It's difficult to understand the principles of electrooptics.

It's difficult to understand his explanation.

This problem is very difficult.

It's difficult to understand the principle of this machine.

It's difficult to understand his explanation.

He is an expert in optics and has a deep knowledge of laser technology.

The electricity in my house has been cut off.

He is known as a fundamentalist believer.

His explanation was impossible to understand.

My specialty is fiber optics.