Question

Draw a ray diagram to show the formation of a rainbow and mark the points where dispersion and internal reflection occurs.

Answer 1

Hint: Examine the factors for the formation of rainbow. Recall the concept of refraction of light, total internal reflection of light and dispersion. All of these characteristics of light cause the formation of a rainbow.

Complete step by step answer:
We know that when sunlight passes through a glass prism or water droplet, it gets refracted and then disperses in seven different colors. We call it rainbow. We know that light of different wavelengths has different velocities. The bending of light at the boundary of another medium varies with wavelength of light. Since each color has different wavelengths, when the sunlight enters the water medium it splits into seven different colors. This is known as dispersion of light.
We see when the sunlight enters the boundary between air and water droplet, it gets refracted due to change in medium. We have shown it in the following figure by point A. Since the light deviates differently for each different wavelengths present in the white light, the dispersion occurs which splits the white light into seven different colors. We have shown it in the following figure by point B. We know that red light has maximum wavelength. Therefore, the red light undergoes minimum deviation while violet light undergoes maximum deviation. Now we can observe the split colors now reflected at the inner boundary between water and air cause it to suffer total internal reflection (point C). These colors now refract at the boundary between water and air before coming out of the droplet (point D).
We observe in the rainbow that violet color is at the top and violet color at the bottom. This is primarily due to different wavelengths of red and violet color. All other colors of rainbow lie between the band of red and violet color.

Note:
There is also the second type of rainbow formation that is secondary rainbow. Secondary rainbow forms when the sunlight enters the droplet from the lower surface of the droplet. Primary rainbows that we discussed in the above solution occur at lower altitudes than the secondary rainbows.