An imaginary line,drawn perpendicularly on the surface of the medium at the point where incident ray falls (point of incident).
It is past time that we rethink what we mean by the words "normal" and "abnormal" as those words apply to the mental and emotional states and behaviors of human beings. Indeed, it is a real question as to whether those words can be sensibly used at all, given their tremendous baggage and built-in biases and the general confusion they create.
This is not an idle question without real-world consequences. The "treatment" of every single "mental disorder" that mental health professionals "diagnose," from "depression" and "attention deficit disorder" on through "schizophrenia," flows from how society construes "normal" and "abnormal." This matter affects tens of millions of people annually; and affects everyone, really, since a person's mental model of "what is normal?" is tremendously influenced by how society and its institutions define "normal."
The matter of what is normal can't be and must not be a mere statistical nicety. It can't be and must not be "normal" to be a Christian just because 95% of your community is Christian. It can't be and must not be "normal" to be attracted to someone of the opposite sex just because 90% of the general population is heterosexual. It can't be and must not be "normal" to own slaves just because all the landowners in your state own slaves. "Normal" can't mean and must not mean "what we see all the time" or "what we see the most of." It must have a different meaning from that for it to mean anything of value to right-thinking people.
Nor can it mean "free of discomfort," as if "normal" were the equivalent of oblivious and you were somehow "abnormal" when you were sentient, human, and real. This, however, is exactly the game played by the mental health industry: it makes this precise, illegitimate switch. It announces that when you feel a certain level of discomfort you are abnormal and you have a disorder. It equates abnormal with unwanted, turning "I don't want to feel sad" into "I have the mental disorder of depression."
In this view "normal" is living free of excessive discomfort; "abnormal" is feeling or acting significantly distressed. Normal, in this view, is destroying a village in wartime and not experiencing anything afterward; abnormal is experiencing something, and for a long time thereafter. The consequences of conscience, reason, and awareness are labeled abnormal and robotic allegiance to wearing a pasted-on smiley face is designated normal. Is that what we really mean? Is that what we really want?
Sadness, guilt, rage, disappointment, confusion, doubt, anxiety and other similar experiences and states are all expected and normal, given the nature and demands of life; except, that is, to mental health professionals, where those states and experiences become markers of abnormality and cash cows. It is simply not right to call the absence of significant distress normal and the presence of significant distress abnormal: that just isn't right.