Advanced Sunrise and Delayed Sunset: Refraction of Light, Effect of Refraction of Light on Sunrise and Sunset (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Do you know that we see the sunrise about 2 minutes before the sun is at that perceived position and do you know that the sunset that we see is of a sun that has already set?
  • So, why these different perceptions? It is because of a phenomenon termed as refraction of light.

What is Refraction of Light?

As simply we can say that, “a ray of light ‘bends’ when it travels from one medium to another” . This ‘bending of light’ is known as refraction of light. The below figure 1 shows the refraction of Light.

Refraction of Light

Refraction of Light
  • If the light ray travels from a rare medium to a denser medium, it bends towards the imaginary normal and if it travels from a dense medium to a rarer medium, it bends away from the imaginary normal.
  • Depending on the density of the different medium, the speed of the travelling light ray keeps changing, and because of this it slow down or speed up, therefore bending in the process.
  • Refractive index of the second medium with respect to the First medium is defined as the ratio of the velocity of light in the first medium to the velocity of light in second medium.
  • So if, Refractive index of the second medium with respect to the First medium
  • = velocity of light in the first medium
  • = velocity of light in the Second medium

Effect of Refraction of Light on Sunrise and Sunset

  • Sunrise and sunset are the instants when the upper edge of the Sun appears and disappears over the horizon respectively as a result of Earth՚s rotation.
  • If we imagine the journey of light rays from the sun then firstly, the journey of light is through vacuum and then through the atmosphere of the earth and then it is finally seen by us.
  • So, at first stage, vacuum will be act as a rarer medium and the earth՚s atmosphere with all its temperature changes, winds, different gases, will be a denser medium in comparison to that.

Successive Atmospheric Refraction Occurs when Sunlight Passes through the Atmosphere

Successive Atmospheric Refraction

During sunrise, the light rays bend due to our atmosphere and we see the sun early even though the sun is just below the horizon as shown in figure 3 below

Sunrise Occurs Before the Sun Actually Reaches the Horizon

The Sun Actually Reaches the Horizon

Same as at sunset, due to the same bending of light rays, we see the apparent position of the sun, not the actual position as shown in figure 4 below

The Sun Actually Sinks below the Horizon Before Sunset Happens

The Horizon Befor Sunset Happens
  • By summing up, due to refraction one can see the sun rise about two minutes before it՚s there and during sunset one can see it for around two minutes more, even though it has already moved from that position.
  • One of the consequences of this effect of refraction is that contrary to what most people think, the day and night is not exactly 12 hours long on the days of the Equinoxes.
  • That causes refraction of sunlight, daytime is just slightly longer than night time on March Equinox and September Equinox.
  • The extent of the refraction depends on air temperature and atmospheric pressure: higher the pressure and lower the temperature, larger the refraction angle.
  • If you watch the Sun set in an area of high pressure on a cold day, you may have to wait several seconds for the upper edge of the Sun to disappear behind the horizon, compared to a day with average pressure and temperature.

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