UNIT – IV MODERN PHYSICS
PART – A QUESTIONS
1. What is black body?
A perfect black body is the one which absorbs and also emits the
radiations completely.
2. What is black body radiation?
Black body is said to be a perfect absorber, since it absorbs all the
wavelengths of the incident radiation. The black body is the perfect radiator
because it radiates the entire wavelength absorbed by it. This phenomenon
is called black body radiation.
3. State Planck’s quantum theory (or) State Planck’s hypothesis (or) What
are the postulates of Planck’s quantum theory? (or) What are the
assumptions of quantum theory of black body radiation? (or) Give the
special features of Quantum theory.
(i) The electrons in the black body are assumed as simple harmonic
oscillators.
(ii) The oscillators will not emit energy continuously.
(iii) The emit radiation in terms of quanta of magnitude ‘hν’, discretely.
i.e. E = nhν, where n = 0, 1, 2, 3, …..
4. Give the importance of Planck’s radiation formula.
• It explains all regions of black body radiation.
• It is based on quantum theory.
• It is used to derive other laws related to black body radiation.
5. State Wien’s displacement law. Give its limitation.
The product of wavelength (λm) of maximum energy emitted and the
absolute temperature (T) is a constant.
λm T = Constant.
Limitation: It holds good only for shorter wavelength.
6. State Rayleigh-Jeans law. Give its limitation.
The energy is directly proportional to the absolute temperature and is
inversely proportional to the fourth power of the wavelength.
T
E
4
Limitation: It holds good only for longer wavelength.
7. What is meant by photon?
Photons are discrete energy values in the form of small quantas of
definite frequency or wavelength.
8. State the properties of photon.
• Photons are similar to that of electrons.
• They do not have any charge.
• They do not ionize.
• They are not affected by electric and magnetic fields.
,9. What is wave function?
Wave function (ψ) is a variable quantity. It is associated with a moving
particle at any position (x, y, z) and at any time‘t’. It relates the probability of
finding the particle at that point and at that time.
10. Mention the physical significance of wave function of matter waves
(or) de Broglie waves.
(i) A variable quantity which characterizes waves is known as wave
function.
(ii) It relates the particle and the wave statistically.
(iii) It gives the information about the particle behavior.
(iv) It is a complex quantity.
(v) Ѱ represents the probability density of the particle which is real and
positive.
11. State de-Broglie hypothesis (or) Explain the concept of wave nature
(or) What is meant by matter waves? (or) Give the origin of concept of
matter waves.
The light exhibits the dual nature. It can behave as a particle and the
wave. de Broglie suggested that an electron which is a particle can also
behave as a wave and exhibits the dual nature.
Thus, the waves associated with a material particle (electron) are called
as matter waves.
If ‘v’ is the velocity and ‘m’ is the mass of the particle,
h
de Broglie wavelength =
mv
12. For a free particle moving within a one-dimensional potential box,
the ground state energy cannot be zero. Why?
For a free particle moving within a one dimensional potential box, when
n = 0, the wave function is zero for all values of x, i.e., it is zero even within
the potential box. This would mean that the particle is not present within
the box. Therefore, the state with n = 0 is not allowed. As energy is
proportional to n2, the ground state energy cannot be zero since n = 0 is not
allowed.
13. Define Eigen value and Eigen function.
Eigen value is defined as energy of the particle. It is denoted by En.
Eigen function is defined as the wave function of the particle. It is
denoted by n.
14. What is photoelectric Effect?
When a photon hits an electron on a metal surface, the electron can be
emitted. The emitted electrons are called photoelectrons. The effect is called
Photoelectric Effect.
15. Write down the Schroedinger time dependent wave equation.
E = H
Where E – Total energy of the particle
H – Hamiltonian operator
- Wave function
, 16. Write down the Schroedinger time independent wave equation.
2m
2 0 + (E − V ) 0 = 0
2
Where E – Total energy of the particle
V – Potential energy of the particle
m – Mass of the particle
h
= - Planck’s Constant
2
17. Mention the applications of Schroedinger wave equation.
• It is used to find the electrons in metals.
• It is used to find energy levels of an electron in an infinite deep
potential well.
