What We Mean by the Word “Shadow”

We will be using the term “shadow” frequently during the beginning of the semester. We need to define what we mean, so everyone is using it the same way in class. 

shadow |ˈ sh adō|

noun

a region or surface form which some light is blocked and does not strike the surface

Note the distinction between the following two questions and answers:

(a) What color is that shadow?  That shadow is blue.

(b) What is that a shadow of?  That is a shadow of blue.

The first answer uses the word blue as an to describe the color of the shadow.

The second sentence uses the word blue to describe the color of light that is being blocked. 

Use (a) when there is a region where light is being blocked and the only color of light that “makes it” to the surface is blue.  Use (b) when there is a region where blue light is being blocked, regardless what other colors of light might make it to the surface. 

A shadow of blue can be black, red, green, yellow but probably not blue, cyan, magenta or white.  Can you see why?

We will usually speak in terms of (b) but occasionally, in class and lecture, the usage in part (a) will slip out.  That is ok.  When it happens, hopefully someone can raise their hand in class and ask for some clarification.  In fact, expect your teacher to purposely ‘slip’ once in a while.