A submarine produces an ultrasonic wave of velocity 1500 $\dfrac{\text{m}}{\text{s}}$ in water. The officer receives signal after 50 s of emission of ultrasonic waves. Find the distance of object which is present in the bottom of sea.

1. 25.7 km
2. 37.5 km
3. 37.5 m
4. 50 km

Assertion A ray of light passing through the centre of curvature of a concave mirror is reflected back along the same path.

Reason The incident ray, the reflected ray and the normal at the point of incidence lie on the same plane.

1. Both Assertion and Reason are true.
2. Both Assertion and Reason are false.
3. Assertion is false but Reason is true.
4. Assertion is true but Reason is false.

The focal length does not depend on

1. refractive index
2. radius of curvature
3. direction of light
4. None of the above

If current through a resistance is doubled and its resistance made one-fourth, then the value of power dissipation will

1. remain same
2. double
3. become 4 times
4. become 1⁄4 time