Why do we see colours when we close our eyes and press our hands against them?

Why do we see colours when we close our eyes and press our hands against them?

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Whenever we close our eyes and press our hands against them we either see some colours or sometimes some weird patterns(similar to hypnotic patterns).

What is the cause of this phenomenon?

Also, is this the right place to ask this question? Does this question have more to do with biology than physics?

Yes, this is a biological phenomenon, not a physical one, because the phenomenon depends on a cellular response. Pressing your eyeballs (mechanically stimulating them) causes you to see phosphenes.

What is the cause of this phenomenon?

The increased pressure in the eye causes activation and inhibition of cells in your retina (retinal ganglion cells). These cells are intermediates between the light-sensitive sensory neurons and the brain which interprets information into an image.

Quoting a well known study from 1989:

Eyeball deformation [pressing] in total darkness led to an activation of the on-center ganglion cells, while the off-center ganglion cells were inhibited. The latency and strength of this activation or inhibition varied considerably between different neurons, but were fairly constant in the same neuron when the eyeball indentation was repeated after a pause of 1-3 min. The latency and strength of neuronal activation or inhibition seemed to be dependent mainly upon the neuron location relative to the point of eyeball indentation. Some on-center neurons also exhibited a short activation at "deformation off". [… ] We assume that the activation of on-center and inhibition of off-center ganglion cells by eyeball deformation are caused by retinal stretching, which also leads to horizontal cell stretch. Stretching the horizontal cell membrane probably generates an increase in membrane sodium conductivity and a depolarization of the membrane potential [neuron firing]. This depolarization of the horizontal cell membrane potential is transmitted either directly or indirectly (via receptor synapses) from the horizontal to the bipolar cells.

This is a phenomenon at the retina (periphery), i.e. not in the brain region associated with vision processing (occipital lobe). You can achieve the same visual effect by vigorously sneezing or blowing your nose, stimulating the same cells electrically (by injecting current with electrodes) or magnetically (with coils that generates a local magnetic flux, whereby currents are locally induced). It is also possible to duplicate the effect by stimulating only the visual cortex, but it is likely due to the fact you are simply approximately replicating the retinal firing patterns coming to the brain via the visual nerve.

The first successful clinical test of optogenetics lets a person see for the first time in decades, with help from image-enhancing goggles

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First, prepare students for lab activities by giving background information according to your teaching practices (e.g., lecture, discussion, handouts, models). Because students have no way of discovering sensory receptors or nerve pathways for themselves, they need some basic anatomical and physiological information. Teachers may choose the degree of detail and the methods of presenting color vision, based on grade level and time available.

Offer students the chance to create their own experiments

While students do need direction and practice to become good laboratory scientists, they also need to learn how to ask and investigate questions that they generate themselves. Science classrooms that offer only guided activities with a single "right" answer do not help students learn to formulate questions, think critically, and solve problems. Because students are naturally curious, incorporating student investigations into the classroom is a logical step after they have some experience with a system.

The "Try Your Own Experiment" section of this unit (see the accompanying Teacher and Student Guides) offers students an opportunity to direct some of their own learning after a control system has been established in the "Class Experiment." Because students are personally vested in this type of experience, they tend to remember both the science processes and concepts from these laboratories.

Use "Explore Time" before experimenting

To encourage student participation in planning and conducting experiments, first provide Explore Time or Brainstorming Time. Because of their curiosity, students usually "play" with lab materials first even in a more traditional lab, so taking advantage of this natural behavior is usually successful. Explore Time can occur either before the Class Experiment or before the "Try Your Own Experiment" activity, depending on the nature of the concepts under study.

Explore before the Class Experiment

To use Explore Time before the Class Experiment, set the lab supplies out on a bench before giving instructions for the experiment. Ask the students how these materials, along with the information they have from the lecture and discussion, could be used to investigate color vision. Give some basic safety precautions, then offer about 10 minutes for investigating the materials. Circulate among students to answer questions and encourage questions. After students gain an interest in the materials and subject, lead the class into the Class Experiment with the Teacher Demonstration and help them to formulate the Lab Question. Wait until this point to hand out the Student Guide and worksheets, so students have a chance to think creatively. (See the accompanying Guides.)

How is the new coronavirus related to your eyes?

