Question about shadows and what this shows about light.

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In summary, when an object casts a shadow, there are two regions - the umbra and the penumbra. The umbra is where the source is completely blocked, while the penumbra only partially blocks the source. As two shadows cast by the same object move closer together, their penumbra regions meet and create a darker region, causing the shadows to appear to "melt" together before the objects actually touch. This is due to the blocking of the source becoming more significant when the penumbra regions unite.
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wasteofo2
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I've noticed that the shadows of objects seem to melt together before the objects casting the shadow touch. For instance, I hold my index fingers a foot apart and begin to move them together. The two shadows will meet and alter the appearance of themselves before my fingers have actually touched. Try it to see what I mean more specifically.

Why does this happen?
 
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You have probably heard the terms umbra and penumbra associated with eclipse, these are the names given to the regions of a shadow. In the umbra region the source is completely masked while in the penumbra region a portion of the source is visible. All shadows produced by a extended source (in other words all shadows) have umbra and penumbra regions. What you are seeing is the meeting of the penumbra regions. By itself neither penumbra region is dark enough to be visible but when they meet they block enough of the source to become visibly dark regions.
 

1. What is a shadow?

A shadow is an area of darkness created when an object blocks light from a light source. It is the absence of light in a particular area.

2. How are shadows formed?

Shadows are formed when an object blocks some or all of the light from a light source. The shape and size of the shadow depend on the angle at which the light hits the object and the distance between the object and the light source.

3. What causes shadows to change in size and shape?

The size and shape of a shadow can change depending on the position of the light source, the angle at which the light hits the object, and the distance between the object and the surface on which the shadow is cast.

4. What does the shape of a shadow tell us about the light source?

The shape of a shadow can tell us about the direction and intensity of the light source. For example, a long and stretched out shadow could indicate that the light source is far away, while a short and sharp shadow could indicate that the light source is close and intense.

5. How do shadows show us that light travels in a straight line?

Shadows prove that light travels in a straight line because they are only formed when an object blocks the path of light. If light did not travel in a straight line, we would not see distinct shadows, but rather a diffused area of light around an object.

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