DUAL NATURE OF RADIATION AND MATTER

Very Short Question Answers

Q1. What are “cathode rays”?

Ans. Fast moving negatively charged particles.The beam of fast-moving electrons.


Q2. What important fact did Millikan’s experiment establish?

Ans. Millikan’s experiment established the fact that electric charge is quantized. $ i.e. \:\:Q =± ne$


Q3. What is “work function”.?

Ans. Work function: The minimum energy required for the electron to escape from the metal surface is called work function.       $$ \phi_{o}= W_{o}   = h\nu_{o}$$


Q4. What is “photo electric effect”?

Ans. Photoelectric effect: When the surface of a metal is illuminated by light of suitable frequency, electrons are emitted from the surface of the metal. This phenomenon is called photoelectric effect.


Q5. Give examples of “photo sensitive substances” why are they called so?

Ans. Photo sensitive substances: $Li,Na,K,Rb$ and $Cs$ etc. The work function of alkali metals is very low.


Q6. Write down Einstein’s photo electric equation. Einstein’s photoelectric equation.

Ans.

$K_{max}= \frac{1}{2} mv_{max}^{2}= h\nu-ϕ_{o}$
$K_{max}$= maximum kinetic energy of the photoelectron, $h$=planks constant, 
$\nu$ = frequency, $ϕ_{o}$= work function. 


Q7. Write down De Broglie’s relation and explain the terms therein.

Ans. De Broglie’s relation:      $\lambda=h/p = h/mv$
$\lambda$ = De Broglie’s wavelength,  $h$= Planks constant,  $P$= momentum,    $m$= mass,   $v$=velocity


Q8. State Heisenberg’s uncertainly principle.

Ans. It is impossible to measure both position and momentum of an electron (or any other particle) at the same time exactly.

$$\bigtriangleup x\,.\bigtriangleup p =  h$$


Q9. What is the De Broglie wavelength associated with an electron, accelerated through a potential difference of 100 volts?

Ans. Accelerating potential$ (V)=100V$
The De Broglie wave length  $\lambda=h/p=1.227/\sqrt{V}$

$=(1.227  )/\sqrt{100}$

$= 1.227/10 $

$\lambda= 0.123\,\: nm$


Q10. An electron, an α – particle, and a proton have the same kinetic energy.Which of these particles has the shortest De Broglie wavelength?

Ans. De Broglie wave length $λ=h/p$ ,  

kinetic energy $K=p^{2}/2m$

$\therefore \lambda=\frac{h}{\sqrt{2mk}}$

Alpha particle has more mass as compared to electron and proton.
Hence De Broglie wavelength is shortest for alpha particle. 


Q11. The work function of caesium is 2.14ev. Find the threshold frequency for casesium.

Ans. Work function $ϕ_{o} = 2.14 eV $, threshold frequency $\nu_{\circ}=  ?$
$ \nu_{\circ}=\frac{\phi_{\circ}}{h} $

$ \nu_{\circ}=\frac{2.14}{6.63\times 10^{-34}} $

$ \nu_{\circ}= 5.16\times 10^{14} \,\, Hz $


Q12. What is the de Broglie’s wavelength of a ball of mass $0.12 kg$ moving with a speed of $20ms^{-1} $ ?

Ans. $\lambda=h/p=h/mv $

$\lambda = 6.63X10^{-34}/(0.12 \times 20)$
 $\lambda=    2.76 ×10^{-34}   m$