Pupils most often dilate as a response to changing light. Larger pupils make it easier for more light to be processed by the eye in low-light situations, resulting in a clearer image.
Pupils also grow smaller to protect the inner parts of the eye from bright lights, such as sunlight or bright lamps. If pupils remain dilated, or pupils dilate unevenly, a medical issue may be the reason, notes Healthgrades.com. Abnormal dilation can be due to brain injuries, cranial pressure, poisoning or other head traumas. Additionally, some drugs, such as LSD, eye drops, marijuana and amphetamines, can cause pupils to dilate. Pupils also change size due to mental stimuli, such as intense thinking, interest, arousal and pain. Focusing on things that are farther away or nearer can also cause a change in pupil size.
When pupils dilate, it is because the iris expands or contracts. Unlike the colored iris, the pupil is not actually a membrane, and is instead a hole through which light passes, to hit the retina at the back of the eye. In medicine, the word mydriasis is used for pupils that are dilated. If mydriasis is paired with negative symptoms, such as rapid heart rate, muscle weakness or a severe headache, a health problem may be indicated.