Have you ever seen the sun rising just above the horizon? Do you really believe that? Nature can play tricks with you. You might be aware of optical illusions in nature like mirages, sundogs, etc. The sunrise is also kind of an optical illusion. We tend to visually see the sunrise above the horizon well before the actual sunrise takes place, this is called advanced sunrise. In the same, manner we tend to see the Sun even after the actual sunset has taken place, this is called delayed Sunset. These optical illusions are based on the refraction of light, as light bends when it travels from one substance to another. In this article, we discuss the optical phenomenon behind the advanced sunset and the delayed sunset.
What is refraction?
When light moves from one substance to another, there can be a difference in the density of the substance. So when a light ray moves from less dense material to a more dense material it tens to bend towards the normal. And similarly, when the light ray moves from denser to less dense, it tends to move away from the normal. This phenomenon gives rise to an optical illusion where the object seems to be at a different location compared to the actual position. But the angle of incidence is 90 degrees, then the light does not bend and there is no refraction of light then.
Atmospheric refraction:
The refraction of light by the Earth’s atmosphere is known as atmospheric refraction. Atmospheric refraction is caused by the bending of light rays when they pass through the layers of the Earth’s atmosphere, which are of different gases and dust particles that have different optical densities. The light coming from a star undergoes atmospheric refraction due to varying optical densities of air at various altitudes.
Advanced sunrises and delayed sunsets are phenomena caused by atmospheric refraction. In this, the Sun appears to rise early by 2 minutes and set late by 2 minutes.
Whenever the rays from the Sun hit the atmosphere, they get refracted. Due to this, the Sun appears to be in an apparent position, which suggests that it has already risen. The same happens at sunset. After setting, the Sun still appears in the sky due to this refraction phenomenon.
What do you mean by advanced Sunrise?
• Before Sunrise, the Sun is below the horizon. Sun’s rays are coming from more dense air towards less dense air. This causes the sun’s rays to refract and appear to come above the horizon. Our eyes see the sun’s rays as a straight line, it appears to us that the sun has risen but it has actually not yet risen. You can see clearly in the below schematic, that the Sun we see is not the actual position of the Sun.
What do you mean by advanced sunset:
After sunset, Sun is below the horizon. Sun’s rays are coming from more dense air towards less dense air. This causes sun rays to refract and appear to come above the horizon. From our eyes, It appears to us that the sun has not yet set.
Similar optical illusions based on the refraction of light
- Sundog
- Mirages
- Sparkling of diamonds
- Bent pencils when put inside water
