  The radiation reaction (RR) is expected to play a critical role in
light-matter interactions at extreme intensity. Utilizing the theoretical
analyses and three-dimensional (3D) numerical simulations, we demonstrate that
electron reflection, induced by the RR in a head-on collision with an intense
laser pulse, can provide pronounced signatures to discern the classical and
quantum RR. In the classical regime, there is a precipitous threshold of laser
intensity to achieve the whole electron bunch rebound. However, this threshold
becomes a gradual transition in the quantum regime, where the electron bunch is
quasi-isotropically scattered by the laser pulse and this process resembles a
water splash. Leveraged on the derived dependence of classical radiation
rebound on the parameters of laser pulses and electron bunches, a practical
detecting method is proposed to distinguish the quantum discrete recoil and
classical continuous RR force.
