Before class, students read 13.2 Discharge Lamps in How Things Work the Physics of Everyday Life and watched:
There are two different types of light ‘detectors’ on our retina, rods and cones. Rods are very sensitive to low light but not to color, and cones are sensitive to different wavelengths (colors) of light. Humans have three types of cones, one for predominately blue light, one for green and one more sensitive to red light (photo below). Cones need fairly bright light so in low light conditions we are mainly detecting light with the rods, which is why we don’t see colors very well in dimly lit rooms.
But not everyone sees colors in the same way. Color blindness, or to be more accurate, color vision deficiency happens when one set of cones is not as sensitive as it should be. This is an inherited trait and affects men more often than women because the responsible gene lies on the X chromosome, which men only have one of and women have two. Its unlikely for women to have the recessive gene on both of their X-chromosomes. My husband is red-green color blind which means his vision isn’t as sensitive to red light, so a pink shirt might look white to him. There are some great apps, including ColorDeBlind, that let you see what the world looks like with different color deficiencies. The photo below shows how the fruit appears when you have ‘normal’ color vision and the photo on the right is what it looks like if your red-green color blind. You can see the blue is pretty much the same but reds, oranges and greens all look like different shades of yellow and brown.
Animals can have different numbers of cones then us and they can cover different wavelengths or colors. Some birds, fish and insects have 4 different cones, giving them tetrachromatic (4-color) vision. Here’s a link to an article on color vision in animals in Cosmos magazine.
I happen to have a book of pseudoischromatic plates for testing color blindness where if you have ‘normal’ color vision you will see a number among the dots, but if you are color blind then you will not see any numbers, or you might see another number. Enchroma, a company producing glasses that enhance the color vision of people with color vision deficiencies, has an online color vision test you can take for free here.
For the lab portion of the class, I did the colored flame demonstration, putting different chemicals (strontium chloride, lithium chloride, etc) over a butane burner and showed how they produced different colored flames. Students looked at the flames with the handheld spectrometers and saw the light had different wavelengths. The different chemicals produce different colored light because their energy levels are different and you can use a flame test to help identify chemicals. You can find a more detailed lab in this post from my chemistry class last year.
Students also did the color viewing box that I did with the middle school class a few weeks ago. They made predictions for how different colored objects would appear under different colored light and then did the experiment to see if they were correct. We used different colored filters and a flash light to produce different colors of light.
Since this chapter also discussed fluorescence, I brought out our collection of fluorescent rocks and a shortwave Ultraviolet (UV) light. The rocks look very boring in normal sunlight (photo on the left), but under UV light the rocks give off visible light. The UV light excites the atoms in rock and when they relax to the ground state they give off visible light. Just like the flames, the color depends on the elements involved. The light I used was an old Raytech which doesn’t seem to be available anymore, but this one by UVP looks similar and is about the same price ($60) I paid for mine. Not all rocks fluoresce but you can buy fluorescent rock collections online or buy them at local rock and gem shows. You can read more about fluorescent rocks in this geology.com article. This article also has some safety tips for using these UV lamps – they can damage your eyes and skin, so you need to make sure its only pointed at the rocks.
In class we watched a video on blackbody radiation by Physics Girl
and Why is Blue so Rare in Nature? by It’s Okay to Be Smart, which is a very cool video on how animals appear to be blue without using blue pigment.