Educator How-To: The eyes have it in this DIY optical illusion

Your eyes are amazing sensory organs. They help you understand shape, color and form, judge distance and alert you to potential dangers. What you perceive as “seeing” is actually the result of a complex series of events that occur between your brain, your eyes and the world around you.

Light reflected from an object passes through the cornea of the eye and moves through the lens, which focuses it. The light then reaches the retina at the very back of the eye, where it meets a thin layer of color-sensitive cells called the rods and cones. Information from the retina travels from the eye to the brain via the optic nerve.

Because eyes see from slightly different positions, the brain must mix the two images it receives to get a complete picture. The light also crisscrosses while going through the cornea so the retina “sees” the image upside down. The brain then “reads” the image and turns it right-side up.

The rods and cones are what you call photoreceptors. When they are overworked, they lose sensitivity. Normally the small movements of your eyes that you make unconsciously, or regular blinking, will keep these photoreceptors sharp and happy. If you are looking at a large enough image, where your eyes can’t rest, or if you purposely hold your eyes still, you will tire out your poor rods and cones and they will adapt to this overstimulation by no longer responding. When you move your eyes to a blank space, your worn out photoreceptors create an “afterimage”.  An afterimage is where your eyes produce a ghost image, like when you stare at something a little too bright and you see dark spots in your field of vision. In an afterimage, light portions of the original image are replaced by dark portions and dark portions are replaced by light portions.

Try this out for yourself by doing the following activity. 

You will create the Texas state flag in some unusual colors. After you stare at this incorrectly colored flag and have worn out your photoreceptors, looking at a blank wall will create a ghost image of the Texas state flag in red, white and blue!

Activity:  Negative Afterimage

Materials:
Scissors
Glue
Paper
Green construction paper
Black construction paper
Yellow construction paper 

Ed How To Optical Flag 1

Procedure:

1. Cut your green and yellow papers in thirds, width-wise.

Ed How To Optical Flag 2

2. Cut a star out of the middle of your yellow piece.

Ed How To Optical Flag 3

3. Glue the yellow piece to one end of the black piece.
4. Turn the black paper so that your yellow piece is placed on the left.

Ed How To Optical Flag 4

5. Glue the green piece to the bottom of the black piece.
6. Trim off any extra green.

Ed How To Optical Flag 5

Now stare at the flag for a minute or so. Try not to have much in your peripheral vision so that you can concentrate on the flag.

Look away from the flag at a neutral colored wall or piece of paper.  You should be able to see the flag in red, white and blue!

Ed How To Optical Flag 6

Have a school group and want to know more about how your eyes work?  Sign up for an Eyeball Dissection with our Labs on Demand.  These labs make a great addition to a field trip, but are also available to come to your school.

Interested in knowing more about how your body works?  Visit Body Carnival, a carnival-themed interactive exhibit that explores the connections between perception and the laws of physics in the human body, at HMNS Sugar Land. Enjoy learning about the human body while investigating force, pressure, light, and color. Crawl through a giant artery to see and hear the effects of restricted blood flow, test your balance in the 10-foot Dizzy Tunnel or don a pair of vision-distorting goggles and discover how sight affects your ability to walk straight. There’s a lot to explore!

 

Save Our ‘Scope: A Gueymard fundraising update & why telescope mirrors matter

If you didn’t already know, we’re in the process of raising money to repair the Gueymard telescope at the George Observatory in the Save Our ‘Scope campaign. And thanks to you, we’ve raised over $65,000 of our $80,000 goal through donations ranging from $1 to $5,000 — and we’re grateful for every last one of ‘em.

But what’s the big deal? I mean, you can totally just buy a new mirror at Target or something, right? Well, in a word: nope.

Telescopes are amazing pieces of equipment – dauntingly large (the Gueymard weighs 10 tons) and yet incredibly delicate. If the mirror in a telescope warps just a few centimeters, it can vastly distort the images you see. Therefore, it’s extremely important to keep these machines in tip top condition.

