Understanding Why We See the Sun Before Rise and After Set
The phenomenon that allows us to see the Sun a few minutes before it actually rises above the horizon and a few minutes after it has set is due to a process called atmospheric refraction.
What is Atmospheric Refraction?
Atmospheric refraction is the bending of light rays as they pass through the Earth's atmosphere. Unlike a vacuum, the Earth's atmosphere consists of layers of air with varying densities, temperatures, and pressures. Light travels at slightly different speeds through these different layers. When light passes from a denser medium to a less dense medium (or vice versa) at an angle, it bends. Since the Earth's atmosphere gets less dense with increasing altitude, light rays from celestial objects entering our atmosphere are bent or refracted.
How Refraction Affects Sunrise and Sunset Visibility
- Before Sunrise: When the Sun is actually just below the horizon, its light rays travel towards the Earth. As these rays enter our atmosphere, they are bent downwards towards the observer's eye. This bending makes the Sun appear slightly higher in the sky than its actual geometric position. Consequently, we can see the sunlight and perceive the Sun even when it is geometrically still below the horizon. This effect typically makes the Sun visible about 2 to 4 minutes before its actual sunrise time.
- After Sunset: Similarly, after the Sun has passed below the horizon (geometrically set), its light rays still pass through the atmosphere. The same bending effect occurs, making the Sun appear slightly above the horizon for a few minutes after it has actually set. This extends the period of daylight visibility. This effect typically lasts for about 2 to 4 minutes after the actual sunset time.
Evaluating the Options
- Option 1 & 2: The Earth's rotation speed is relatively constant, and the Sun's apparent motion is primarily due to this rotation, not a slowing down or specific movement below the horizon that explains this pre-sunrise/post-sunset visibility.
- Option 3: While clouds can scatter and reflect sunlight, making the sunrise or sunset colours more vivid, they are not the primary cause for seeing the Sun when it's geometrically hidden below the horizon. The bending of light by the atmosphere itself is the key factor.
- Option 4: Atmospheric refraction perfectly explains why light from the Sun can reach our eyes even when the Sun is geometrically below the horizon, making it appear visible earlier and disappear later.
Therefore, the reason we can observe the Sun slightly before its rise and after its set is the bending of sunlight by the Earth's atmosphere, known as atmospheric refraction.
Important Questions from Basic Concepts in Physics
$50 \ \Omega$, $50 \ \Omega$ and $100 \ \Omega$ resistors are connected in series in a circuit. They can be replaced with a single resistor of ______________in the circuit.
- Which of the following statements correctly describes the relationship among the incident ray, the refracted ray, and the normal at the point of incidence when light passes from one transparent medium to another?
- Work done by a force on a body is said to be negative when:
- What is the value of the force exerted by the Earth on a satellite, if the mass of the Earth is $6 \times 10^{24}$ kg and the mass of the satellite is 400 kg. The satellite is orbiting the Earth at a distance of 400 km from its surface. The radius of the Earth is 6400 km and G = $6.67 \times 10^{-11}$ N m$^2$/kg$^2$.
- A person with eye defect of ___________ cannot see nearby objects distinctly, which is corrected using a ___________ lens of suitable power.
