Explanation
This is a sound-alike memory trick, not a letter acronym, for the lens/mirror equation. Say it out loud: "If I Do I Die" sounds like "1/f = 1/do + 1/di".
- 1/f = 1/do + 1/di
- "f" is focal length
- "do" is object distance
- "di" is image distance
Example: a lens has a focal length of 10 cm (f = 10), and an object is placed 15 cm in front of it (do = 15). Plug those into the formula:
- 1/10 = 1/15 + 1/di
- 1/di = 1/10 − 1/15 = 3/30 − 2/30 = 1/30
- di = 30 cm
So the image forms 30 cm from the lens.