A hydrogen atom makes a transition from $$n=5$$ to $$n=1$$ orbit. The wavelength of photon emitted is $$\lambda$$. The wavelength of photon emitted when it makes a transition from $$n=5$$ to $$n=2$$ orbit is

The curve of binding energy per nucleon as a function of atomic mass number has a sharp peak for helium nucleus. This implies that helium nucleus is