Question

To determine frequency of AC supply by CRO.

Answer 1

The CRO has a 'time/division value' that indicates what time corresponds to one division of the scale. So, you can measure (count) how many divisions make up one complete wave, from peak to peak or crest to crest, multiply the number of divisions by the time/division value, and find the reciprocal of this value. This is how you find the frequency of the AC source using the cathode-reay oscilloscope.