Key ideas1 of 3
A filter selects light
A colour filter absorbs some wavelengths and transmits others. In the idealised model, a red filter transmits red light and absorbs other visible colours. It does not change blue photons into red ones or add missing red light. White light through the filter emerges red because the red wavelengths were already present in the incident light. Real filters transmit a band of wavelengths with varying efficiencies.
