The human eye and the telescope


Our eyes have a light gathering aperture of a few millimeters only. The eyes day time opening is 1-2 millimeters across and when the eyes are dark-adapted it expands to about 6 millimeters to allow for more light.

In contrast, the planned James Watt’s space telescope will have an aperture of 6 m. Both the eyes and the telescope have circular openings. A telescope in Kavalur can see stars of the 11th magnitude. a healthy person with normal eyesight can detect stars up to 6th magnitude.


The eye is quite fast: it takes about 10 pictures every second, whereas the just for instance, can linger on an image for several minutes. But the eye’s field of vision is indeed wide viz. 180 degrees, as against a mere by 60th of a degree for the JWST.

Light passes through the cornea in the eye (0.5 mm thick). The iris, a muscle tissue with variable circular opening pupil, provides the effective apertures. It controls the amount of light that goes into pupil, by expanding and contracting. The eye lens is controlled by certain muscles, which change its shape. Flexing of these muscles controls the lens and the focal length, as the distance between the lens and the retina is fixed. The muscles lose power as we grow old. In telescopes, however, the focal length can be varied

The retina has two types of sensitive receptors: cones numbering about 5 million in the area called fovea. They help us sense color from violet to red (400 nm to 700 nm) but they function best under bright light. In contrast to cones, rods numbering about 100 million provide low light vision. The eye is sensitive to a wide range of light levels. You can read this book in brilliant sunlight as well as in fairly bright moonlight, without an artificial light.

Recent research holds that the constant jittery motion of the eye gives it the ability to see even complex sights like the grooves on the rim of a coin . in contrast , a telescope has to be rock steady to make a good image!

In terms of distance, we can see a book in front of us or a distant star. Our ability does not depend on distance but on the size and brightness of the objects. Of course, even when we see some distant object, our naked eyes cannot fully resolve its details. Several innovations such as computers and photonic device come to our rescue. But the eyes have a supercomputer called the brain to make sense of what they resolve!

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