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The eye adjusts to different light levels through the processes of dilation and constriction of the pupil.
The human eye is an incredibly complex organ that has evolved to function optimally in a wide range of light conditions. This is achieved through the dilation and constriction of the pupil, the black circular opening in the centre of the eye. The size of the pupil is controlled by the iris, the coloured part of the eye, which expands or contracts to allow more or less light into the eye.
In bright light, the iris contracts, reducing the size of the pupil. This is known as constriction. By limiting the amount of light that enters the eye, constriction prevents the retina, the light-sensitive tissue at the back of the eye, from being overwhelmed and potentially damaged by too much light.
In contrast, in low light conditions, the iris expands, increasing the size of the pupil in a process known as dilation. Dilation allows as much light as possible to enter the eye, maximising the ability to see in the dark. However, this also makes the eye more sensitive to sudden increases in light, which is why a bright light can seem blinding if you've been in the dark.
The process of adjusting to different light levels is not immediate. It can take several minutes for the eyes to fully adapt from bright light to darkness, a process known as dark adaptation. This is why it can be difficult to see when you first enter a dark room from a brightly lit area. Over time, however, the eyes adjust and you are able to see more clearly.
In addition to pupil dilation and constriction, the eye also adjusts to different light levels through the use of different types of photoreceptor cells in the retina. Rod cells are more sensitive to light and allow us to see in low light conditions, while cone cells function best in bright light and enable us to see colour.
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