Study Guide

Optics Themes

  • Anatomy and Health

    Photoreceptors: How We See Roy.G.Biv

    We learned about the 7 flavors of visible light in the electromagnetic spectrum since preschool days: red, orange, yellow, green, blue, indigo, and violet (They Might Be Giants wrote a song just for remembering the order). Visible light is actually a continuum of colors, though. So how do we see all of these colors? Is there a cell in our eye for every possible color? And what happens in our eye and brain that turns a certain wavelength of light hitting our eye, into a color?

    We see light using photoreceptors, special cells in the retina of our eye. Light, as we learned in this chapter, is an electromagnetic wave. When light enters our eye, photoreceptors translate this electromagnetic wave into a signal that our brain can process and turn into the sensation of color.

    There are two types of photoreceptors, cones and rods. Rods are very sensitive to light – as few as 6 photons can activate a rod cells to send a signal to our brain that says LIGHT. But rods don't see color, they just say, "Yes, light" or "No, light." Because they are sensitive to even just a tiny bit of light, rods are very important for seeing in the dark. This is why things seem to lose their color when it is dark. After dusk, we see mostly with our rod cells and since they don't see color, colors seem to fade.

    The type of photoreceptors that allow us to see color are called cones. We don't have a cone for each of the hundreds of colors we see. Instead, they come in just three flavors that essentially correspond to the colors red, green, and blue. The rainbow of colors we see are generated by our brain mixing the colors red, green, and blue.

    Each of these three cone receptors has a certain wavelength of light that activates it. We call the "red cone" red because the light that cone absorbs best is the wavelength of red light–564 nm. The "red cone" also gets activated by light with wavelength that are a little bit longer or a little bit shorter. The same for blue, except the blue cones absorb light with shorter wavelength, around 420 nm. The green cones are in between. Green cones are activated by light with a 534 nm wavelength.

    How do we see turquoise which has a wavelength of about 500 nm?

    What happens is that 500 nm is between 420 nm (blue) and 534 nm (green), so 500 nm light activates both the blue and the green receptor and the green receptor a little more than the blue because 500 nm is closer to green than to blue. The red cones don't get activated at all. Our brain reads this particular combination of green and blue as turquoise.

    All of the colors we can see are just three types of cone photoreceptors signaling by different degrees. TVs and computer screens take advantage of this. The color pictures generated by televisions are made from just three colors, red, green, and blue. To make a rainbow of colors TV folk just mix different numbers of green, red, and blue pixels.

    Check out a TED-ED video for a cartoon explanation of how we see color.

    People who are colorblind have defects in their cone cells. People that can't see color at all have defects in one to all of the three cone types. The most common form of color blindness is caused by a defect in the "green" cone cells, which makes it difficult to distinguish green and red. A rarer form of color blindness makes people not be able to tell the difference between blue and yellow; these people have defective "blue" cone cells.

  • Biology and Chemistry

    Cheese! Using Diffraction To Take Pictures of DNA

    We remember from biology class that DNA is a double helix, but how do we know? DNA is too small to be able to see it. We may be able to see single cells of bacteria under a microscope, but DNA is much tinier than that.

    We took a photo of it. That's how.

    Rosalind Franklin was the scientist who took the first photo of DNA, not with a Polaroid or digital camera, but using X-rays, which are very short wavelength light. She knew that shining light on something small causes diffraction, as the light makes its way around and through that object,. It's much like the Double Slit Experiment, but in the case of DNA a more complicated pattern results.

    When we observe how light diffracts, we can tell something about the shape of the object changing the path of the light. This is exactly what Rosalind did. She made crystals of DNA and studied how these crystals diffract X-rays.

    She didn't use visible light, as Young did in the Double Slit Experiment, because visible light's wavelength is too big to be affected by a crystal of DNA. Imagine a tiny pebble, the size of a grain of sand, lying on the beach. A wave comes and washes over this grain. Does the path of the wave change? By looking from far away, could we tell that the wave encountered the tiny pebble? No, and no. It's the same idea.

    What if we replaced that tiny pebble with a rock the size of a Lazyboy chair? Would the path of the wave change? Yes. For an object to alter the path of a wave, the sizes of the wavelength and object have to be really close to each other. A 1 m water wave won't be impacted by a 1 mm pebble, but it sure will interact with a 3 m Lazyboy. When that wave hits that recliner, we'll definitely see a diffraction pattern.

    The same is true for light. DNA is so tiny that visible light would not interact with it enough to generate a diffraction pattern. Rosalind used light of a smaller wavelength, X-rays, which are near in size to the width of DNA.

    Biologists (and physicist and chemists) use this method of taking pictures, called X-ray crystallography, for very small things. Biologists use X-ray crystallography to snap shots of not only DNA but also proteins. By taking photos of the proteins, we can learn things about how they work, and just as importantly, why some faulty proteins cause syndromes and conditions.

  • Nature

    How Do Chameleons Change Color?

    First off, we have to dispel this myth: for the most part chameleons change color to communicate with each other. They have a color for being cool as a cucumber and one for when they're absolutely furious. Kind of like the bright red that moms turn when they're pushed just a little too far, or that lovely shade of purple that kids turn when they hold their breath to get their way.

    But we digress. How do chameleons change color?

    Under their outer layer of skin, which is essentially transparent, they have three layers of special skin cells. The cells of the layer closest to the surface contain yellow and red pigment. The layer underneath this "yellow" layer is a layer of cells that reflects all light using crystals. Yup, crystals.

    That's the tools they use, but how do they use these tools to change colors? That's the job of the crystals.

    The yellow pigment cells never change, but the spacing of the cells holding the crystals, which reflect light, does. When the crystals are held really close to each other, they are arranged in such a way that blue light is reflected. Blue from the crystal cells and yellow from the top layer, mix to make the chameleon look green.

    Green is a chameleon's color for "relaxed."

    When the chameleon is angered, the cells containing the crystals are allowed more room. As the crystals get more room, the light reflected changes from blue to green to yellow to orange to red.

    Yellow is a chamelon's "I want to appear threatening" color.

    Hormones in the chameleon signal these crystal-holding cells whether it is time cheerfully arrange the crystals tightly or whether it's time to rage and spread the crystals.

    Again, what matters is how light the crystals reflect and scatter light, not the pigment of the cells. As a result, a chameleon can tell his rival to back off or his lady-love to come hither. You can read more about this fascinating chameleon-trick