See-Through: How Does Mercury's Atmosphere Affect Images of its Surface?

PROBLEM

How does Mercury's atmosphere affect images of its surface?

Materials

• red pen
• ruler
• drawing compass
• two 3-by-5-inch (7.5-by-12.5-cm) white index cards
• masking tape
• flashlight
• walnut-size piece of modeling clay
• magnifying lens
• two 9-ounce (270-ml) clear plastic glasses
• helper

Procedure

1. Draw a red circle with a diameter of about ¹/₂ inch (1.25 cm) in the center of one of the index cards. Color in the circle.
2. Tape the index card to a wall just above a table so that the bottom of one long edge of the card rests on the table.
3. Turn on the flashlight and darken the room.
4. Lay the flashlight on the table at an angle to and about 4 inches (10 cm) from the index card. Position the flashlight, using clay as a support if necessary, so that its light shines on the red circle.
5. Holding the magnifying lens in one hand, position the lens opposite the flashlight at the same angle and distance from the index card on the wall. Hold the second card in the other hand behind the lens and move both the lens and the card back and forth until the lens projects the best image of the red circle on the card.
6. With the lens and card in this position, ask your helper to set one of the empty glasses on the table between the flashlight and the red circle, and the second glass between the red circle and the magnifying lens. Observe any change in the appearance of the image of the red circle on the card.

Results

The image of the red circle on the second index card was clearer before the glasses were set in place.

Why?

The red circle represents a location on the surface of Mercury. The flashlight is sunlight. The magnifying lens represents the lens of a camera used in spacecraft, and the second card is the photo (light)-sensitive electronic surface inside the camera, where the image is projected when a photograph is taken. The empty glasses represent the extremely thin atmosphere of Mercury, which contains helium, sodium, hydrogen, and possibly oxygen gases.

Part of the light reaching the red circle is absorbed by the paper and the rest is reflected away from the red circle. Some of this reflected light passes through the magnifying lens, and an image of the red circle is projected on the card in your hand. In the same way, light passes through a camera lens and is projected on the photo-sensitive surface inside the camera. If enough light from each part of the red circle is projected on the card, the image of the red circle will be bright and well defined.

The plastic glasses, like the atmosphere around Mercury, change the direction of light passing through them. Some of the light is reflected, some is refracted in different directions, and some passes straight through. Because of the change of the direction of the light, less light from the red circle passes through the lens, so the image of the red circle is not as clear and well defined as without the glasses. In a similar way, images of Mercury's surface are less clear because of its thin atmosphere.

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