A metal rod cools at the rate of $$4{ }^{\circ} \mathrm{C} / \mathrm{min}$$ whon its temperature is $$90^{\circ} \mathrm{C}$$ and the rate of $$1{ }^{\circ} \mathrm{C} / \mathrm{m}{\text {in }}$$ when its temperature is $$30^{\circ} \mathrm{C}$$. The temperature of the surrounding is

On replacing a thin film of mica of thickness $$12 \times 10^{-5} \mathrm{~cm}$$ in the path of one of the interfering beams in Young's double slit experiment using monochromatic light, the fringe pattern shifts through a distance equal to the width of bright fringe. If $$\lambda=6 \times 10^{-5} \mathrm{~cm}$$, the refractive index of mica is

In the hysteresis curve the value of magnetization (B) which will be present in a substance when value of magnetizing force $$(\mathrm{H})$$ is made zero $$(\mathrm{H}=0)$$ is called as

The output of following combination is same as that of