# Consider the De-broglie Wavelength of an Electron and a Proton. Which Wavelength is Smaller If the Two Particles Have (A) the Same Speed (B) the Same Momentum (C) the Same Energy? - Physics

Short Note

Consider the de-Broglie wavelength of an electron and a proton. Which wavelength is smaller if the two particles have (a) the same speed (b) the same momentum (c) the same energy?

#### Solution

de-Broglie wavelength,

λ = h/(mv)

where h = Planck's constant

m = mass of the particle

v = velocity of the particle
(a) It is given that the speed of an electron and proton are equal.
It is clear from the above equation that λ ∝ 1/m

As mass of a proton, mp > mass of an electron, me, the proton will have a smaller wavelength compared to the   electron.

(b) λ = h/p  (∵ p = mv)

So, when the proton and the electron have same momentum, they will have the same wavelength.
(c) de-Broglie wavelength (λ) is also given by

λ = h/sqrt(2mE) ,

where E = energy of the particle.
Let the energy of the proton and electron be E.
Wavelength of the proton,

λ_p = h/sqrt(2m_pE)      ....(1)

Wavelength of the electron,

λ_e = h/sqrt(2m_eE)      ....(2)

Dividing (2) by (1), we get :

λ_e/λ_p = sqrt(m_e)/sqrt(m_p)

⇒ λ_e/λ_p < 1

⇒ λ_e < λ_p

It is clear that the proton will have smaller wavelength compared to the electron.

Concept: de-Broglie Relation
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 20 Photoelectric Effect and Wave-Particle Duality
Short Answers | Q 12 | Page 363