91影视

A cathode ray tube (CRT) was traditionally used in computer monitors.The CRT accelerates the electrons by a high voltage and sweeps the electrons across the screen regularly with the help of magnetic coils or electric deflection plates.

The exposure time of the camera is, \(T = \frac{1}{4}{\rm{s}}\)

Number of cursor arrows seen in the photograph, \(N = 15\)

Since the CRT sweeps the electrons across the screen at regular intervals, the screen in the monitor is recreated after regular intervals. The mouse moves the cursor arrow continuously, but its location is created at regular intervals only while the screen is being recreated. The screen must be recreated 14 times within the exposure time and one initial position of the curser. Thus, you see the cursor 15 times.

Let us consider that the screen is recreated in every t seconds and at t &= 0, the cursor is at the first position on the screen. So, after time t, the screen is recreated, and the arrow is seen at the second position. The arrow is seen at the third position after time 2t and at the fourth position after time 3t.

Proceeding in the similar manner, the arrow will be seen at the fifteenth position after time 14t. But 15 arrows are seen in the photograph in the exposure time T. Thus,

\(\begin{aligned}{c}T &= 14t\\t &= \frac{T}{{14}}\\ &= \frac{1}{{14}}\left( {\frac{1}{4}\;{\rm{s}}} \right)\\ &= \frac{1}{{56}}\;{\rm{s}}\end{aligned}\)

So, the monitor screen refreshes after every \(\frac{1}{{56}}\;{\rm{s}}\).

The refresh rate of the screen is:

\({\rm{Refresh rate}} = \frac{1}{t} = 56\;{\rm{Hz}}\)

Thus, the refresh rate of the screen is \(56\;{\rm{Hz}}\).