An open U tube pressure gauge containing water shows a difference in level of 15cm when connected to a gas supply. Find in N/m^2 the excess pressure of the gas above the atmospheric pressure.

blue light is scattered in different directions by the tiny molecules of air. blue is scattered more than other colours because it has a shorter distance to travel which results in smaller waves. this is why we see a blue sky most of the time.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.