Science (English Medium) कक्षा १२ - CBSE Important Questions

A hydrogen atom makes a transition from n = 5 to n = 1 orbit. The wavelength of photon emitted is λ. The wavelength of photon emitted when it makes a transition from n = 5 to n = 2 orbit is ______.

Write the ionisation energy value for the hydrogen atom.

How is the size of a nucleus found experimentally? Write the relation between the radius and mass number of a nucleus.

A nucleus with mass number A = 240 and BE/A = 7.6 MeV breaks into two fragments, each of A = 120 with BE/A = 8.5 MeV. Calculate the released energy.

Consider the fission of `""_92^238"U"` by fast neutrons. In one fission event, no neutrons are emitted and the final end products, after the beta decay of the primary fragments, are `""_58^140"Ce"` and `""_44^99"Ru"`. Calculate Q for this fission process. The relevant atomic and particle masses are

`"m"(""_92^238"U")` = 238.05079 u

`"m"(""_58^140"Ce")` = 139.90543 u

`"m"(""_44^99"Ru")` = 98.90594 u

Asha's mother read an article in the newspaper about a disaster that took place at Chernobyl. She could not understand much from the articles 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.

(a) What was the installation at Chernobyl where the disaster took place? What according to you, was the cause of this disaster?

(b) Explain the process of release of energy in the installation at Chernobyl.

(c) What according to you, were the values displayed by Asha and her mother?

Write the basic nuclear process underlying β⁺ and β^– decays.

Define the terms (i) half-life (T_1/2) and (ii) average life (τ). Find out their relationships with the decay constant (λ).

 In a typical nuclear reaction, e.g.

`"_1^2H+"_1^2H ->"_2^3He + n + 3.27 \text { MeV },`

although number of nucleons is conserved, yet energy is released. How? Explain.

Write the relationship between the size of a nucleus and its mass number (A)?

In a given sample, two radioisotopes, A and B, are initially present in the ration of 1 : 4. The half lives of A and B are respectively 100 years and 50 years. Find the time after which the amounts of A and B become equal.

Define one Becquerel.

Distinguish between isotopes and isobars.

Two nuclei have different mass numbers A₁ and A₂. Are these nuclei necessarily the isotopes of the same element? Explain.

Explain the release of energy in nuclear fission and fusion on the basis of binding energy per nucleon curve.

How mass defect is related to the stability of the nucleus?

With the help of a neat circuit diagram, explain the working of a photodiode. 

Meeta's father was driving her to school. At the traffic signal, she noticed that each traffic light was made of many tiny lights instead of a single bulb. When Meeta asked this question to her father, he explained the reason for this.

Answer the following questions based on above information:

(i) What were the values displayed by Meeta and her father?

(ii) What answer did Meeta's father give?

(iii) What are the tiny lights in traffic signals called and how do these operate?

(i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.

Draw energy band diagrams of an n-type and p-type semiconductor at temperature T > 0 K. Mark the donor and acceptor energy levels with their energies.