The Passive Voice can be used :
a) without the doer of the action being mentioned (the doer is either unknown or unimportant)
e.g. The telegram was sent yesterday. (Телеграму відправлено вчора.)
b) with the doer of the action being mentioned. This occurs only when the doer of the action is to some extent emphasized.
The noun or pronoun denoting the doer of the action is introduced by the preposition by.
e.g. This beautiful poem was written by Taras Shevchenko. (Цей чудовий вірш був написаний Тарасом Шевченком.)