A car is moving on a circular path of radius 600 m such that the magnitudes of the tangential acceleration and centripetal acceleration are equal. The time taken by the car to complete first quarter of revolution, if it is moving with an initial speed of 54 km/hr is $$t(1-e^{-\pi/2})s$$. The value of t is ____________.

Unpolarised light is incident on the boundary between two dielectric media, whose dielectric constants are 2.8 (medium $$-1$$) and 6.8 (medium $$-2$$), respectively. To satisfy the condition, so that the reflected and refracted rays are perpendicular to each other, the angle of incidence should be $${\tan ^{ - 1}}{\left( {1 + {{10} \over \theta }} \right)^{{1 \over 2}}}$$ the value of $$\theta$$ is __________.

(Given for dielectric media, $$\mu_r=1$$)

For a charged spherical ball, electrostatic potential inside the ball varies with $$r$$ as $$\mathrm{V}=2ar^2+b$$.

Here, $$a$$ and $$b$$ are constant and r is the distance from the center. The volume charge density inside the ball is $$-\lambda a\varepsilon$$. The value of $$\lambda$$ is ____________.

$$\varepsilon$$ = permittivity of the medium

When two resistance $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ connected in series and introduced into the left gap of a meter bridge and a resistance of 10 $$\Omega$$ is introduced into the right gap, a null point is found at 60 cm from left side. When $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ are connected in parallel and introduced into the left gap, a resistance of 3 $$\Omega$$ is introduced into the right gap to get null point at 40 cm from left end. The product of $$\mathrm{R_1}$$ $$\mathrm{R_2}$$ is ____________$$\Omega^2$$