KnowHOW team explains: A shadow is a region where light is unable to reach. The more the darkness, the clearer the shadow. To produce a shadow, two conditions need to be fulfilled. First, the source of light must be limited so that the shadow area does not receive light from anywhere else. Second, the light source which is responsible for the shadow should be strong enough to nullify the effect of other sources.

A neon tube does not produce a clear shadow, while an ordinary filament bulb does, because the latter is a limited or point source of light and in most cases smaller than the object. So, the shadow region is what is known as umbra, an area of complete darkness.

However, neon lights come in the form of thin tubes and are, generally, an extended source of light. More often than not, in case of neon lights the object is smaller than the source of light. As a result, when an area is cut off from the light emitted from one end of the tube, it is lit up by the light from another end. Thus, the shadow area is ‘penumbra,’ or an area of partial darkness.

Also, the light received from neon bulbs are of low intensity, hardly suitable for a sharp shadow. Formation of the shadow has got nothing to do with the quality of light. It is decided by the size of the light source.

The question was sent by Savita Verma from Dhanbad