Answer : 4. "Refraction of light & Total Internal Reflection of light"
Explanation :

Answer: D) Refraction of light & Total Internal Reflection of light Explanation: A Mirage is a naturally occurring optical phenomenon in which light rays bend to produce a displaced image of distant objects or the sky. It is caused due to Total internal Reflection. It is an optical illusion observed in deserts or hot summer days due to which a person sees a pond of water some distance ahead of him in which the surrounding objects appear inverted. How Mirage Forms:: 1. The temperature of other layers of atmosphere goes on decreasing as we go up.2. Therefore density and refractive index of air changes as one goes up.3. Thus a ray of light,gets refracted more and more away from the normal.4. At a particular angle when a ray of light exceeds critical angle, total internal reflection takes place and ray of light gets reflected in the same medium.5. when the reflected ray reaches our eye, it appears as coming from tree or sky .6. Hence the inverted image of tree creates an impression from a pond of water.