(i) Lyman series has least wavelength.
(ii) Lyman series lies in ultraviolet region.

K.E of the electron = 13.6 eV
P.E is equal to twice its K.E
P.E of the electron = -13.6 x 2 = -27.2 eV 

When the surrounding temperature reaches around 10⁷K, lighter nuclei start fusing to form heavier nuclei and this resulting reaction is called thermonuclear fusion reaction.

When the hydrogen is burnt out, the sun will enter into a new phase called the red giant where helium will fuse to become carbon. During this stage, the sun will expand greatly in size and all its planets will be engulfed in it

Neutrons are classified according to their kinetic energy as
(i) slow neutrons (0 to 1000 eV)
(ii) fast neutrons (0.5 MeV to 10 MeV).

Neutrons are stable inside the nucleus. But outside the nucleus they are unstable. If the neutron comes out of the nucleus (free neutron), it decays with the emission of proton, electron, and antineutrino with a half-life of 13 minutes.

The moderator is a material used to convert fast neutrons into slow neutrons
(i) A billiard ball striking a stationary billiard ball of equal mass would itself be stopped but the same billiard ball bounces off almost with same speed when it strikes a heavier mass.
(ii) This is the reason for using lighter nuclei as moderators.

The process of breaking up of the nucleus of a heavy atom into two fragments with the release of large amount of energy is known as nuclear fission.

It is defined as the quantity of radiation which produces 1.6 x 10¹² pair of ions in 1 gram of air.

In nuclear fusion reaction, the energy liberated per unit mass of the nuclei taking part in the reaction is many times larger than the energy liberated in a fission reaction.

Nuclear fuel, neutron source, moderator control roads, coolant and shielding.

(i) A nuclear chain reaction dies out, when the neutrons produced so fast they escape from the uranium without interacting.
(ii) When the neutrons are lost due to the. extremely small size of uranium (i.e, smaller than critical size).

A nuclear reaction in which the neutron used to carry out the nuclear fission reaction gets multiplied as more and more such fission reaction take place is called a nuclear chain reaction.

The material which can undergo nuclear fission easily is called fissile material.
Example: \(_{ 92 }^{ }{ { U }^{ 235 } }\)

(i) Alpha ray has high ionizing power.
(ii) α - particle has a large mass and large nuclear cross-section. So it has high ionizing power.