Question

Question 65: (II) Figure 17–44 is a photograph of a computer screen shot by a camera set at an exposure time of \(\frac{{\bf{1}}}{{\bf{4}}}{\bf{s}}\). During the exposure the cursor arrow was moved around by the mouse, and we see it 15 times. (a) Explain why we see the cursor 15 times. (b) What is the refresh rate of the screen?

Short answer

(a) You see the arrow 15 times because the screen recreates after regular intervals of time.

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

Step-by-step solution

Understanding of Computer Monitors

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.

Given information

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

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

(a) Evaluation of reason of watching the cursor 15 times

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.

(b) Determination of refresh rate of the screen

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 \(56\;{\rm{Hz}}\).