Equivalent circuit is shown at right side.
The green resistor is input impedance of opamp.
----- Virtual short --------
Ideally the voltage across input impedance is zero. ie. Voltage difference between two input terminals is zero.
ie Vd=0
ie V1-V2=0
ie V1 = V2
This is possible only if the two terminals are actually shorted. As they are not, they are said to be "virtually shorted".
----- Virtual ground -----
Now if one of the terminal is actually grounded. Then the other terminal appears to be grounded. This is referred as "virtual ground".
Keep circuiting. Take care. ☺
