Questions - Answers

Short Answer Questions (4 Marks)

1. What is Moseley's Law? Discuss briefly its importance.
A: Moseley's law: The square root of frequency of a particular X-ray spectral line ( ) is proportional to atomic number (Z) of target element used.

                                    
                Where a, b, are constants for a given series.
Importance of Moseley's law:
1) Moseley's law helped in the rearrangement of elements according to their atomic numbers instead of atomic weights. He founds that cobalt comes before nickel and argon before potassium.
2) This law helped in the discovery of new elements like Masurium and Illinium.
3) It enabled in perfecting periodic table by determining the atomic numbers of rare earth elements.

2. Define the terms work function and threshold frequency. Explain the relation between them.
A: Work function:
              The minimum photon energy required to remove the electron from the atom of metal surface is called work function.
                           It is denoted by "W₀".
Threshold frequency:
              The minimum frequency of the incident radiation, which can not set free electron from the emitter is called Threshold frequency.
             Work function and Threshold frequency are related as W₀ = h 
 

Very Short Answer Questions        (2 Marks)
 

1. If a proton and an electron have same de Broglie wavelength, which has more momentum and kinetic energy?
A: de Broglie wavelength is   

* When de Broglie wavelength is same, then momentum 'P' is same for proton and electron.
* When 'P' is same, the body with less mass has more K.E.

 
 So, KE of electron is more than proton.
 

2. What are de-Broglie waves?
A: According to de Broglie hypothesis matter will exhibit wave nature are called de Broglie waves.
Wavelength of De Broglie waves is