A plano-convex lens fits exactly into a plano-concave lens.Their plane surfaces are parallel to each other.If lenses are made of different materials of refractive indices $\mu_1$ and $\mu_2$ and $R$ is the radius of curvature of the curved surface of the lenses,then the focal length of the combination is
$\frac{R}{\left(\mu_1-\mu_2\right)}$
$\frac{2 R}{\left(\mu_1-\mu_2\right)}$
$\frac{R}{2\left(\mu_1+\mu_2\right)}$
$\frac{R}{2\left(\mu_1-\mu_2\right)}$
Consider a fuse wire of length $l$ and radius $r$ .The time of heating $(t)$ for passing the maximum current will depend on
$t \propto r^2 l$
$t \propto r^3 l^2$
$t \propto r^4 l^0$
$t \propto r^2 l^3$
Density and volume of a body are given as $(20 \pm 4) \mathrm{gm} / \mathrm{cm}^3$ and $(10 \pm 1) \mathrm{cm}^3$ respectively. The absolute error in measurement of mass is
20 gm
30 gm
45 gm
60 gm
If a vector $\vec{v}=3 \hat{i}$ is rotated in the $x-z$ plane by an angle $\theta$ with respect to $x$-axis in the clockwise direction,then for an observer at $+y$ axis the vector will be
$3 \sin \theta \hat{i}$
$3 \cos \theta \hat{i}$
$3 \sin \theta \hat{i}+3 \cos \theta \hat{k}$
$3 \cos \theta \hat{i}+3 \sin \theta \hat{k}$
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