Explanation
Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the earth atmosphere.
Stars appear as tiny dots in the sky; as their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions (light is bent when it hits a change in density - like a pocket of cold air or hot air). This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling).
Option A is correct.
A concave mirror can form following types of images;
1. Real and diminished when object is kept beyond radius of curvature so, $$m$$ is negative and $$0<|m|<1$$.
2. Virtual and enlarged when the object is kept between focus and the pole so, $$m$$ is positive and $$|m|>1$$.
3. Real and enlarged when the object is kept between focus and radius of curvature so, $$m$$ is negative and $$|m|>1$$
4. Real and of same size when the object is kept on the radius of curvature so, $$m = -1$$.
As hot air rises above cold air descends so as we move upward refractive index also decreases .
For exhausted travelers in the desert, an inferior mirage may appear to be a lake of water in the distance. An inferior mirage is called "inferior" because the mirage is located under the real object. The real object in an inferior mirage is the (blue) sky or any distant (therefore bluish) object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground in the distance.
Light rays coming from a particular distant object all travel through nearly the same air layers and all are bent over about the same amount. Therefore, rays coming from the top of the object will arrive lower than those from the bottom. The image usually is upside down, enhancing the illusion that the sky image seen in the distance is really a water or oil puddle acting as a mirror.
$$ \bf{Convex\ Mirror} $$ is a curved mirror where the reflective surface bulges out, towards the light source.
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