When it's too hot to play outside, catch rainbows indoors using a glass jar and some plain white paper. Any size jar will do, as long as it is crystal clear. Simply fill the jar to three-quarters full with water and set it on a sunny windowsill. Find a sunny patch on the floor where the light is shining through the window and sit on the floor facing the windowsill. Lay a piece of white paper out in front of you. Slide the paper along the floor until it captures the rainbow that is reflecting from the jar. The rainbow that you see on the paper forms just like real rainbows do -- sunlight shining through water droplets. As the rainbow is reflecting on the paper, trace the colors using crayons or markers to preserve the beauty of the rainbow forever.
Sunlight is not always necessary when it comes to indoor rainbows. In fact, for the rainbow mirror experiment, the darker the better. Simply fill a glass half full with water and slide a small cosmetic mirror inside. Lay the mirror on an angle against the sides of the glass. If you only have a hand mirror available, fill a clear baking dish half full with water and lay the mirror inside, angling it against the walls of the pot. For maximum effect, the container that holds the water should be crystal clear. Shut out the lights and it's show time -- or rainbow time in this case. Once the room is pitch dark, shine a flashlight onto the mirror. Watch as colorful rainbows dance on the ceiling and walls opposite the angle of the mirror. Try shifting the angle of the mirror inside the water to change the direction of the rainbow. Light refraction is what makes the rainbows in this experiment. As light from the flashlight strikes the water droplets surrounding the mirror, the water bends the light at various angles, creating the rainbow effects.
Milk is for more than drinking -- it's also for making indoor rainbows. Experiment with creating milk rainbows by filling a cereal bowl half full with room temperature milk. Add one drop of red, blue and yellow food coloring to the center of the bowl of milk. Follow up by squeezing a drop of dish soap into the center of the bowl of milk. Watch as the colors of the rainbow begin to appear before your eyes in the bowl of milk. Dish soap is the catalyst that creates the swirly, blend of rainbow colors. The dish soap separates the fats in the milk, creating minor currents that carry the food coloring along the surface, forming a rainbow effect.
Plants are capable of pulling water into their cellular tissues with and without roots. Such is the case with a stalk of celery. If placed in a glass of water, xylem cells within the celery stalk suck water upward. A few drops of blue food coloring added to the water will create streaks of blue that span the length of the celery stalk. For the indoor rainbow celery experiment, line up seven glasses on a window sill or countertop and fill each glass half full with water. Add two drops of red food coloring to one glass. Add one drop of red food coloring and one drop of yellow food coloring to the next to make orange. Add two drops of yellow food coloring to the third, green to the fourth, blue to the fifth, one drop of red and a drop of blue to the sixth to make indigo, and finally, add two drops of blue and one drop of red to create violet water in the seventh glass. Set one stalk of celery in each glass, leafy side up. Observe the celery over the next few days. As the xylem cells transport the colored water, the celery stalks will turn every color of the rainbow -- right up to their curly tops.