The ground state energy of hydrogen atom is $$-13.6 ~\mathrm{eV}$$. The energy needed to ionize hydrogen atom from its second excited state will be :
The escape velocity of a body on the earth surface is $$11.2 \mathrm{~km} / \mathrm{s}$$. If the same body is projected upward with velocity $$22.4 \mathrm{~km} / \mathrm{s}$$, the velocity of this body at infinite distance from the centre of the earth will be:
A lens is made up of 3 different transparent media as shown in figure. A point object O is placed on its axis beyond $$2f$$. How many real images will be obtained on the other side?
The diameter of a spherical bob, when measured with vernier callipers yielded the following values : $$3.33 \mathrm{~cm}, 3.32 \mathrm{~cm}, 3.34 \mathrm{~cm}, 3.33 \mathrm{~cm}$$ and $$3.32 \mathrm{~cm}$$. The mean diameter to appropriate significant figures is :