Explanation:
a)Snell's law states that when light travels from a rarer to a denser substance, like air to water or from a less dense layer of the atmosphere to a denser layer, it bends towards the normal{an imaginary line that is perpendicular to the surface of both media}. However,the opposite occurs when light moves from a more dense to a less dense medium. The angle between the normal and the refracted light ray is known as the angle of refraction.
In case of earth as light from the stars enters the earth atmosphere it bends towards smaller angle because the earth density increases as the light travel towards the earth troposphere from the exosphere as per the Snell's law described above.
b)Light rays that travel straight down do not bend, while rays that enter the Earth's atmosphere at a shallower angle get refracted and bend towards the normal, roughly following the direction of the Earth's curvature.
This means that celestial objects in the zenith position directly above you appear in the correct position, while objects closer to the horizon appear to be higher up in the sky than they actually are.
(a) The speed of light decreases in the denser medium.
(b) The actual position of stars is lower than the apparent position in relation to the horizon.
According to Snell's Law when light rays enter from a rarer medium, let's say vacuum, to a denser medium like air, it bends towards the normal as the speed of light decreases in the denser medium. As shown in the image attached.
Similarly, when light from stars enters our atmosphere it gets refracted multiple times, since the atmosphere is made of different layers, and each layer further refracts the light toward the normal. And hence the apparent position of the stars is higher than the actual position as compared to the horizon, you can refer to the image attached for better understanding.
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