Maxine walked to the bus stop, fearing the math test she had today. She’d studied really hard for the test, but she was still worried.

“Whoa!” Adam said. “You stepped on a crack (裂缝).”

Maxine looked down. “So what?”

“It’s Friday the 13th. That’s bad luck already and you just added to it by stepping on that crack.”

Bad luck? Maxine didn’t need bad luck. She needed good luck, and a whole lot of it, if she was going to get a good grade on her math text.

“Do you really believe in that?” Maxine asked.

“There has to be a reason so many people are superstitious about Friday the 13th, right?” Adam said.

Maxine thought about that all morning and was careful not to step on any more cracks before her math test.

Maxine’s hand shook as she wrote her name on the test paper. Mr. Saunders told everyone to begin. She looked through the problems, and to her surprise, they didn’t look too hard. She took her time and worked out each problem. When she finished, she brought her paper up to Mr. Saunders’s desk.

Maxine was worried when Mr. Saunders started grading it right away. Maxine sat down and looked out of the window. She’d thought the problems weren’t bad at all, but what about her bad luck?

“Maxine, could you come here?” Mr. Saunders asked.

Maxine held her breath as she walked up to Mr. Saunders’s desk. She secretly looked at her paper, expecting to see a lot of red marks on her incorrect answers. Instead she saw a big letter A.

“I got an A?”

“Yes,” Mr. Saunders said. “I’m very surprised you have made so much progress. I thought you should see this right away.”

Maxine smiled for the rest of the day. Friday the 13th certainly wasn’t bad luck for her.

The sun shines again after rain, and you see red, orange, yellow, green, blue, indigo,and purple light in the sky—a rainbow.

Refraction in Action

Sunlight has all the colors of the rainbow, but some go faster when sunlight passes through water. From top to bottom are the faster ones to the slower. Red is the fastest and purple is the slowest.

As light enters water, it refracts. Red, traveling fastest, is refracted (使折射) least. Orange is refracted a little more, and so on. Different colors are separated in this way and that’s what you need to get a rainbow.

How to See a Rainbow

Stand with your back to the sun and search the sky, because a rainbow appears opposite the sun. If the sun is too high, no rainbow can appear. Look at your shadow, the dark shape your body makes on the ground. If it’s taller than you, you have a good chance of seeing a rainbow. The longer your shadow is, the bigger the rainbow.

Guess the Weather

If a rainbow appears in the morning, that means the rainbow is in the west, which is where the rain is. Weather usually moves from west to east, so the rain is coming your way. If it’s in the afternoon, things will be different. Here’s a saying:

Rainbow in the morning, shepherd (牧羊人)’s warning.

Rainbow at night, shepherd’s delight.

See Two at Once

Look for a second rainbow after you see one, but it’s hard to see. The main rainbow has red on top and purple on bottom, but the secondary has red on bottom and purple on top.

Unseen Rainbow Parts

Sunlight has infrared radiation (红外线), which we can’t see. Still, infrared radiation gets refracted by raindrops, and lies above the main rainbow’s red light. Ultraviolet radiation (紫外线) is also part of a rainbow. It’s just below purple.