http://www.nikonchat.com/nikkor-1-8-lens/

D3100 Nikon – Nikkor 35mm 1.8 or 50mm 1.4 lens?
Can not seem to decide – I am looking to take good pictures indoors at night or pleasant pictures (of low light) and listen to these lenses are very good. All suggestions are welcome!
Yes, the 35 mm is a "normal" lens on the D3100. I love using the lens of 35 mm multi-use my D300, which has the same size sensor, but the 50mm would be a better portrait lens. Translates into an equivalent 75mm and this is a longitude of better working conditions for portraits. The f / 1.4 IS better than the control f/1.8 to blur the background, too.
nikon 50mm 1.8D AF Nikkor Lens for Nikon DSLR Cameras