Formation of a Mirage
A mirage is an optical illusion that occurs when light rays are bent or refracted due to the variation in the refractive index of air layers. It often appears as a shimmering pool of water or a distant object in the desert or on a hot road. The phenomenon responsible for the formation of a mirage is a combination of refraction and total internal reflection.

Refraction
Refraction is the bending of light as it passes from one medium to another with a different refractive index. In the case of a mirage, the air near the ground is heated more than the air above it, creating a temperature gradient. This temperature gradient causes a variation in the refractive index of air, which results in the bending of light rays.

Total Internal Reflection
Total internal reflection occurs when light traveling in a medium with a higher refractive index encounters a boundary with a medium of lower refractive index at an angle greater than the critical angle. Instead of being refracted, the light is reflected back into the medium with the higher refractive index.

Formation Process
1. Temperature Gradient: The formation of a mirage begins with a temperature gradient near the ground, where the air is hotter than the air above it. This temperature difference causes the air to have a different refractive index at different heights.

2. Light Ray Bending: As light rays from distant objects pass through the layers of air with different refractive indices, they are bent or refracted. The bending of light makes the objects appear displaced from their actual position.

3. Virtual Image: The bent light rays reach our eyes, creating an illusionary image of the object. This virtual image appears as if it is coming from a position different from its actual location.

4. Total Internal Reflection: In some cases, when the temperature gradient is steep enough, the light rays traveling close to the ground encounter a boundary with the hotter air above it at an angle greater than the critical angle. This causes total internal reflection, where the light rays are reflected back towards the ground.

5. Shimmering Effect: The total internal reflection of light rays creates a shimmering effect, giving the mirage a watery appearance. The reflection and refraction of light rays contribute to the illusion of water or objects that are not actually present.

Therefore, a mirage is formed due to the combined phenomenon of refraction and total internal reflection. The temperature gradient in the air causes the bending of light rays, while the steep temperature variation near the ground can lead to total internal reflection, enhancing the illusionary effect of a mirage.