There are four waveforms given in page no 36.Mind well that IT SHOWS THE TURN ON TIME OF SCR. That is, SCR takes 1 to 4 uS to trigger that is
1*10^-6 to 4*10^-6seconds. Although it is just blink of an eye, it is divided into 3 time zones..But u can read that from book. I will try to just explain the graphs.
1.Gate current vs Time This graph is pretty simple. The gate current slowly increases upto certain value
(Ig) and then it is constant.
2.Anode Current vs Time Here the anode current slowly starts increasing because the SCR was non conductive at first and then it starts conducting.
3.Anode Cathode Voltage vs Time Now when no current was conducting, there was no voltage drop across SCR. So Vak was equal to supply voltage.But as the SCR starts conducting, the voltage across SCR falls and so the value of Vak is seen as decreasing.
4.Power loss vs Time As
Power=(Vak)*(It) , it is obvious that power is maximum when both Vak and It are in middle that is when Vak is at middle of rise time and It is also at middle of rise time. During starting, the Vak is high but It is low and at the end of turn on of SCR the Vak is low while It is high.So power is low at both the cases.