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Contents of Physics - 1: Heat and Light : 315
shadow in shape is always black, the size of the object varies with the direction and distance of the source of light. D. C. Jaina2 has called this type as a form of darkness though this shadow is visible. Jaina and Sikdar have overlooked this point. However, Samavāyānga mentions the change in size of shadow at different times and position of the sun as below63 :
(a) When sun is north of equator, length of man's shadow is 24 Angulas in the first 1/8th of the day.
(b) On seventh day of later Srāvaņa month, length of man's shadow is 27 Angulas.
(c) in the month of Caitra and Āśvina, length of man's shadow is 36 Angulas.
(d) On seventh day of early Kārttika, length of man's shadow is 37 Angulas.
(e) On 15th day of later Phālguna, length of man's shadow is 40 Angulas.
If an Angula unit of length is taken roughly as equal to 1.75 cms., the above shadow lengths per eighth part of the day during specific times have to be verified. The phenomenon of shadow formation represents indirectly the postulate of linear propagation of light. We can experience this fact through our own shadows in sunlights.
(ii) The second type of shadows are those in which images of the objects are formed. These are called Pratibimbas ( images ). There are two ways of image formation :
(a) Formation of images by reflecting surfaces like mirror. These are virtual and laterally inverted ( side-wise ).
(b) Formation of images through the interspersion of different types of transparent concave or convex lenses between the light source and objects. The nature of such images depends upon the nature of the lens and their surfaces. The size and distance of the image may vary. The images through lenses are generally known as real. Sometimes they may be inverted also. Cinema screens give un-inverted real images.
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