# physics mock question

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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.