Our eyes may play a role in the spread and prevention of the COVID-19 coronavirus pandemic, but the relationship between the coronavirus and your eyes is a complicated one.

Coronavirus mainly spreads through the airborne respiratory droplets that leave your mouth when you cough, sneeze, laugh or even talk. The droplets are inhaled through another person’s nose or mouth and can lead to an infection.

This is how the flu virus usually spreads, too.

Droplets can also enter through your eyes in one of two ways:

Droplets land directly on your eyes.

You touch your eyes without washing your hands, spreading the virus from your hand to the area near or on either eye.

The fluid-lined surfaces of your eyes, nose and mouth are known as mucous membranes and offer an easy pathway for viruses to enter your body.

This is why the CDC and the World Health Organization (WHO) recommend regularly and thoroughly washing your hands with warm water and soap.

The Daring Racism Experiment That People Still Talk About 20 Years Later (VIDEO)

More than 20 years ago, "The Oprah Winfrey Show" conducted an experiment about racial prejudice that audiences will never forget. The year was 1992 -- in the wake of the deadly Los Angeles riots that erupted after the acquittal of police officers on trial for the beating of Rodney King -- and racial tensions in the country were running high. Yet, the "Oprah Show" audience members didn't suspect a thing when they arrived at the studio and were immediately separated into two distinct groups.

The division wasn't based on skin color, but eye color. "What we did was treat each group differently, discriminating against the people who have blue eyes, catering to those people with brown eyes," Oprah explained back then.

As the audience lined up to enter the studio, the blue-eyed people were pulled out of line, told to put on a green collar and wait outside. The brown-eyed people were told to step to the front of the line. Once indoors, the brown-eyed group was then treated to coffee and doughnuts, while the blue-eyed group could only stand around and wait. When the blue-eyed group saw that the brown-eyed group was going to be seated first, some became upset.

"Look at those people! What are they doing in there?" one woman shrieked.

When the show began, Oprah welcomed diversity expert Jane Elliott to the stage. Elliott helped set up the experiment, and she knowingly added fuel to the fire when she spoke. "I've been a teacher for 25 years in the public, private and parochial schools in this country, and I have seen what brown-eyed people have done as compared to what blue-eyed people do. It's perfectly obvious," she said. "You should have been here this morning when we brought these people in here."

Feeling discriminated against, the blue-eyed audience members stood to voice their frustrations.

"She was rude to us! All of us!" one woman said. "Yelled at us, called us names, pushed us aside. She was rude!"

"Why doesn't Jane have a green collar on? She's got blue eyes," another pointed out.

Elliott didn't hesitate in her answer. "Because I've learned to act brown-eyed," she said. "And the message in this room is, act brown-eyed and you, too, can take off your collar."

The blue-eyed people were flabbergasted, but it wasn't long before the brown-eyed people bought into the idea that they were superior. "People, I had a girlfriend in school who was blue-eyed. She was so stupid, she was always copying off of my papers," said one brown-eyed woman. "These [blue-eyed] people were so rude and so noisy today, we couldn't hear ourselves even talk!"

Eventually, the audience figured out that the show was really about race. "God created one race: the human race," Elliott told them. "Human beings created racism."

Twenty-two years after that memorable episode, "Oprah: Where Are They Now?" caught up with Elliott, who still gets emotional when talking about the catalyst that led her to create the blue-eyed-brown-eyed experiment in 1968.

"Martin Luther King, Jr. had been one of our 'heroes of the month' in February in my third-grade classroom, and he was dead at the hands of an assassin," Elliott says, getting choked up. "I hate to talk about this because every time I talk about it, I remember how it felt that day. I was going to have to go into my classroom and explain to my students why the adults in this country had allowed somebody to kill hope. Martin Luther King, for me, was hope for this country."

In an effort to get her small-town, all-white class to experience what it was like to walk in someone else's shoes, she created the eye-color experiment. "I decided the next day that I was going to do what Hitler did. I was going to pick out a group of people on the basis of a physical characteristic over which they had no control, separate them. treat one group badly and treat the other group very well, and see what would happen," Elliott says.

Why eye color? "Eye color and skin color are caused by the same chemical: melanin," Elliott explains. "There's no logic in judging people by the amount of a chemical in their skin. Pigmentation should have nothing to do with how you treat another person, but unfortunately, it does."

