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Pupil dilation

Dilation means becoming wider, or opening up. The eye’s pupil dilates when it opens wider to let in more light. This happens in darker or dimly lit conditions.

When the pupil aperture gets bigger is the definition of pupil dilation

She dialled late (dilate) in the evening to see if the takeaway was still open.

Image showing one pupil at its normal light position and one at its dilated position in low light

The pupil on the right is very wide – this is often referred to as dilation of the pupil. On the left, the pupil has reduced in size, which is known as constriction of the pupil.

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