Dark lines appearing in monitor screen ,when viewed through mobile camera

The refresh rate of the CRT monitor is faster than what the sensor on your mobile camera can handle, hence it will show an aliasing effect 😀 This depends on the scanning method of the sensor, for example, some CMOS sensors will sweep the image from top to bottom. But during the sweep, the CRT monitor would have initiated the vertical blanking signal intermittently showing the dark lines.

The vertical and horizontal scanning in TV monitors or CRT monitors should have been so quick ,that it wont be realised easily(usually by naked eye).

In that case ,is my mobile camera so much sensitive to catch these scanning??

@bharatpb-'Sensitive'?See here we don't talk about anything such as sensitivity of the mobile camera.What ash is saying that your mobile camera has a face-plate which has cells which generate an electrical signal when light falls on them. There will be an internal electronic beam which will scan this plate and produce a picture on your mobile screen.Now the rate at which this plate is scanned is slower than the scanning rate of the CRT.So before your CMOS sensor completes scanning of the entire face plate once,the CRT electron beam has scanned the CRT display screen multiple times.So since the refresh rate of the sensor is not fast enough the earlier image of the CRT screen is still present on the mobile screen.As the CRT has a blanking pulse n between the horizontal lines, the sensor displays it with the dark lines.
If you did not understand, just think on the basic fact that any display takes advantage of the fact that you can perceive changes in the image only after 0.1sec, so all the screen has to do is to display the image again before 1/10th of a sec.