What she found with the experiment is how incredible its impact can be.

"Give me a child at the age of 8 and let me do that exercise, and that child is changed forever," Elliott says.

Throughout January, OWN hosts a month-long celebration honoring civil rights legends, as we approach the 50th anniversary of the historic Selma to Montgomery marches led by Dr. Martin Luther King, Jr.


You can watch the pupil of your eye change size in response to changes in lighting. You can also experiment to determine how light shining in one eye affects the size of the pupil in your other eye.

Tools and Materials

  1. Magnifying glass that is at least 1 inch (2.5 cm) in diameter
  2. Any size handheld or wall mirror (note that plastic mirrors are safer than glass)
  3. Flashlight


To Do and Notice

Place the magnifying glass on the surface of the mirror. Look into the center of the magnifying glass with one eye. If you wear contact lenses or glasses, you may either leave them on or remove them.

Adjust your distance from the mirror until you see a sharply focused and enlarged image of your eye. You may need to adjust the position of the magnifier to get the clearest image of your eye. Notice the white of your eye, the colored disk of your iris, and your pupil, the black hole in the center of your iris.

Shine a light into the pupil of one eye. If you are using a small mirror, hold the flashlight behind the mirror and shine the light around the edge of the mirror into your eye. If you are using a large mirror, bounce the flashlight beam off the mirror into your eye. Observe how your pupil changes size.

Notice that it takes longer for your pupil to dilate than it does to contract. Notice also that the pupil sometimes overshoots its mark. You can see it shrink down too far, and then reopen slightly.

Observe changes in the size of one pupil while you, or a partner, shine a light into and away from the other eye.

In a dimly lit room, open and close one eye while observing the pupil of the other eye in the mirror.

What's Going On?

The pupil is an opening that lets light into your eye. Since most of the light entering your eye does not escape, your pupil appears black. In dim light, your pupil expands to allow more light to enter your eye. In bright light, it contracts. Your pupil can range in diameter from 1/16 inch (1.5 mm) to more than 1/3 inch (8 mm).

Light detected by the retina of your eye is converted to nerve impulses that travel down the optic nerve. Some of these nerve impulses go from the optic nerve to the muscles that control the size of the pupil. More light creates more impulses, causing the muscles to close the pupil. Part of the optic nerve from one eye crosses over and couples to the muscles that control the pupil size of the other eye. That’s why the pupil of one eye can change when you shine the light into your other eye.

In this experiment, the light reflecting from your eye passes through the magnifying lens twice—once on its way to the mirror and once on its way back. Therefore, the image of your eye is magnified twice by the magnifying glass.

Going Further

The size of your pupils actually reflects the state of your body and mind. Pupil size can change because you are fearful, angry, in pain, in love, or under the influence of drugs. Not only does the pupil react to emotional stimuli, it is itself an emotional stimulus. The size of a person’s pupils can give another person a strong impression of sympathy or hostility.

The response of the pupil is an involuntary reflex. Like the knee-jerk reflex, the pupillary response is used to test the functions of people who might be ill or injured. You may have seen a doctor shine light into the eyes of a person with a suspected head injury—they are looking at the pupillary response.

The pupil of your eye is also the source of the red eyes you sometimes see in flash photographs. When the bright light of a camera flash shines directly through the pupil, it can reflect off the choroid, which supplies red blood to the retina (the light-sensitive lining at the back of your eye), and bounce right back out through the pupil. If this happens, the person in the photograph will appear to have glowing red eyes. To avoid this, photographers move the flash away from the camera lens. With this arrangement, the light from the flash goes through the pupil at an angle, illuminating a part of the retina not captured by the camera lens. Many cameras are equipped with red-eye reduction features, such as a pre-flash that causes pupil constriction before the actual flash that illuminates the photo.

This Science Snack is part of a collection that highlights Black artists, scientists, inventors, and thinkers whose work aids or expands our understanding of the phenomena explored in the Snack.

Dr. Patricia Bath (1942-2019), pictured above, was an ophthalmologist and laser scientist, and was the first woman chair of ophthalmology at a US university. She studied the causes of and cures for blindness, and invented a widely used method of using laser surgery to treat blindness caused by cataracts. Dr. Bath also co-founded the American Institute for the Prevention of Blindness. This Science Snack can help you investigate the structures in the eye that help you see, so you can understand the eye like Dr. Bath did.