Eye see you

Think of telescopes as an extension of your eyes. We can’t see things far away very well. When you look at the sky at night (and it’s dark enough) you see tons of tiny specks of light – some are stars, some are planets, others are galaxies, or even galaxy clusters. The light can travel for thousands (or millions or billions) of light years. The light heads right into your eye, onto your retina, and sends a message to the brain that says, “Hey, that’s neat, a speck of light.”

However, due to the distance involved, we can no longer appreciate the scale or detail of the images. The further away an object is, the smaller the space it takes up on the retina.

Telescopes fix all of this so that a bright, detailed image can reach your eye as it captures more light and then focuses and magnifies it.

Light-bending lenses

Lenses bend light waves, either causing them to converge (focusing light) or diverge (spreading out light). Glass lenses were used in the creation of the first telescopes, called refracting telescopes.

In this model, light passes through the objective lens, which collects the light, causing it to converge on the eyepiece where it is then magnified. These images would become distorted, however, as different wavelengths of light bend at different angles and focus at different points.

Mirror, mirror on the wall

So, how do you solve a problem like bent light? In a word: mirrors. With reflecting and compound telescopes, the light doesn’t pass through the objective lens. Instead, it is reflected (via concave mirror) back to a smaller mirror, directing the light to the eyepiece where it is magnified. Ta-da! Beautiful, clear images of faraway objects.

The cool thing here is that the larger your mirror, the more light you capture, giving you higher resolution images with better detail.

The bigger, the better?

If your goal is to capture as much visible light as possible, then yes, bigger is better. But this comes with special problems too. Mirrors can get heavy – very heavy. Think about it: our Gueymard telescope mirror is 36 inches in diameter, and some mirrors can be several hundred inches in diameter. Now that’s a lot of mirror! All of this weight can change the shape of the mirror so that, over time, they sag and no longer properly focus light to another point.

Oh, honey

This issue can be solved with honeycomb mirrors. No, they’re not made by bees. Rather, they’ve been influenced by the structure of a honeycomb. This allows the face of the mirror to be well supported, while reducing the weight of the mirror up to 80 percent.

Now that you’re (sorta) a telescope expert, come see the big stuff at the George Observatory in Brazos Bend State Park! Houston’s really lucky to have this observatory in its backyard, so to speak, since the Gueymard is the largest telescope in the country open to public viewings.

Can you spare a George for the George? We’d greatly appreciate if you pitched in to help save our ‘scope. Your efforts ensure that Houstonians can continue to stargaze through the most incredible telescope they’ll ever get to use for many years to come.

I can’t find my reading glasses, but I’ll manage.

You’ve probably noticed the magnifying effect of a glass of water or any other clear beverage (the black text to the right of the glass is the same size as the black text behind the glass):

And you probably have some idea that the magnification has to do with the curved shape of the glass and the water it contains: The water in the glass bends light so it appears to us to be coming from an object that is bigger or closer than it really is.

To explore this more, try making differently sized water drops on top of a sheet of waxed paper (the waxed paper helps the water ‘bead up,’ which improves the effect):

You’re aiming for a large drop about 2 centimeters or 1 inch across, and medium and small drops that are, well, smaller.  If you don’t have an eyedropper to help you, you can either pour extremely carefully or dip a pencil or spoon in water and let the water drip off of it.

Look at a page with words through the drops (don’t use your first editions of The Old Man and the Sea or Einstein’s General Theory of Relativity, because the water will eventually seep through the waxed paper and make you very, very sad).  Do you see any differences between the larger and smaller drops?

This looks much clearer if you try it yourself, so go do it! 

You may be thinking “My large drops (possibly puddles) don’t seem to change anything; why do the small drops work so much better?”  To explain this, try looking at your drops from the side (your eyes should be level with the surface of your table:

The shapes are different: The largest drop looks almost flat across the top, while the smallest drop makes a very tidy little dome shape.  Another way to say this is that the smallest drop’s surface is more sharply curved, or is more convex than the larger drops (convex surfaces bulge out, concave surfaces “cave in.” And it turns out that the less convex the surface of the drop, the less it magnifies.  If you want a more in depth explanation with diagrams, check out this site.

Convex and concave lenses are used in all kinds of cool equipment. For more information on lenses and the anatomy of your eyeballs, check out The Anatomy of the Eye.