JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Modern Physics

An electron is travelling with a velocity

v

in free space and when it enters a medium, its velocity is reduced by

20%

. The de Broglie wavelength of electron in the medium is

α λ_{0}

, where

λ_{0}

is its de Broglie wavelength in free space. The value of

α

is ______

(A)1.20
(B)1.0
(C)1.25✓ Correct
(D)0.75

Answer: (C)

Solution

The de Broglie wavelength is

λ = \frac{h}{m v}

, inversely proportional to the speed. In free space

λ_{0} = \frac{h}{m v}

, while in the medium the speed falls by

20%

0.8 v

, so

λ = \frac{h}{m ( 0.8 v )} = \frac{1}{0.8} λ_{0} = 1.25 λ_{0}

. The value of

α

1.25

JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Single correct option

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