Lets say there is a Web page http://www.example.com/account/login. Notice this page is using the unencrypted HTTP protocol. We can follow these steps in Wireshark to capture the unencrypted packets:
Using the same example, instead of signing in at http://www.example.com/signin, if we signed in the HTTPS page, https://www.example.com/signin, and run Wireshark, we can see the traffic is definitely encrypted, and we can not see the username or password.
Note: If we have access to the private key, we can decrypt the SSL traffic.