The pupil of the eye is the circular opening through which light enters. Its dia

Question

The pupil of the eye is the circular opening through which light enters. Its diameter can vary from about
2.0 mm to about 8.0 mm to control the intensity of the light reaching the retina. (a) Calculate the angular
resolution, ?R, of the eye for light that has a wavelength of 550 nm in both bright light and dim light. In which
light can you see more sharply, dim or bright? (b) You probably have noticed that when you squint, objects that
were a bit blurry suddenly become somewhat clearer. In light of your results in part (a), explain why squinting
helps you see an object more clearly.

Explanation / Answer

(a) To figure out the angular resolution of the eye (in radians), the formula is a=(1.22*w)/e where w is the wavelength of light in mm, and e is the exit pupil in mm.

e = (2+8)/2 = 5mm

When you convert the nm to mm, you get a=(1.22*0.00055)/5, which gives you a value for a of 0.0001342 radians. Convert that to seconds of arc, and you get 27.6 (Answer)

(b) Squinting blocks out the peripheral rays that are not in focus and allows the central rays to be in focus on your retina. If you have a refractive error (need for glasses, contacts or laser vision correction) squinting blocks out more of the "out of focus light rays", whereas opening your eyes wider lets in more of "out of focus light rays." The squinting effect is similar to the pinhole effect. Try it, make a small hole (or several holes) in a piece of paper and look through it. The holes should be very small (made with a pin or tip of a pen or sharp pencil). Objects will be in better focus if you have a need for glasses (refractive error) if you squint or look through a pin-hole.

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