of minimum deviation.
* The splitting of white light into different colours (VIBGYOR) is called dispersion.
* The relation between the speed of wave (v), wavelength (λ) and frequency (f) is v = fλ.
* Speed of the wave increases with increase in wavelength of light and vice versa.
* Rainbow is a good example of dispersion of light.
* The beautiful colours of the rainbow are due to dispersion of sunlight by millions of tiny water droplets.
* The process of re-emission of absorbed light in all directions, with different intensities by atoms or molecules is called scattering of light.
* The blue colour of the sky is due to scattering of light.
* The molecules of N2 and O2 present in atmosphere are responsible for the blue colour of sky.
* During Sun rise and Sun set, Sun appears red in colour and during noon hours Sun appears white in colour.
Writer: C.V.Sarveswara Sarma
