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Sensitivity to light is common in the animal word. Even the one-celled euglena have eyespots that enable them to sense light. Only animals with eyes can sense differences in the light coming from different objects in their environment (contrast). Humans sense the intensity of light, but they are much more sensitive to contrast. What this means is that when intensities change slowly, as at sunset, we can tell that the intensity of light is changing, but we have a much better sense of differences in brightness between one object and another (contrast perception). Sensitivity to contrast makes it possible for us to see in detail what is around us and to do so over a huge range of intensities. This ability depends critically on the ...

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