Tricky questions

## Dual Nature Of Radiation And Matter

- Some questions are often misinterpreted by the students. Let's explore them!
1
Some questions are asked based on characteristics of photon. Students get confused here because the properties of photon are not totally similar to that of ordinary particle. For illustration, an ordinary particle has mass but a photon has no mass. Therefore it is important to understand the properties of photon.
Assertion
The energy and momentum of a photon are related as
Reason
The photon behaves like a particle.
A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C
Assertion is correct but Reason is incorrect
D
Both Assertion and Reason are incorrect
2
A classic problem is to count the number of photons normally striking per unit area in a photoelectric effect. The pro tip here is the intensity of radiation is related to the number of photons. There if we know intensity or power we can calculate the number of photons.
A point of source of 6 watts emits monochromatic light of wavelength . The number of photons striking normally per second per unit area of the surface distant 5 m from the source will be
A
B
C
D
3
Sometimes students mix up the energy of incident photons and the kinetic energy of photoelectrons. Remember that the photoelectrons are coming out of the surface because of the striking of incident photons. Therefore it is obvious that the kinetic energy of photoelectrons must less that the energy of incident photons.
Anode voltage is at . Incident radiation has frequency 1.410 and work function of the photo cathode is 2.8 . Find the minimum and maximum of photo electrons in
A
3, 6
B
0, 3
C
0, 6
D
2.8, 5.8
4
The concept of work function is understood by quantum theory. Since it is not there in classical physics, students find it difficult to crack. But questions can be asked to calculate the work function of a metal undergoing photoelectric emission.
In fig, electromagnetic radiations of wavelength are incident on a metallic plate A. The photoelectrons are accelerated by a potential difference of .These electrons strike another metal plate B from which electromagnetic radiations are emitted. The minimum wavelength of emitted photons is . If the work function of metal A is found to be , then find the value of . (Given
5
Questions are asked based on temperature dependence of de Broglie wavelength. Students may be clueless to establish the relationship. But a particle can have energy due to temperature (thermal energy). This is the same energy of the associate matter wave. Now finding the de Broglie wavelength when energy is known is a trivial calculation.
De Broglie wavelength for an average helium atom in a furnace at 400 K is . Find the value of k. Here, Given the mass of helium
6
Students may wonder knowing the de-Broglie wavelength of a projectile particle. But is technically possible. a projectile has its own momentum. Therefore, using de Broglie hypothesis one can easily calculate the de Broglie wavelength.
A particle of mass m is projected from ground with velocity u making angle with the vertical. The de Broglie wavelength of the particle at the highest point is
A
B
C
D