First, let’s take a look at how our eyes work. The eye is basically a ball filled with a jelly-like substance called the vitreous humor. At the front of your eye (the part you can see when you look in the mirror) is a thin, rounded dome of tissue called the cornea; this is like a window that lets light pass through the eye. Once past the cornea, the light passes through the pupil (the black part of your eye), an opening surrounded by the iris (the colored part of your eye). Inside the pupil is the lens, which focuses the entering light on the tissues at the back of your eye which are called the retina. The retina turns the light into an electrical signal, which is transmitted by the optic nerve to the brain; the brain then interprets these electrical signals, allowing us to see all the things around us.
TEST YOUR EYESIGHT!
NOTE: While this is an easy and usually accurate way to test your eyesight, it is NOT a substitute for getting your eyes checked by a professional! Only an eye doctor can provide you with the correct prescription for glasses, or diagnose if you have any other problems with your eyes.
Here’s what you’ll need:
1. Masking tape
2. A tape measure at least 20 feet long
3. An accurate Snellen Chart, which you can download and print by clicking here, or go to http://www.i-see.org/block_letter_eye_chart.pdf.
4. A friend to help you
Here’s what to do:
1. Download and print the Snellen Chart from the link above. Be sure to disable the printer’s “Fit to Page” option. The first page contains instructions, the last page we’ll get to later!
2. Check t o be sure the chart has printed correctly—the letter ‘E’ at the top of the chart should be 3.49 inches (88.7 millimeters) high.
3. Tape the chart on a wall at eye level, with the largest letter at the top.
4. Measure 20 feet from the wall using your tape measure, and mark that spot with masking tape.
5. First cover your left eye, and read the numbers on the chart aloud, starting at the top and reading every line until the letters are too small for you to read. Have your friend keep track of which line you could not read.
6. Now cover your right eye, and repeat step 5 with your left eye.
7. When you have finished, ask your friend which lines were the smallest you could read.
8. To calculate your visual acuity for each eye, take the number to the left of the smallest print you could read for that eye, and put it under the number 20. For instance, if the smallest line you could read was the seventh line from the top letter, your vision in that eye is 20/30—you can see at 30 feet what a person with normal vision can see at 20 feet.
What is your visual acuity? Do you wear glasses? Do you think you should? If you think you may need glasses, visit an eye doctor to have your eyes professionally checked!
MORE THINGS TO TRY!
The final page of the eye chart is called a Near Vision Test Card, and it tests how good your vision is close up. Tape this page to a wall and stand 16 inches away, and see how many rows of letters you can read!
If you do need glasses, and you are nearsighted, here’s a really cool trick to try!
Take a piece of paper (any piece large enough to cover your eye will do), and poke a small hole in it with a pen.
References for further reading:
1. H. Snellen, Probebuchstaben zur Bestimmung der Sehschärfe, Utrecht 1862.
2. "What Does 20/20 Vision Mean?" Eye Care Associates of East Texas. http://www.eyecaretyler.com/2020.htm. Accessed 28 May 2014.