Final answer:
The line uses a simile, which is a comparison between two different things using 'like' or 'as' to describe the father's pain vividly.
Step-by-step explanation:
The literary device used in the line "At first, my father doubled over in pain, like a lightning strike..." is a simile. A simile is a figure of speech that compares two different things using the words 'like' or 'as.' In this instance, a comparison is drawn between the sudden and intense pain experienced by the speaker's father and the quick, powerful impact of a lightning strike. This comparison helps to convey the immediacy and severity of the father's pain to the reader, creating a vivid and relatable image.