Why do halogens get darker?

Hint:Halogens are a group of those elements which have high electronegativity and these are placed on the right side of the periodic table. When we move down the group, the color of the halogens gets darker which is related to the ionization energy of the halogen.

Complete step-by-step answer:Halogens are a group of those elements which have high electronegativity and these are placed on the right side of the periodic table. The halogens are fluorine, chlorine, bromine, and iodine. These are the elements in which there is only one electron less to complete the octet of the element.
The first element is fluorine having atomic number 9, its color is yellow, second is chlorine having atomic number 17, its color is greenish-yellow, third is bromine having atomic number 35, its color is red, fourth is iodine having atomic number 53, its color is violet.
So, we can see that when we move down the group, the color of the halogens gets darker which is related to the ionization energy of the halogen. This color of the compounds is due to the absorption of light from the visible region by the electrons which excites the higher energy, and when it comes back to the ground state it will reflect the complementary color.
The color becomes darker because as we move down the group, the ionization energy decreases and the electron can easily excite to the higher energy level and the atomic radii also increases which will increase the number of shells that will absorb more visible light.

Note:In halogens, fluorine and chlorine are gases but the bromine is liquid while iodine is a solid which means that as we move down the group the boiling point increases steadily.