I have watched light sources such as incandescent lamps and other lamp sources; they have always made shadows. But a fluorescent lamp doesn't make any shadow. What is the reason behind the non-appearance of prominent shadow?
[Physics] Why don’t fluorescent lights produce shadows
everyday-lifevisible-light
Related Solutions
There are multiple ways in which you can in principle disguise the source of light, but photons still must be emitted in order to illuminate something. So a shadow can only really be generated if there is a source of light, although the specific location of the source can be easily masked.
The simplest example is the case of knife-edge diffraction, which is used to send signals over obstructions, particularly in radio wave propagation. Essentially, the transmitter is behind a relatively sharp edge object (like a mountain in the case of radio waves), and the light will actually "bend" around the object to a receiver on the other side.
More advanced cases look at "bending light" from a laser around an object. Experimentalists now claim to be able to "bend light" an arbitrary number of degrees.
Much of the research into metamaterials is focused on different applications involving refracting light using materials with negative refractive indexes. These materials play key roles in the concepts of modern "cloaking devices" where the idea there is to mask an object in a cocoon of meta material well into the visible spectrum.
If you get into the metamaterial realm, the idea might be that one could not only bend the light around the object, but amplify the light in some way so that in some arbitrary direction light passing through the metamaterial exits with greater intensity than the background ambient light such that an object in the path of the amplified light would cast a shadow. I would imagine that the impression of the observer would be that some region they are looking at somehow appear brighter, although the exact location of the light source might not be identified exactly.
So in general it is possible to mask the specific location of a source, but you simply shouldn't be able to generate a shadow without there being some change in intensity between the front and back of the object casting the shadow.
A Fluorescent lamp ballast provides a spike of high voltage that starts electrons flowing from one electrode to the other. After ignition, once an arc of electrons is flowing through the gas, the voltage requirement drops significantly as current begins to pick up on its own. The function of the ballast is to regulate voltage from high at startup to low during operation. If there were no ballast, the self-increasing current would burn out the tube in short order.
Instant-start and rapid-start fluorescent lamps require about 1.85 to 1.70 times more voltage to start the lamp than is required to keep it glowing. The ratio of starting to average voltage is called the "crest factor". A high crest factor causes great wear on the electrodes of the lamp.
This is why: Electrodes generally are made of tungsten and are coated with an emission mix that generates electrons that arc from electrode to electrode inside the tube. High voltage at startup really pummels the emission mix. As the tungsten electrodes lose their coating, the fluorescent light may darken near the electrodes. When the electrodes no longer emit sufficient electrons, the light burns out.
As startup rapidly depletes the coating on the electrodes due to the high voltage required to get an arc going, frequent startups will shorten the life of the bulb.
Best Answer
To complement Floris's answer, here's a quick animation showing how the shadow changes with the size of the light source. In this animation, I've forced the intensity of the light to vary inversely with the surface area, so the total power output is constant ($P \approx 62.83 \, \mathrm{W}$). This is why the object (Suzanne) doesn't appear to get any brighter or darker, but the shadow sharpness does change:
In this scene, the spherical lamp is the only light source, except for the dim ambient lighting. This makes the shadows very easily visible. In a real-world scenario with other light sources (windows, for example), the effect would be less pronounced because the shadows would be more washed out.
The following animation shows the scenario Floris described, with a rotating long object: