Assertion (A) : The shift in the position of the bright spot on the CRO screen per unit change of voltage across the deflection plates is called deflection sensitivity. Reason (R) : The path traveled by the spot on a CRO screen during the retrace time is called retrace path.
Both (A) and (R) are true, but (R) is not correct explanation of (A).
Both statements are correct definitions, but they define two unrelated terms — option 2.
A — deflection sensitivity, and where it comes from. The definition given is exactly right, and the quantity follows from the geometry of the tube:
\(S=\dfrac{D}{V_{d}}=\dfrac{L\,l}{2\,d\,V_{a}}\qquad\text{mm/V}\)
with L the distance from the plates to the screen, l the plate length, d their separation and \(V_{a}\) the accelerating voltage. Two consequences are worth noting. Sensitivity is inversely proportional to the accelerating voltage — a faster electron spends less time between the plates and is deflected less — so a brighter trace costs sensitivity, which is the fundamental compromise in CRT design. And it depends only on fixed dimensions, so it is a constant for a given tube, typically a fraction of a millimetre per volt; the volts-per-division control is a calibrated attenuator ahead of the vertical amplifier, not a change in the tube.
R — retrace, and why it is blanked. The definition is also correct. The timebase sweeps the spot left to right and must then return it to start the next sweep. That return is the retrace or flyback, and it is made as fast as possible. During it the Z axis is driven negative — the blanking pulse — so the beam is cut off and the return leaves no visible line. Without blanking, a bright diagonal would cross every trace and confuse the display.
| Term | Concerns | Axis |
|---|---|---|
| Deflection sensitivity | How far the spot moves per volt | Vertical (or horizontal) |
| Retrace | The return sweep between traces | Horizontal timing, with Z blanking |
Why R is not an explanation of A. Sensitivity is a static property of the deflection system, fixed by the tube's dimensions and its accelerating voltage. Retrace is an event in the timebase's cycle. Neither causes nor constrains the other — the sensitivity would be exactly the same in a tube with no timebase at all, as in an X-Y display. The two statements are correct definitions placed side by side, which is precisely the situation option 2 describes.
Hence, both are true, but (R) is not the correct explanation of (A).
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