15 eV is given to electron in 4th orbit then find its final energy when it comes out of H-atom.

If half life of an element is $$69.3 \mathrm{~h}$$, then how much of its percent will decay in 10th to 11th $$\mathrm{h}$$. Initial activity $$=50 ~\mu \mathrm{~Ci}$$

If the speed of sound in air is 330 m/s then, find the number of tones present in an open organ pipe of length 1 m whose frequency is $$\le$$ 1000 Hz.

A light of wavelength $$500 \mathrm{~nm}$$ is incident on a Young's double slit. The distance between slit and screen is $$D=1.8 \mathrm{~m}$$ and distance between slits is $$d=0.4 \mathrm{~mm}$$. If screen moves with a speed of $$4 \mathrm{~m} / \mathrm{s}$$, then with what speed first maxima will move?