General Physics Question 5
Question 5
- Estimate the wavelength at which plasma reflection will occur for a metal having the density of electrons $N \approx 4 \times 10^{27} \mathrm{~m}^{-3}$. Take $\varepsilon_{0} \approx 10^{-11}$ and $m \approx 10^{-30}$, where these quantities are in proper SI units. (a) $800 \mathrm{~nm}$ (b) $600 \mathrm{~nm}$ (c) $300 \mathrm{~nm}$ (d) $200 \mathrm{~nm}$
Passage
Phase space diagrams are useful tools in analysing all kinds of dynamical problems. They are especially useful in studying the changes in motion as initial position and momentum are changed. Here we consider some simple dynamical systems in one-dimension. For such systems, phase space is a plane in which position is plotted along horizontal axis and momentum is plotted along vertical axis.
The phase space diagram is $x(t) v s p(t)$ curve in this plane. The arrow on the curve indicates the time flow. For example, the phase space diagram for a particle moving with constant velocity is a straight line as shown in the figure. We use the sign convention in which position or momentum upwards (or to right) is positive and downwards (or to left) is negative.
(2011)
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Answer:
Correct Answer: 6. (b)
Solution:
$$ \begin{aligned} \omega & =2 \pi f=\frac{2 \pi c}{\lambda} \ \therefore \quad \lambda & =\frac{2 \pi c}{\omega}=\frac{2 \pi c}{\sqrt{\frac{N e^{2}}{m \varepsilon_{0}}}} \end{aligned} $$
Substituting the values, we get $\lambda \simeq 600 \mathrm{~nm}$.
