Refraction Through A Prism

To determine which statement regarding rainbows is incorrect, let's evaluate each option:

1. When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed.

   This statement is correct. A secondary rainbow occurs when light is reflected twice inside the water droplets before coming out, resulting in a pattern with reversed colors compared to the primary rainbow.

2. The order of colours is reversed in the secondary rainbow.

   This statement is also correct. In a secondary rainbow, the order of colors is indeed reversed because the light is reflected an additional time inside the droplets, which alters the sequence of colors.

3. An observer can see a rainbow when his front is towards the sun.

   This statement is incorrect. Rainbows are typically seen with the observer's back to the sun because they form due to the refraction, dispersion, and reflection of sunlight in water droplets which occurs opposite to the sun. Therefore, the correct answer to this question is that this statement is the wrong one.

4. Rainbow is a combined effect of dispersion, refraction and reflection of sunlight.

   This statement is correct. Rainbows result from the dispersion (splitting of light into colors), refraction (bending of light as it enters water droplets), and reflection (bouncing of light inside the droplets), ultimately creating the colorful arc seen by observers.

The concept of rainbows involves understanding that they are created when sunlight disperses upon entering and exiting raindrops, causing a spectrum of light to appear in a circular arc. Observing a rainbow requires having the sun at the observer's back, casting light into the droplets ahead.