One day Shyam noticed that his younger brother was engaged in solving a question related to light emission from vapour lamp. Shyams brother was anxious to know about different colours being emitted by different light with mercury and also keep to have idea about what is inside the sodium vapour lamps kept in physics lab. With this, Shyam explained to his brother about absorption of energy and re-emission of photons in visible region. While explaining to his brother, Shyam advised his brother not to touch or break any items in physics lab for the thirst of knowledge.
(i) What is the moral you derive from Shyam?
(ii) Which series in the hydrogen spectrum is in the visible region?

(i) Draw a schematic arrangement of Geiger-Marsden experiment showing the scattering of a-particles by a thin foil of gold. Why is it that most of the a-particles go right through the foil and only a small fraction gets scattered at large angles?
Draw the trajectory of the a-particle in the coulomb field of a nucleus. What is the Significance of impact parameter and what information can be obtained regarding the size of the nucleus?
(ii) Estimate the distance of closest approach to the nucleus (Z = 80) if a 7.7 MeV a-particle before it comes momentarily to rest and reverses its direction.

Asha's mother read an article in the newspaper about a disaster that took place at Chernobyl. She could not understand much from the article and asked a few questions from Asha regarding the article.
Asha tried to answer her mother's questions based on what she learnt in Class XII Physics?

On the basis of Bohr's theory of H-atom, total energy of electron in a stationary orbit is given by the relation
Where, n is the number of orbits. It is clear that total energy of electron in a stationary orbit is negative which implies that electron is bound to the nucleus and is not free to leave it. The value of negative energy decreases as n increases.
Read the above p[assagr and answer the following questions:
(i) What is the total energy of electron in ground state of hydrogen atom? What does it indicate?
(ii) Why does energy required to remove an electron is smaller when atom is in any one excited state?

According to Bohr's theory of hydrogen atom, total energy of electron in a stationary orbit is \(E=-\frac { 13.6 }{ { n }^{ 2 } } eV,\) where n is the number of orbit. Clearly, total energy of electron in a stationary orbit is negative, which means the electron is bound to the nucleus and is not free to leave it. An n increases, value of negative energy decreases, i.e., energy is progressively larger in the outer orbits.
Read the above passage and answer the following questions :
(i) What is total energy of electron in ground state of hydrogen atom? What does it imply?
(ii) Energy required to remove an electron is smaller when atoms is in any one excited state. Comment.
(iii) How is this concept translated in day to day life?

A nuclear reactor is a powerful device, wherein nuclear energy is utilised for peaceful purposes. It is based upon controlled nuclear chain reaction. The nuclear chain reaction is controlled by the use of control rods (of boron or cadmium) and moderators like heavy water, graphite, etc. The whole reactor is protected with concrete walls 2 to 2.5 metre thick, so that radiations emitted during nuclear reactions may not produce harmful effects.
Read the above passage and answer the following questions:
(i) Give any two merits of nuclear reactors.
(ii) What is radiactive waste?
(iii) Why do people often oppose the location site of a nuclear reactor? What do you suggest?

The energy of an electron in a hydrogen atom is \({ E }_{ n }=\frac { -13.6 }{ { n }^{ 2 } } eV\), where n = 1, 2, 3,... Show that
(i) the electron in a hydrogen atom cannot have an energy of -6.8eV.
(ii) spacing between the lines within the given set of observed hydrogen spectrum decreases as n increases.