Arghh me maties. Me name’s Captain Houkk. I have this here patch on me eye due to my darned arrrstigmatism that I ain’t be correcting soon argh. Well lads, some people are told that they have astigmatism, but what does it be exactly? After this lab you should have a good idea about the optics behind it.

First, we have to do the “Arrrrstigmatism Test."

Step 1: Astigmatism Test + Materials

Cover one eye like a PIRRRATE (like me) and look at the chart which lies below. Do any of the lines look darker than the others? If you say “AYE,” rotate the chart 90 degrees to convince yourself that the lines are the same -- your eye is what causes the effect.

You see, the lens ‘n your eye is similar to the lens in me telescope. When light hits the cornea, or the lens of your eye, it is focused onto one single point in the retina, where the image is detected and communicated to the brain. However, in people with astigmatism, the light is focused by the lens onto more than one point in the retina, causing t’ blurry effect. Now, let’s try it out with an eye model. I gots it when I searched for treasures in the Caribbean. Just kiddin’.

You will need the following materials:
PASCO Human Eye Model OS-8477
+62 mm lens
-128 mm lens
+307 mm lens

Step 2:

Adjust ye eye model to normal by putting the +62 mm lens in the SEPTUM slot, removing other lenses, and putting the retina screen in the NORMAL position. Adjust the eye-source distance so that the image is in focus.

Step 3:

Now place yer -128 mm cylindrical lens in slot A. The side of t’ lens handle marked with the focal length should be directed towards the light source. This eye has an astigmatism matie.

Step 4:

Turn the cylindrical lens about. What happens to yer image? Aye, matie. This shows that astigmatism can have different directions depending on which direction the defect in the eye’s lens system is oriented.

Step 5:

You will now correct the astigmatism with eyeglasses, which I, the captain, cannot afford due to me horrendous crew. Place the +307 mm cylindrical lens in slot 1. The side of the lens handle marked with the focal length should be towards the light source.

Step 6: Optional

If someone in your lab group be wearin’ eyeglass, try borrowin’ those and holden’ ‘em in front of t’ eye model. What vision problems do you have cause give the eye model so that the eyeglasses actually help de’ darn vision?