The frequency of the electromagnetic radiation produced when an electron transitions between energy levels is directly related to the energy difference between those levels. According to the formula \( E = h \cdot f \) (where \( E \) is the energy difference between the levels, \( h \) is Planck's constant, and \( f \) is frequency), a larger energy difference corresponds to a higher frequency, while a smaller energy difference produces a lower frequency.
In this question, you need to analyze the given transitions:
A. an electron falling from the 6th to the 2nd energy level: This transition involves a larger energy difference than many of the others; thus, it would emit a higher frequency photon.
B. an electron falling from the 3rd to the 2nd energy level: This transition has a moderate energy difference.
C. an electron falling from the 5th to the 2nd energy level: This transition has a larger energy difference than transition B, but not as large as transition A.
D. an electron jumping from the 1st to the 2nd energy level: This transition has the smallest energy gap of the options provided.
Comparing these transitions, the smallest energy transition corresponds to transition D. Therefore, the transition that would produce a wave with the lowest frequency is:
D an electron jumping from the 1st to the 2nd energy level.