Question

Why do lanthonoids from coloured compounds?

Answer 1

HEYAA..!
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Because of so-called f-f transitions.

Answer 2

At the time of D- d transition process and the electron absorb certain energy from the radiation and also emit the remainder of energy as colored light with no trouble of it.

Then the can know which when electrons jump from the one orbital to another light is emitted due to the compound of transition element to colored compounds in a fine manner.