A body of density'$\rho$'is dropped slowly on the surface of a lake of depth $d$ .If the density of the lake water be'$\rho^{\prime \prime}\left(\rho^{\prime}<\rho\right)$ then the time taken by the body to reach the bottom of the lake is ‘
$\left[\frac{2 d \rho}{g\left(\rho-\rho^{\prime}\right)}\right]^{\frac{1}{2}}$
$\left[\frac{2 g d}{\rho\left(\rho-\rho^{\prime}\right)}\right]^{\frac{1}{2}}$
$\left[\frac{2 d \rho^{\prime}}{\rho g\left(\rho-\rho^{\prime}\right)}\right]^{\frac{1}{2}}$
$\left[\frac{g\left(\rho-\rho^{\prime}\right)}{2 d \rho}\right]^{\frac{1}{2}}$
Beyond what distance,the ray optics is sufficiently valid when the aperture is 6 mm wide and the wavelength is $6000 \mathop {\rm{A}}\limits^{\rm{o}}$ ?
50 m
40 m
60 m
10 m
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
$t \propto r^2 l$
$t \propto r^3 l^2$
$t \propto r^4 l^0$
$t \propto r^2 l^3$
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