∵I= U R ，∴灯泡电阻R= U I = 4V 0.2A =20Ω；灯泡额定功率P额= U R = (6V)2 20Ω =1.8W，当电压为2V时的实际功率：P实= U R = (2V)2 20Ω =0.2W；故答案为：1.8；0.2．
