PhysicsQ51–54 of 90 questions
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51
PhysicsMediumMCQNEET · 2020
The power of a biconvex lens is $10 \; D$ and the radius of curvature of each surface is $10 \; cm$. The refractive index of the material of the lens is:
A
$3/2$
B
$4/3$
C
$9/8$
D
$5/3$
Solution
(A) The lens maker's formula is given by $\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$.
Given power $P = 10 \; D$,the focal length $f = \frac{1}{P} = \frac{1}{10} \; m = 0.1 \; m = 10 \; cm$.
For a biconvex lens,$R_1 = +10 \; cm$ and $R_2 = -10 \; cm$.
Substituting these values into the formula:
$\frac{1}{10} = (\mu - 1) \left( \frac{1}{10} - \frac{1}{-10} \right)$
$\frac{1}{10} = (\mu - 1) \left( \frac{1}{10} + \frac{1}{10} \right)$
$\frac{1}{10} = (\mu - 1) \left( \frac{2}{10} \right)$
$1 = 2(\mu - 1)$
$0.5 = \mu - 1$
$\mu = 1.5 = \frac{3}{2}$.
Given power $P = 10 \; D$,the focal length $f = \frac{1}{P} = \frac{1}{10} \; m = 0.1 \; m = 10 \; cm$.
For a biconvex lens,$R_1 = +10 \; cm$ and $R_2 = -10 \; cm$.
Substituting these values into the formula:
$\frac{1}{10} = (\mu - 1) \left( \frac{1}{10} - \frac{1}{-10} \right)$
$\frac{1}{10} = (\mu - 1) \left( \frac{1}{10} + \frac{1}{10} \right)$
$\frac{1}{10} = (\mu - 1) \left( \frac{2}{10} \right)$
$1 = 2(\mu - 1)$
$0.5 = \mu - 1$
$\mu = 1.5 = \frac{3}{2}$.
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52
PhysicsMediumMCQNEET · 2020
The variation of electrostatic potential $V$ with radial distance $r$ from the centre of a positively charged metallic thin shell of radius $R$ is given by the graph:
A
B
C
D
Solution
(C) For a positively charged metallic thin shell of radius $R$:
$1$. Inside the shell $(r < R)$,the electric field is zero,which implies that the electrostatic potential $V$ is constant and equal to the potential at the surface.
$V = \frac{q}{4 \pi \varepsilon_{0} R} = \text{constant}$
$2$. Outside the shell $(r \geq R)$,the shell behaves as a point charge placed at the centre. Thus,the potential varies inversely with distance.
$V = \frac{q}{4 \pi \varepsilon_{0} r} \implies V \propto \frac{1}{r}$
Therefore,the graph shows a constant potential for $r < R$ and a hyperbolic decrease for $r \geq R$,which matches the graph in option $C$.
$1$. Inside the shell $(r < R)$,the electric field is zero,which implies that the electrostatic potential $V$ is constant and equal to the potential at the surface.
$V = \frac{q}{4 \pi \varepsilon_{0} R} = \text{constant}$
$2$. Outside the shell $(r \geq R)$,the shell behaves as a point charge placed at the centre. Thus,the potential varies inversely with distance.
$V = \frac{q}{4 \pi \varepsilon_{0} r} \implies V \propto \frac{1}{r}$
Therefore,the graph shows a constant potential for $r < R$ and a hyperbolic decrease for $r \geq R$,which matches the graph in option $C$.
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53
PhysicsMediumMCQNEET · 2020
Which of the following gates is called a universal gate?
A
$NOT$ gate
B
$OR$ gate
C
$AND$ gate
D
$NAND$ gate
Solution
(D) universal gate is a logic gate that can be used to implement any other logic gate or Boolean function without the need for any other type of gate.
$NAND$ and $NOR$ gates are known as universal gates.
Since $NAND$ gate is listed among the options,it is the correct answer.
$NAND$ and $NOR$ gates are known as universal gates.
Since $NAND$ gate is listed among the options,it is the correct answer.
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54
PhysicsEasyMCQNEET · 2020
The wave nature of electrons was experimentally verified by,
A
Davisson and Germer
B
de Broglie
C
Hertz
D
Einstein
Solution
(A) The wave nature of electrons was experimentally verified by the Davisson-Germer experiment in $1927$.
This experiment demonstrated electron diffraction,which provided direct evidence for the wave-like behavior of electrons as proposed by Louis de Broglie.
This experiment demonstrated electron diffraction,which provided direct evidence for the wave-like behavior of electrons as proposed by Louis de Broglie.
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