Lab Manual

Step 1: Make money grow and shrink.

Fill a glass with water. Hold a penny so that it is half-submerged, about two-thirds of the way back in the glass. Look at the penny’s flat side so you can see both the part above and below the water, as shown in Figure 14.4 (a)). It may be easier to see the penny if you set the glass on white paper. What do you see? Does the part of the penny below water look larger or smaller than the part above? Does it look closer or farther away? Is it inverted? Record your results in Table 14.1.

Partly fill a clear rectangular container with water. Place an empty cup in the container; hold it down if it floats. Place a penny behind the empty cup so that it is half-submerged. Look down at the penny, so you can see it through the backside of the glass. See Figure

14.4 (b)). Make sure you can see both the part above and below the water. What do you see? Record your results in Table 14.1.

Figure 14.4: (a) Partly submerge a penny in a glass of water and view it from the side. (b) Partly submerge a penny in a rectangular container and view it from behind the backside of an empty cup.

Why does the penny grow and shrink? When the partly-submerged penny is inside the glass, you view it through a boundary that curves outward. The outward curvature causes rays of light to diverge as they travel from the penny to your eye. Thus, the glass acts as a convex lens that magnifies the image of the penny and makes it appear farther away.

When the partly-submerged penny is behind the glass, you view it through a boundary that curves inward. The inward curvature causes rays of light to converge as they travel from the penny to your eye. Thus, the glass acts as a concave lens that shrinks the image of the penny and makes it appear closer.

Step 2: Make money multiply.

Partly fill a rectangular container with water. Look at the penny through a corner of the container, so you can see the top surface and both sides. How many pennies do you see? Move the penny around. How much money can you make?

Step 3: Make money disappear.

Set an empty glass on top of a penny. Make sure both the penny and the bottom of the glass are dry. Look through the side of the glass. Can you see the penny?

Fill the glass with water, looking at the penny through the side as you fill the glass. Does the penny disappear?

Now, wet the penny and put it back under the glass. Can you see it now?

Why is the penny sometimes visible and sometimes not?

Glass has an index of refraction of about 1.5 and air has an index of refraction of about

1.0. When the glass is empty, light bends upon entering the glass bottom, but bends back again upon entering the air in the glass. Its path to your eye is nearly straight, so you can see the penny (Figure 14.5 (a)).

When the glass is full and the penny is dry, light bends upon entering the glass bottom, but it doesn’t bend back as much upon entering the water because the refractive index of water (1.33) is higher than the index of refraction of air. Since the light doesn’t bend back as much, light travels upward toward the top of the glass. If you look straight down, you will be able to see the penny (Figure 14.5 (b)).

When the glass is full and the penny is wet, light bends upon traveling from water to glass, but bends back upon traveling from glass to water. Its path to your eye is once again nearly straight so you can see the penny (Figure 14.5 (c)).

Figure 14.5: (a) Light bends upon entering the glass bottom, but bends back again upon entering the air in the glass. (b) Light bends upon entering the glass bottom, but it doesn’t bend back as much upon entering the water because the refractive index of water is higher than that of air. (c) Light bends upon traveling from water to glass, but bends back upon traveling from glass to water.