How to recognize your own biases

Another way to lift the curtain on your hidden prejudices is to honestly examine all parts of your life. If, for no conscious reason, you have no friends of a different race, sexual orientation, or religion than yours if your office hasn&rsquot hired an employee who is disabled, queer, or speaks English with an accent or if you&rsquore a teacher or a landlord and your favorite students or tenants are people like yourself, it&rsquos worth questioning whether implicit biases are at least partly to blame.

Unfortunately, becoming aware of biases is not enough to dislodge them. Biased thoughts are &ldquosticky&rdquo because they get reinforced by the world around us, which is fueled by the dominant culture. The conceptualizing of scientists as white men comes from the reality that in the course of history most have been&mdashthough that&rsquos starting to change, Pietri says.

Biased thoughts are &ldquosticky&rdquo because they get reinforced by the world around us.

Himmelstein notes that fat phobia results from society&rsquos widespread (albeit mistaken) notion that body weight is under a person&rsquos complete control and that we all wish to be thin, so anyone who can&rsquot drop pounds must lack willpower. (Actually, many factors, including biology and environment, are involved in BMI, itself a biased measure of health.) Stereotypical images and biased representations exist in advertising, TV shows, movies, song lyrics, what we hear friends and family members say and share on social media, and in many other realms.

When societal changes do eventually take hold, however, they can affect our unconscious thoughts rather powerfully. Take sexual orientation: As recently as in 1994, nearly half of Americans said that people who were gay or lesbian shouldn&rsquot be accepted by society, a figure that plummeted to 21% in 2019.

This change has been mirrored by a drop in implicit bias toward LGBTQ people in the past decade. And with more Black actors being cast in roles beyond those of criminals and support staff, our perceptions about race continue to shift, says Lai, since media exposure to Black people in positive roles (such as those of athletes and politicians) has been shown to reduce anti-Black bias. Still, with limited positive imagery about age and disability, related prejudices have been harder to shake prejudice against people of above-average weight has increased.

News About Human Body

The appendix, a safe house for good bacteria, is not a useless evolutionary leftover, but shows God’s careful design in humans.

A recent article highlights the supposed “flaws” in the design of the human body. But are these really “flaws”?

Rod and Cone Density on Retina

Cones are concentrated in the fovea centralis. Rods are absent there but dense elsewhere.

Measured density curves for the rods and cones on the retina show an enormous density of cones in the fovea centralis. To them is attributed both color vision and the highest visual acuity. Visual examination of small detail involves focusing light from that detail onto the fovea centralis. On the other hand, the rods are absent from the fovea. At a few degrees away from it their density rises to a high value and spreads over a large area of the retina. These rods are responsible for night vision, our most sensitive motion detection, and our peripheral vision.

The above illustration does make it appear that there are no cones outside the fovea centralis, but that is not true. The blue cones in particular do extend out beyond the fovea.

Cone details Rod details

Red Sky at Night: The Science of Sunsets

A meteorologist explains why the sky is sometimes so colorful.

On a recent autumn night here in Washington, D.C., the sun seemed to personify a Dylan Thomas poem: Do not go gentle into that good night . Rage, rage against the dying of the light. The scarlet skies inspired many viewers to grab their cameras, and prompted a question: Why are some sunsets so spectacular, and others a mere muddle?

We asked Stephen Corfidi, a National Oceanic and Atmospheric Administration (NOAA) meteorologist who's written about the science of colorful sunsets, to help us see the light.

In simple terms, what makes a good sunset happen?

I guess it depends on how you define "good," but I'm going to assume you mean a strikingly colorful one, where the colors are spectrally pure—say, vivid orange or red—as opposed to a more muted palette.

Keep in mind that what we see with our human eyes is just a tiny part of the electromagnetic radiation that's given off by the sun. That radiation contains a wide spectrum of wavelengths, but your eyes are only sensitive to certain parts of it: the so-called visible wavelengths. Different colors are associated with different wavelengths.

And depending on what happened to the light before it got to you, some of those visible wavelengths don't even reach your eye. Portions of it are absorbed and filtered out in the atmosphere.

