1. HOW A RAINBOW FORMS

2. INTRODUCTION : 1. What is rainbow? 2. Two physical phenomena: Reflection and Refraction 3. Why does a rainbow have the seven colors? 4. Conclusion HOW A RAINBOW WORKS

3. WHAT IS RAINBOW  Author Donald Ahrens said that ‘ rainbow is one of the most spectacular light shows observed on earth’.  A rainbow is a phenomenon that causes a spectrum of light to spread out in the sky by water droplets in the Earth’s atmosphere.  A rainbow is a group of circular arcs of colors.

4. REFLECTION AND REFRACTION TThe body of water can be seemed as a prim. According to the concept which we learn on Class, we can know that after white light enters prism, light will be refracted by a prism.

5.  The following diagram shows the geometry of reflection in a sphere. Note that angles alpha and beta are related by Snell’s law, that triangles ABO and BCO are two congruent isosceles triangles, and that the angle between incident and reflected rays is 180 - 2 alpha + 4 beta. The total deviation is expressed mathematically by: or: The extremum of deviation will happen when: Which can be solved:

6. WHY DOES A RAINBOW HAVE SEVEN COLORS  The index of refraction values are different for the frequency of light. For instance, the n value for frequencies of violet light is 1.53; and the n value for frequencies of red light is 1.51. Because of the absorption and reemission, these two processes cause the lower frequency (higher wavelength) red light travel faster through the glass than the higher frequency (lower wavelength) violet light. Violet light refracts more than red light by the absorption and reemission. When the white light entry the boundary of a triangular prism, the white light will be separated into seven component colors of the spectrum.

7. THE ANGLE OF DEVIATION

8. CONCLUSION  Though seen by many people, few can explain rainbow with physics.  With the knowledge of Reflection and Refraction, we can answer the question about the formation of a rainbow.  Last but not the least, physics should not be considered an isolated subject, but should be used in everyday just like explaining the formation of a rainbow.