Introduction:
The energy levels of atoms are quantized, meaning that electrons can only exist in certain energy levels or orbitals. When an electron moves from a higher energy level to a lower one, it emits energy in the form of electromagnetic radiation, such as visible light.

Frequency of Light Emitted:
The frequency of light emitted when an electron in a helium atom transitions from n=4 to n=2 can be calculated using the formula:

ΔE = -Rhc(1/nf² - 1/ni²)

Where R is the Rydberg constant, h is Planck's constant, c is the speed of light, nf is the final energy level (n=2), and ni is the initial energy level (n=4).

Substituting the appropriate values gives:

ΔE = -Rhc(1/2² - 1/4²)
ΔE = -2.18x10⁻¹⁸ J

The frequency of light emitted can be calculated using the formula:

E = hf

Where E is the energy of a photon, h is Planck's constant, and f is the frequency of the light.

Solving for f gives:

f = E/h
f = ΔE/h
f = (-2.18x10⁻¹⁸ J)/6.63x10⁻³⁴ J.s
f = 3.09x10¹⁵ Hz

Therefore, the frequency of light emitted when an electron in a helium atom transitions from n=4 to n=2 is 3.09x10¹⁵ Hz.

Corresponding Transition in Hydrogen Atom:
The transition in a hydrogen atom that corresponds to the helium transition from n=4 to n=2 is the n=5 to n=2 transition. This is because the energy difference between the n=5 and n=2 energy levels in hydrogen is equal to the energy difference between the n=4 and n=2 energy levels in helium.

Conclusion:
The frequency of light emitted when an electron in a helium atom transitions from n=4 to n=2 can be calculated using the energy and frequency formulas. The corresponding transition in a hydrogen atom is the n=5 to n=2 transition.