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4. Total internal reflection is the complete reflection of a ray of light within a medium such as water or glass from the surrounding surfaces back into the medium. The phenomenon occurs if the angle of incidence is greater than a certain limiting angle, called the critical angle.
5. Refer Grade 10 NCERT textbook
6. A rainbow is a natural spectrum of sunlight in the form of bows appearing in the sky when the sun shines on rain drops. It is combined result of reflection, refraction and dispersion of sunlight from water droplets, in atmosphere. It is always formed in the direction opposite to the sun. To see a rainbow, the sun must behind us and the water droplets must fall infront of us. When sunlight enters into a spherical raindrop, it is refracted and dispersed. The different colours of light bend in different angles.
When different colours of light fall back on the inner surface of drop, it (water drop) reflects (different colours of light) internally (total internal reflection).
The water drops again refract into different colors when it comes out from the raindrop. After leaving this different colours from the raindrop as rainbow, they reach our eyes. Thus, we see a rainbow.