Imagine that you are watching an airplane flying by at a constant velocity relative to you. This time, you are in your own airplane. As you fly past each other, you decide to turn on your overhead lamp so that you can read a book that is 1 meter directly below your lamp. The passenger in the other plane sees your lamp turn on and sees the light hit the book. From his point of view: a) the photons that hit the book do not travel straight down (i.e., vertically), but instead travel on a diagonal path from the location that the lamp was at when it emitted the photons to the location of the book when the photons hit the book. b) the photons that hit the book travel straight down (i.e., vertically) from the lamp to the book.

Imagine that you are watching an airplane flying by at a constant velocity relative to you. This time, you are in your own airplane. As you fly past each other, you decide to turn on your overhead lamp so that you can read a book that is 1 meter directly below your lamp. The passenger in the other plane sees your lamp turn on and sees the light hit the book. From his point of view: a) the photons that hit the book do not travel straight down (i.e., vertically), but instead travel on a diagonal path from the location that the lamp was at when it emitted the photons to the location of the book when the photons hit the book. b) the photons that hit the book travel straight down (i.e., vertically) from the lamp to the book.