As Expected..

I went in class early today ard 7.20am and slept straight. The next thing I heard was Mr.John calling "Cheng Kok Mun". In my half-asleep mood I had to do my presentation on Solar Power - Photovoltaic Cell. I expected the class to ask me hard questions, but apparently only YeeSiew asked a very pathetic and easy question. But after that, as expected, I got shot with a very hard question by Mr. John. Zzz

The question:

You are standing 50meters away from a microwave tower that transmit microwaves with a uniform frequency of 1800MHz. At 50m away, the photovoltaic cell in your hand cannot produce any current. How far do you have to move toward the tower in order for the photovoltaic cell to produce a current?

PM me on msn if you wanna know the answer. The answer is kinda, erm, tricky. After Mr. John told me the answer, I felt as though I just got conned for 4 marks.