In the case of Ca, Sr, Ba and Ra, the electrons can be excited by the supply of energy to higher energy levels. When the excited electrons return to the original level, the energy is released in the form of light. In Be and Mg, the electrons are tightly held and excitation by mere flame is rather difficult, thus, they do not show flame colouration. Ca imparts brick red colour, Sr imparts crimson colour, Ba imparts apple green colour and Ra imparts crimson colour to the flame.