Which type of electromagnetic radiation has the longest wavelength?

A) ultraviolet

B) infrared

C) X-ray

D) visible

B. Infrared.

Explanation:

Referring to the electromagnetic spectrum, ultraviolet rays can be measured with a frequency of 10-⁸, infrared has a frequency of 10-⁵, visible radiation has a frequency of 0.5 x 10-⁶ meanwhile X-rays show a frequency of 10-¹⁰.

Hence, the largest magnitude among the rest goes to infrared rays, which makes B the correct answer.