The number of the following statement/s which is/are incorrect is ______ [JEE Main 2023, 8 Apr (Shift 1)]

Line emission spectra are used to study the electronic structure

The emission spectra of atoms in the gas phase show a continuous spread of wavelength from red to violet.

An absorption spectrum is like the photographic negative of an emission spectrum

The element helium was discovered in the sun by the spectroscopic method