21. If galvanometer shows null deflection in the given figure then the value of Y is
22. The energy of 25 eV is required to remove one of the electrons from the neutral helium atom. The energy required to remove both the electrons from the helium atom .
23. White light is incident normally on an oil film of thickness 10–4 cm and refractive index 1.4. Which wavelengths will not be seen in the reflected system? .
24. A plano convex lens is silvered at its convex surface. The refractive index and the radius of curvature of the lens are and 30 cm respectively. Then the effective focal length will be .
25. The magnetic flux linked with a coil is given by a equation f (in webers) = 8t2 + 2t + 20. The induced emf in the coil at the end of third second will be .