Two large, perfectly parallel plane mirrors face each other and are separated by a distance of 1.5m. A photon is released exactly midway between the mirrors so that it travels perpendicularly toward one of the mirrors.
(i) Explain why the photon continues to move back and forth along the same straight line after each reflection.
(ii) Derive an expression for the total distance Dn travelled by the photon after it has undergone n reflections (hits on either mirror).
(iii) Using your expression, calculate the total distance travelled by the photon after it has struck a mirror 50 times.