So really, there's a good sunset every night we just can't always see it from the ground. You may have noticed this if you've ever taken off in an airplane at sunset. It might not look like anything special from the ground, just a whitish-pink sky, because you're still within the atmosphere's "boundary layer." That's where all the large particles are trapped, things like dust and pollution. But as the plane gets above the boundary layer, into cleaner air, suddenly the sunset looks very vivid. It's all a matter of perspective.

Okay, so let's talk about the typical Earthling's perspective. Why do we see more orange and red colors in the sky during sunrise and sunset than we do at other times of day?

When a beam of sunlight strikes a molecule in the atmosphere, what's called "scattering" occurs, sending some of the light's wavelengths off in different directions. This happens millions of times before that beam gets to your eyeball at sunset.

The two main molecules in air, oxygen and nitrogen, are very small compared to the wavelengths of the incoming sunlight—about a thousand times smaller. That means that they preferentially scatter the shortest wavelengths, which are the blues and purples. Basically, that's why the daytime sky is blue. The daytime sky would actually look purple to humans were it not for the fact that the sensitivity of our eyes peaks in the middle [green] part of the spectrum—that is, closer to blue than to purple.

But at sunset, the light takes a much longer path through the atmosphere to your eye than it did at noon, when the sun was right overhead. And that is enough to make a big difference as far as our human eyes are concerned. It means that much of the blue has scattered out long before the light reaches us. The blues could be somewhere over the West Coast, leaving a disproportionate amount of oranges and reds as that beam of light hits the East Coast.

So the same ray of sunlight is hitting people in both the Rockies and the Appalachians? Basically, the East gets the West's leftovers at sunset?

Yes, I think a lot of people don't realize that. Everything is connected. And as humans, we like to think color is concrete: "Oh, that's a blue sky," or "That's a brown table." But the colors you see depend on the light's path before it got to you, how the object you are viewing reflects that light, and what your eyes are sensitive to. Absolutes don't really exist in color perception. It's rather disquieting when really you start thinking about it!

Do dust and air pollution make sunsets more dramatic?

No, you often hear that, but—assuming you mean typical pollution in the lower atmosphere—it's a myth. It's actually the opposite: Large particles in the lower atmosphere tend to mute and muddy the colors because they absorb more light and scatter all the wavelengths more or less equally, so you don't get that dramatic filtering effect. In areas with a lot of haze, you don't typically see the types of sunsets that are likely to appear on a wall calendar—or in, say, National Geographic.

Do the seasons affect sunsets?

You see bright ones in the fall and winter particularly, especially in the East, because the air along the path of the ray of sunlight tends to be dryer and cleaner.

I grew up in Baltimore, and this is part of why I got interested in weather. I would wonder: Why is the sunset so pretty tonight? And there weren't answers to questions like this in standard weather books, because it's more about physics than forecasting.

Speaking of forecasting, what about the saying: "Red sky at night, sailor's delight red sky in morning, sailors take warning." Any scientific truth to that?

Absolutely. Those spectrally pure colors are telling you there's a sizable swath of clear air off to your west that's likely to be over you the next day.

So conversely, could local weather forecasters predict a pretty sunset?

Yeah, you can forecast them to a certain degree. I guess it's a question of who cares—maybe filmmakers or photographers would find that information useful, but most people just want to know if it's going to rain or not.

Why are sunsets sometimes more dramatic after a major storm?

There's often a slanting band of clouds on the back side of the departing weather system, and that can act as a sort of projection screen for the low-sun colors, better than a horizontal band would. The slant means it captures more of the orange and red light, and if the cloud is thin enough, it will reflect those colors down to you. Also, storms wash a lot of the big particles out of the air.

Is it true that by the time we see a sunset, the sun is actually already gone?

Yes, true sunset occurs a minute or so before you see the sun disappear. What you see is a kind of mirage the light is getting bent around the horizon by the effect of refraction.

Sounds like there's a lot of science to sunsets, but it's also a very subjective experience.

Yes. Our eyes are sensitive to a very tiny part of the spectrum of the sun's wavelengths, and that's responsible for the way we see our environment. Other creatures seem able to see the ultraviolet area of the spectrum. We can only see a tiny part of what's going on.

So a butterfly or a reindeer, which can perceive ultraviolet light, might be seeing a different, perhaps more colorful sunset than we do?

Absolutely. The more you look at things, the more you realize how unique your own experience is as a human on this planet, at this particular place and time.