Dual Nature of Matter and Radiation | NEET Important Chapter

Just like most of the chapters in part 2 of physics of class 12, the dual nature of matter and radiation also plays an important role in the NEET exam. The chapter revolves around the microscopic behaviour of particles and light. 

The dual nature of matter and radiation starts with the electron emission and photoelectric effect. Moving further, we study the history of the photoelectric effect with the explanation of various experiments performed by a few great physicists. The experimental setup of the photoelectric effect lets us understand the phenomena. With the help of an experimental set-up, we study a few important topics such as photocurrent and stopping potential.

In the chapter dual nature of light and radiation, we got introduced to wave theory. In wave theory, we come across a few remarkable concepts such as wave nature of matter, energy function, etc… In the wave theory of matter, we find the most significant discoveries by De Broglie. We study the De Broglie hypothesis to understand the dual nature of light. In this chapter, we also come across the uncertainty principle, Davisson Germer's experiment.

In this article, we will be focusing on the concepts of the dual nature of matter and radiation that plays a vital role in the NEET exam.

Important Topics of Dual Nature of Matter and Radiation

• Photoelectric effect

• Work function

• Threshold frequency

• Relation between Potential and Photoelectric Current

• Relation between Intensity of the Incident Light and Photoelectric Current

• Electron emission

• Cathode rays

• De Broglie equation

• Wave nature of matter

Important Concepts of Dual Nature of Matter and Radiation

We know that in a chapter there will be many concepts that are being mentioned in the textbook, but when it comes to NEET preparation we have to focus on what is most important among all the concepts in the chapter. Below is the table provided for some most important concepts of the dual nature of matter and radiation.

List of Important Formulae of Dual Nature of Matter and Radiations

Now, let us have a look at a few important dual nature of radiation and matter formulas that will be helpful in solving questions from this chapter.

Solved Examples of Dual nature of Matter and Radiations

1. All electrons emitted from a metal surface by an incident light of wavelength 100 nm can be stopped just before travelling 2 m in the direction of uniform electric field of 6 N/C. Then the work function of the surface is

1. 0.5 eV

2. 2.2 eV

3. 0.3 eV

4. 0.4 eV

Sol:

Given, 

The wavelength of incident light=$\lambda$= 100 nm

The electrons ejected by the metal surface are stopped by the electric field of 6 N/C at a distance 2m.

Now, we are asked to determine the stopping potential of the electrons. Let us assume that the electrons are travelling with a maximum speed Vmax. Then we write:

${k_{max}}=\dfrac {1}{2} m {v^2}=e E d$

${k_{max}}=\dfrac {e\times 6 \times 2}{e}=12 eV$

The energy of incident photo is:
$E=h \nu=\dfrac {hc}{\lambda}=\dfrac {1240 nm eV}{100}= 12.4 eV$

From Einstein's photoelectric equation we have:

${E_k}=h \nu -\phi$

Where,

$\phi$- The work function

${k_{max}}=h \nu -\phi$

$\phi = 12.4-12=0.4 eV$

Therefore, option d is the correct answer.

Key point: While calculating the maximum kinetic energy we should not forget to convert it in terms of eV.

2. The threshold frequency for photoelectric effect on sodium corresponds to a wavelength 8000 Å. Its work function is?

Sol:

Then work function of an emitted electron will be

$\phi= {\nu_o}{h}=\dfrac {hc}{\lambda}$

$\phi =\dfrac {6.6\times {10^{-34}} \times 3 \times {10^8}}{8000\times {10^{-10}}}$

$\phi=2.475\times {10^{-19}} joules$

Key point: We need to modify the formula according to the given data. We know that the work function is written in terms of threshold frequency but here we were given threshold wavelength. We need to use the relation between frequency and wavelength to solve this problem.

Previous Year Questions from Dual Nature of Matter and Radiations

1. The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of $3.3 \times {10^{-3}}$ watt will be: (NEET 2021)

1. ${10^16}$

2. ${10^15}$

3. ${10^18}$

4. ${10^17}$

Sol:

Given,

Wavelength of source of monochromatic light=$\lambda$= 600 nm

Power radiated by the source=$3.3 \times {10^{-3}}$ watt

The energy of the photon=E=$\dfrac {hc}{\lambda}= 3.3 \times {10^{-19}} Joules$

Let n is the number of photons emitted per second such that the energy emitted in one second is $3.3 \times {10^{-3}}$ J.

Thus, we get:

nE=$3.3 \times {10^{-3}}$

$n=\dfrac {3.3 \times {10^{-3}}}{3.3 \times {10^{-19}}}={10^16}$

Therefore, option A is the right answer.

Trick: Use the formula $P=\dfrac {nhc}{\lambda}$ to arrive at the solution easily.

2. Light of frequency 1.5 times the threshold frequency is incident on a photosensitive material. What will be the photoelectric current if the frequency is halved and intensity is doubled? (NEET 2020)

1. Four times

2. One-fourth

3. Zero

4. Doubled

Sol: 

Given,

Frequency of light=$\nu= 1.5 {\nu_o}$

Now, it is said that the frequency is halved, i.e., ${\nu}’ = \dfrac {1.5 {\nu_o}}{2}=0.75 {\nu_o}$

Since, 0.75 𝝼o < 𝝼o, .e. f' is less than threshold frequency, zero current will flow.

Therefore, option C is the right answer.

Key point: Current will flow if and only if threshold frequency is less.

Practice Questions

1. 1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1% of the incident photons produce photoelectrons, the current in the cell is (Ans: 0.48 µA)

2. The frequency of incident light falling on a photosensitive plate is doubled, and then the maximum kinetic energy of the emitted photoelectrons will become? (Ans: More than 2 times of the earlier value)

Conclusion

In this article we have discussed the dual nature of matter and radiation chapter with respect to the NEET exam. In this article we have provided solved examples, previous year questions, important formulae list, etc.