PART – A QUESTIONS
1. What is black body?
A perfect black body is the one which absorbs and also emits the
radiations completely.
2. What is black body radiation?
Black body is said to be a perfect absorber, since it absorbs all the
wavelengths of the incident radiation. The black body is the perfect radiator
because it radiates the entire wavelength absorbed by it. This phenomenon
is called black body radiation.
3. State Planck’s quantum theory (or) State Planck’s hypothesis (or) What
are the postulates of Planck’s quantum theory? (or) What are the
assumptions of quantum theory of black body radiation? (or) Give the
special features of Quantum theory.
(i) The electrons in the black body are assumed as simple harmonic
oscillators.
(ii) The oscillators will not emit energy continuously.
(iii) The emit radiation in terms of quanta of magnitude ‘hν’, discretely.
i.e. E = nhν, where n = 0, 1, 2, 3, …..
4. Give the importance of Planck’s radiation formula.
• It explains all regions of black body radiation.
• It is based on quantum theory.
• It is used to derive other laws related to black body radiation.
5. State Wien’s displacement law. Give its limitation.
The product of wavelength (λm) of maximum energy emitted and the
absolute temperature (T) is a constant.
λm T = Constant.
Limitation: It holds good only for shorter wavelength.
6. State Rayleigh-Jeans law. Give its limitation.
The energy is directly proportional to the absolute temperature and is
inversely proportional to the fourth power of the wavelength.
T
E
4
Limitation: It holds good only for longer wavelength.
7. What is meant by photon?
Photons are discrete energy values in the form of small quantas of
definite frequency or wavelength.
8. State the properties of photon.
• Photons are similar to that of electrons.
• They do not have any charge.
• They do not ionize.
• They are not affected by electric and magnetic fields.
,9. What is wave function?
Wave function (ψ) is a variable quantity. It is associated with a moving
particle at any position (x, y, z) and at any time‘t’. It relates the probability of
finding the particle at that point and at that time.
10. Mention the physical significance of wave function of matter waves
(or) de Broglie waves.
(i) A variable quantity which characterizes waves is known as wave
function.
(ii) It relates the particle and the wave statistically.
(iii) It gives the information about the particle behavior.
(iv) It is a complex quantity.
(v) Ѱ represents the probability density of the particle which is real and
positive.
11. State de-Broglie hypothesis (or) Explain the concept of wave nature
(or) What is meant by matter waves? (or) Give the origin of concept of
matter waves.
The light exhibits the dual nature. It can behave as a particle and the
wave. de Broglie suggested that an electron which is a particle can also
behave as a wave and exhibits the dual nature.
Thus, the waves associated with a material particle (electron) are called
as matter waves.
If ‘v’ is the velocity and ‘m’ is the mass of the particle,
h
de Broglie wavelength =
mv
12. For a free particle moving within a one-dimensional potential box,
the ground state energy cannot be zero. Why?
For a free particle moving within a one dimensional potential box, when
n = 0, the wave function is zero for all values of x, i.e., it is zero even within
the potential box. This would mean that the particle is not present within
the box. Therefore, the state with n = 0 is not allowed. As energy is
proportional to n2, the ground state energy cannot be zero since n = 0 is not
allowed.
13. Define Eigen value and Eigen function.
Eigen value is defined as energy of the particle. It is denoted by En.
Eigen function is defined as the wave function of the particle. It is
denoted by n.
14. What is photoelectric Effect?
When a photon hits an electron on a metal surface, the electron can be
emitted. The emitted electrons are called photoelectrons. The effect is called
Photoelectric Effect.
15. Write down the Schroedinger time dependent wave equation.
E = H
Where E – Total energy of the particle
H – Hamiltonian operator
- Wave function
, 16. Write down the Schroedinger time independent wave equation.
2m
2 0 + (E − V ) 0 = 0
2
Where E – Total energy of the particle
V – Potential energy of the particle
m – Mass of the particle
h
= - Planck’s Constant
2
17. Mention the applications of Schroedinger wave equation.
• It is used to find the electrons in metals.
• It is used to find energy levels of an electron in an infinite deep
potential well.
