Eclipses of the Ring Light
During a solar eclipse, the moon casts two kinds of shadows. A ring light and a disk of black paper can help demonstrate how these shadows are made, and how they differ from one another.
- Cut a small disk approximately 2 inches (5 cm) in diameter out of the black construction paper or cardstock. Color it black with a marker to make it as dark as possible. (Click to enlarge photo below.)
- Tape the disk to the end of a bamboo skewer. Use the marker to make the tape and the top few inches of the skewer black. (Click to enlarge photos below.)
- Punch or cut a hole in a piece of white paper 2 inches (5 cm) square. Draw arms and legs to make a small stick figure using the hole as the head. (Click to enlarge photos below.)
Investigation I: Seeing Shadows
Darken the room. Turn on the ring light and set it to face a white wall. The best distance will depend on the diameter of the ring light. Ring lights that are about 4 inches (10 cm) across should be about 1 foot (30 cm) away. Larger ring lights should be farther away, up to about 3 feet (1 m).
Using the skewer as a handle, hold the disk about 2 inches (5 cm) away from the wall, but parallel to its surface (click image below for a closer view of a setup that uses an optional binder clip to hold the skewer). Look at the shadow made by the disk and make a quick sketch or take a picture. Does the shadow look like it has two parts—a darker section and a lighter section?
If you were to move the disk away from the wall while keeping it parallel to the wall, how do you think the two parts of the shadow would change? Will both get larger, both get smaller, the dark one get larger and the lighter shadow get smaller or vice versa? Give it a thought before trying it out and make a note of your prediction.
Now, slowly move the disk away from the wall. How does the shadow change? Was your prediction correct? (Click to enlarge photos below.)
Investigation II: Looking Back at the Ring Light
Place the punched piece of paper on your cell phone so that the hole is over the front-facing camera. (This is easiest to do if you have the camera app open and the front-facing camera selected.) Use a small piece of tape to hold the paper in place.
Lay the cell phone flat on a table and hold the ring light about 1 foot (30 cm) above the phone. You should be able to see the ring light’s image on the phone’s screen. (Click to enlarge photo below.)
Now, using the skewer as a handle, hold the disk parallel to the table and between the phone and the ring light, so that it makes a similar two-part shadow, as before. You’ll want to hold the disk high enough that it makes a dark shadow about ½ inch (1 cm) across. How much of the ring light can you see on the phone’s screen?
If you move the disk from right to left, a shadow will move across the phone’s camera (click to enlarge animated gif below). How much of the ring light will be displayed on the phone’s screen when the lightest part of the shadow reaches the camera? When the darkest part of the shadow goes across the camera?
A spot—on a wall, for example—is in the dark if no light can get to it. If the light source is small, such as a single candle, many objects will be big enough to block the candle’s light from getting to various places on the wall. (See image below: click to enlarge.) That means a room with only a candle for light will have many dark shadows. Places where no light can reach are called umbras.
If there are two sources of light in a room, there may be places on the wall that can get light from one source, but not the other. This occurs in spots where the object blocks the light from one source, but not the other. (See image below: click to enlarge.) That spot won’t be as bright as a spot that can receive light from all the light sources, but it will be brighter than a spot that can’t get light from any source. These places are called penumbras.
A big glowing source of light can be thought of as many small sources of light put together. The ring light has many sources of light along the ring that send light forward in a wide arc. Most spots on the paper can receive light from every part of the ring, and so are very bright. Looking back at the light source, then, an eye or a camera would be able to see all the light from the whole ring light.
In this case, however, the disk blocks some of the ring’s light from hitting the paper. Where the disk blocks all the ring’s light, the paper is in the darkest shadow, an umbra. An eye or camera in that area looking back toward the ring’s light wouldn’t be able to see any part of the light.
The disk can block some but not all of the ring’s light. The light from some parts of the ring gets through, while other parts are blocked. These partially blocked areas, called penumbras, have an intermediate, lighter shadow—not as dark as the umbra of the darkest shadow, and not as bright as the unobstructed paper. An eye or camera in those areas would see part of the ring light as bright as ever, while in other parts some of the light would be blocked.
These situations are similar to what happens on the side of the earth facing the Sun during a solar eclipse: For most regions of the earth, the moon doesn’t block any of the Sun’s light, and the day isn’t different from any other.
In a smaller region, the moon blocks some of the light from the Sun. The day may be dimmer, but it’s still dangerous to look at the Sun in this region because the sunlight that’s not being blocked by the moon is still just as bright as it is on a normal day.
Finally, in a much smaller area, the moon blocks nearly all the light from the Sun, and the region is very dark. If the moon is close enough to the earth, then all the bright light from the Sun is totally blocked. During that short time (sometimes up to 7 minutes), it’s safe to look at the Sun. While the brightest part of the Sun (its photosphere) is covered by the moon, the much dimmer corona can be seen. The corona is only as bright as a full moon, so it’s safe to directly view the corona during an eclipse.
If the moon is farther from the earth, though, it will still leave a brighter ring around the Sun. This type of eclipse, called an annular eclipse, is named for the word “annulus,” which describes a ring-shaped object (not to be confused with “annual,” for something that happens once a year, which is not the case here). Never look directly at this bright ring of light! An annular eclipse must only be seen through a shield designed for viewing eclipses.
Drawing rays of light can help clarify what’s happening here. Reducing the image to two dimensions can also make things a bit less confusing.
Imagine a light source that sends light in all directions. (Click to enlarge image below.)
Let’s pay attention, though, to the light that hits the wall on the right. The entire wall is illuminated because light hits everywhere on the wall.
An object can block some of the light and create a shadow. (Click to enlarge image below.)
Now imagine there are two light sources. (One is shown by dashed lines here, and one by straight lines, to help you see the difference. Click to enlarge image below.)
The wall is brighter now because twice as much light is hitting it.
Let’s put something in the way. (Click to enlarge image below.)
Three different things happen. First, most of the wall is still bright because those parts still receive light from both sources. Second, some of the wall is black because the object blocks both light sources and makes a dark shadow (umbra). Third, part of the wall gets light from one source but not the other, making a penumbra, a shadow that’s not as dark as the umbra.
Drawing out the model with rays of light can be somewhat tedious, but it‘s a great way to see why closer objects make bigger umbras and smaller penumbras. You can also redraw this type of image showing objects of different sizes and at different distances to see how those changes affect the situation.
If you’d like this activity to look a bit more like the earth and moon during an eclipse, you can use a baseball or Styrofoam ball to represent the earth, and then mount a small rubber ball to a skewer to represent the moon.
If you’d like to investigate the size and scale of a real solar eclipse, see the Eclipse to Scale Science Snack.
The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA.