Question: why 50mm is normal lens?
Answer: The 50mm is "standard" because it's a cheap lens to produce, and in the era of slow films, having a fast max aperture for little money was a good thing for most photographers. Many SLRs were sold with a 50/1.8 or 50/2 as a kit lens. When films became faster most consumers found a slower zoom lens to be more versatile.