If I were to guess, I would say that the present participle of to lighten, lightening, is not much used in everyday discourse. That said, I heard a report earlier this summer on NPR about how the Black Lives Matter movement in the United States has inspired a discussion about colorism and skin lightening potions in India, where colorism apparently runs rampant. Also, if you ever teach Julia Alvarez’s superb novel In the Time of the Butterflies, it makes at least one specific reference to relatively well-known fact that the dictator of the Dominican Republic from from 1930 to 1961, Rafael Trujillo, used skin-lightening cream and was, unsurprisingly, a virulent racist who slaughtered thousands of Haitians in the infamous Parsley Massacre. In fact, there is even a Wikipedia page on Colorism in the Caribbean if you are interested.
Such are the wages, I’m afraid, of the valorization and privileging of white skin.
So, this set of five worksheets on the homophones lightening and lightning might be more of an exercise in ensuring students understand what the lightning is as a meteorological and electrical phenomenon and how properly to spell the word. Still, there is room in these worksheets for fooling around with the verb to lighten, used both intransitively and transitively.
If you find typos in these documents, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.