In the beginning why doesn't the Bible say God said let there be an Electromagnetic Spectrum not just the visible part of that spectrum?

Consider the concept observed but rarely taught by churches: Genesis 1:1 In the beginning God created the Heavens and the Earth.

Okay, I'm not going to copy the Bible here since you likely have one and can read it for yourself but consider: The story in Genesis begins with the entire Universe already crated, including the Earth but the Earth was void and uninhabitable.

The six days were creative periods which sequentially follow the changes to the Earth and written in a way that indicates what would be observed if a man were standing on the surface of the Earth and observing what progressed.

Since the Earth was shrouded in dust and debris nothing could be seen from the surface but after that settled visible light could be observed from the surface. Also the "heavens" in general could be observed after that period.

Also note the Bible was written for the people living over 2 thousand years ago to understand and if it were rewritten today different terminology may have been used. Even today the Truth about Reality can't be spoken to the general population of the world or the person speaking it would likely be charged with some crime like blasphemy and be executed by horrendous means.

Even the prophets of old understood the fact that humanity in general should not have the exact truth revealed to them.

Well, this god had a voice box and had to speak, so maybe he also had eyes like a person. Not that the story is from the imagination of a man who couldn't imagine more than he, himself experienced.

EASY, because the who creation was oriented to the creation of humans and the relating , to humans too.

It's assumed that that's what he meant. Everything is usually assumed, in that realm.

1) If you try actually reading the Bible, you should discern that it is not intended to be a science book.

2) When was the term "Electromagnetic Spectrum" first coined, given, for instance, that some of the rays in the ES were not identified until early in the 20th Century?

Let's imagine, for instance, that you were asking this question of scientists [not Bible based believers] 1,000 years ago, according to the knowledge they possessed of rays at that time. What response do you imagine you would receive?

See how silly your question looks? [frankly, I wonder if you do].

The focus of the Bible is on sin, not science. Where it touches on science, it often beats mainstream science to the punch.

For example, before Einstein's General Theory of Relativity where he describes space-time as a flexible fabric, we have this.

Isa 45:12 I have made the earth, and created man on it.

I, even my hands, have stretched out the heavens;

and I have commanded all their army.

Why does "Light" not cover both the visible portion and the invisible portion of the spectrum?

This question is just silliness.

the bible is for simple folk

if i was god the bible would only say god did it

because god did not need to say that