Rectilinear propagation describes the tendency of electromagnetic waves (light) to travel in a straight line.
[citation needed] The rectilinear approximation is only valid for short distances, in reality light is a wave and have a tendency to spread out over time.
For everyday usages, it remains valid as long as the refractive index in the medium is constant.
This implies that light rays travel along a straight line ABC, and hence, candle flame is visible.
It means that the light emitted by the candle is unable to bend and reach observers eye.