They are blue because of hydrogen helium and methane
Atmospheric electricity and storms, electrostatic control filters, and industrial electrostatic seperation as well as spark discharge. these are just a few. hope it helps.
Answer:
time=4s
Explanation:
we know that in a RL circuit with a resistance R, an inductance L and a battery of emf E, the current (i) will vary in following fashion
, where
max=
Given that, at i(2)=
⇒
⇒
⇒
Applying logarithm on both sides,
⇒
⇒
⇒
Now substitute 
⇒
⇒
⇒
Applying logarithm on both sides,
⇒
⇒
⇒
now subs. 
⇒
also 
⇒
⇒