physics mock question

1 Answer

Rating
  • 天同
    Lv 7
    7 years ago
    Favorite Answer

    1. The longest time occurs when the light ray is under total internal refletion on the cladding-core interface. The angle of incident is the critical angle c, say.

    Hence, by Snell's Law, sin(c) = 1.47/1.5

    Let L be the longitudinal distance L (i.e. distance measured along the axis of the optical fibre) travelled by the light ray if it goes from one side of the core to the opposite side, and d be the corresponding actual distance travelled by the light ray.

    thus, sin(c) = L/d

    i.e. d = L/sin(c) = 1.5L/1.47

    In 1 km (1000 m) of the optical fibre, there are 1000/L sections. Hence, the actual distance travelled by light ray = (1000/L).d

    = (1000/L).(1.5L/1.47) m

    = 1500/1.47 m

    Speed of light in the fibre core = 3x10^8/1.5 m/s

    Hence, time taken for light to go through 1 km of the optical fibre

    = (1500/1.47)/(3x10^8/1.5) s = 5.1 x 10^-6 s

    2. Q: electric potential energy 幾時係正,幾時係負?

    A: In general, an isolated +ve charge produces +ve potential in its surrounding space, an isolated -ve charge produces -ve potential at its surrounding space.

    Thus, a +ve test charge under a +ve potential field (i.e. field associated with a +ve charge) possesses +ve potential energy. The same test charge under a -ve potential field (i.e. field associated with a -ve charge) possesses -ve potential energy.

    In other words, +ve potential energy indicates a repulsive force exists and a -ve potential energy indicates the existance of an attractive force.

Still have questions? Get your answers by asking now.