Spot the Sun

None needed.

With this Snack, you can safely investigate the Sun by looking at its image. Never look directly at the Sun!

Use the hole punch to cut a hole in an index card. On a sunny day, hold the card so you can see its shadow on a light-colored surface, such as a posterboard placed on the ground. Orient the card so you can see the sunlight coming through the hole. What is the shape of the spot of light that shines through? 

Now use a pencil to punch a hole in a new index card (click to enlarge the photo below). 

Hold the card as before. How does it compare with the hole punch? How does the shape of the image compare with the shape of the hole? How does the image of the light change as you move closer and farther away from the viewing surface?

Experiment with other shapes and sizes of holes, such as a square, a triangle, or an irregularly shaped hole. How does the image of the light change as you move closer and farther away from the viewing surface?

Next, grab your mirror.

Caution: It’s dangerous to have sunlight reflected directly into your eyes. Make sure no one is in front of the mirror, and do not point the mirror toward anyone’s eyes or face, including your own.

Tilt the mirror so it reflects the Sun onto a dark-colored surface, such as a wall in the shade or a dark posterboard (click to enlarge photo below). How does the image of the Sun compare with the shape of your mirror? Move closer and farther away from the surface to see if it changes.

Use masking tape to block off all but a small section of the mirror in any shape you choose, as in the photo below (click to enlarge).  

Reflect the mirror’s light onto the dark-colored surface. What shape does it make? How does it compare with the shape you created on the mirror? Move closer and farther away from the surface to see if it changes.

Hold an index card with a hole in it near a viewing surface, and you’ll probably see the spot of light in the shape of the hole. As you pull the card away from the surface, though, that spot of light should gradually look circular, no matter what shape the hole is. You’ve revealed the shape of the light source, the spherical Sun!

When the Sun shines around us, everything looks blanketed in light. This light is the reflection of overlapping images of the Sun. When you block the light with your index card, you reveal only those images of the Sun that pass through the hole. With a small hole held at the right distance, you will see the circular image of the Sun.

Rather than letting an image of the Sun through the hole, the mirror reflects the image of the Sun. If the mirror is very large, many images will overlap and look like a bright spot. A small mirror held far enough away from the viewing surface will reflect only a small area of sunlight, and you’ll see the circular image of the Sun.

Have you ever noticed bright sparkles on the surface of moving water in the sunlight? Little waves in the water act like small mirrors, reflecting images of the Sun toward your eye. If you take a photo, you can capture a moment in time and notice that each sparkle is a circular image of the Sun, as in the photo below (click to enlarge).

During a solar eclipse, the moon blocks part or all of our view of the Sun. This means that, during an eclipse, the images of the Sun that shine onto the earth form partial circles, which we might not notice if they all overlapped. 

Viewing sunlight through a pinhole or reflecting it off a mirror can isolate the image to reveal the true shape of the light source hitting the earth. In a total solar eclipse, the Sun’s image will be a partial circle until it is completely blocked by the moon, or has reached totality. In an annular eclipse, when the moon is too far away to block the full Sun, the image will be a ring of light, called an annulus.

Take a Light Walk, an exploration of sunlight developed by Exploratorium artist Bob Miller, and see light in a whole new way.

This Snack is an easy and inexpensive way for students to observe and investigate the Sun. A nice way to begin is to stand under a tree on a sunny day and have students observe the shadows beneath the leaves. Even though the gaps between the leaves make very irregular shapes, there should be some circular images of the Sun on the ground. This phenomenon can serve as a starting point for students to explore before using index cards and mirrors to investigate their own shapes.

If you are lucky enough to experience a solar eclipse with your students, using holes cut into cards is a safe and simple way for everyone to view the progress of the eclipse and share what they notice.

Watch Exploratorium scientist Paul Doherty project an image of the Sun through an index card, as well as through holes made with his fingers.

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.