An electron with kinetic energy K₁ enters between parallel plates of a capacitor at an angle 'α' with the plates. It leaves the plates at angle 'β' with kinetic energy K₂. Then the ratio of kinetic energies K₁ ∶ K₂ will be: 

Concept: 

In this concept, a parallel plate capacitor is used which can be defined as:

The parallel plate capacitor is a device that uses two parallel plates connected across a battery to create an electric field between them and charge the plates. 

Calculations:

Given:

α, β are the angles made with capacitor plates by electrons. 

Since the electric field inside a parallel plate capacitor is perpendicular to the plates, the parallel component of velocity won't change.

So, we can write it as V₁ cosα = V₂ Cosβ     -----(1) 

Rearranging equation (1) we get: V1 /V2 = Cosβ/ cosα    -----(2)

As we know, kinetic energy (k) = (1/2) mv²

The ratio of kinetic energy can be written as: k₁ / k₂ = (1/2) mv₁² / (1/2) mv₂² 

k1 / k₂ = (v₁/ v₂)²   -----(3) 

Using equations (2) and (3) we get:  k1 / k2 = (Cosβ/ cosα)2 

Hence, option (4) is correct.