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In an electron microscope, the accelerating voltage is increased from 20 kV to 80 kV. The resolving power (initially R) becomes:

1. R/4
2. 4R
3. 2R
4. R/2

Correct answer: 2R

Solution

de Broglie wavelength lambda = h/sqrt(2*m*e*V), so lambda ~ 1/sqrt(V). Resolving power ~ 1/lambda ~ sqrt(V). Increasing V from 20 to 80 kV multiplies V by 4, so resolving power multiplies by sqrt(4) = 2. New value = 2R.

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