Question

Find the initial momentum of electron if the momentum of electron is changed by Pm and the De Broglie wavelength associated with it changes by 0.50%.

Answer 1

answer is 400p∘
λ∼1pλ∼1p⇒Δpp=−Δλλ⇒Δpp=−Δλλ⇒|Δpp|=|Δλλ|⇒|Δpp|=|Δλλ|⇒p∘p=0.25100=1400⇒p∘p=0.25100=1400⇒p=400p∘

Answer 2

As You Know , 

De Brogli Equation :

λ = h/mv 

λ = h/P....(1)

Let P0 be the initial Momentum and P is the decrease in momentum .

When  wavelength changed by 0.5%

λ + 0.5λ/100 = h/P0-P

100.5λ/100 = h/P0-P----(2)

By 1 and 2 

P0 = 1.99P