ANSWER I :Fluorescent tube light produces something called a cylindrical wave front as it is an extended line source. The sun, which is a spherical body, can be assumed as a point source owing to its great distance from the earth. So the sun produces a plane wave front at a large distance. The shadow due to the sun shows a definite umbra and penumbra provided the distance between the object and the screen on which the shadow is cast, is small. Umbra refers to the dark part of the shadow while penumbra refers to the lighter part. When the distance between the object and the screen increases the penumbra grows at the cost of the umbra. In the case of a fluorescent tube, the shadow due to a cylindrical wave front consists of a large part of penumbra and very little of umbra. So the shadow is not sharp. On the contrary, the shadow produced by the sun has more of umbra and less of penumbra. So the shadow is sharp.
ANSWER II : The sharpness of the shadow of an opaque object depends on the angular size of the source of light producing that shadow. In the case of a point source of light, the shadow has to be perfectly sharp with no blurred outline.
An extended source, that is one with finite size, can be treated as consisting of an infinite set of points each of which causes the formation of the object's shadow.
Since these constituent points of the source are in different directions with respect to the object, the corresponding shadows due to them are formed at differing positions, depending on the their relative angular separation.
Usually these individual shadows have some common overlap, which genuinely forms a dark portion of net shadow we see. Technically this is called `umbra'.
But there is also a region of the net shadow where light does reach from one part or the other of the source.
Thus the dark shadow region is usually surrounded by partially lighted region, which is called `penumbra'. This penumbra is to be seen in the form of a blurred outline of the shadow. The size of the umbra and penumbra depend on the angular size of the source.
It is to be noted that even though the sun is a large heavenly body, its angular size is quite small, about half a degree, with respect to all objects on earth, because of its large distance from the earth.
The shadow of an object in the sunlight is thus found to be reasonably sharp when it is observed close to the object. The quality of the sharpness can be seen to fall as the distance of the shadow from the object increases.
When one tries to observe a shadow formed due to a fluorescent lamp, the lamp is usually not very far away from the object. Its angular size with respect to the object is typically several degrees.
This leads to a small umbra in the shadow. Very often this umbra is altogether absent.
Usually it is the penumbra that is visible in the form of a diffused shadow. If the object is taken to larger distances from the lamp then the sharpness of the shadow improves.
Courtesy : The Hindu
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