Advanced Sunrise and Delayed Sunset: Atmospheric Refraction

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The phenomenon of advanced sunrise and delayed sunset is induced by refraction in the air. The Sun appears to rise 2 minutes earlier and set 2 minutes later in this scenario. When the Sun's rays strike the atmosphere, they are refracted. As a result of which, the Sun appears to be in a position that indicates it has already risen. Due to the same refraction effect, the Sun appears in the sky after it has set. The variation in the refractive index of the atmosphere is responsible for this entire phenomenon. In this article, we will further explore the reason behind the delayed sunset and advanced sunrise.

Read More: Air Composition

Table of Content

What is Refraction?
Refraction Terminology
Atmospheric Refraction
Advanced Sunrise Explanation
Delayed Sunset Explanation
Things to Remember
Sample Questions

Keyterms: Phenomenon, Sunrise, Sunset, Air, Refraction, Reflection, Sun, Sky, Light, Optical Density, Refractive Index, Refracted ray, incident ray

What is Refraction?

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The bending of a light wave's path as it passes through the border separating two mediums is known as refraction. When a wave changes medium, it experiences a shift in speed, which causes refraction.

When a light wave moves from a medium where it travels slowly (relatively speaking) to a medium where it travels quickly, it refracts away from the normal. The refracted ray will be further away from the normal line than the incident light in this situation.
When a light wave moves from a media in which it travels quickly (relatively speaking) to a medium in which it travels slowly, the light wave refracts back to the normal. The refracted ray will be closer to the normal line than the incident light in this situation.

$Refraction of Light$

Refraction of Light

The video below explains this:

Human Eye One Shot Detailed Video Explanation:

Refraction Terminology

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Normal: The normal is a line that runs perpendicular to the border between the two substances (creating a 90-degree angle). Because light travels more slowly in a denser medium, it bends.
Incident Ray: The incident ray is a ray of light that falls directly on the surface separating two mediums.
Refracted Ray: The refracted ray is a ray of light that has changed direction while going through another medium.
The angle of Incidence: The angle of incidence is the angle formed by the incident beam and the normal.
The angle of Refraction: The angle of refraction is the angle formed by the refracted beam with the normal.
Optical Density: The optical density of a transparent substance is a characteristic that gauges the speed of light passing through it. The optical density describes how far any optically dense medium bends the transmitted light rays towards or away from the normal.
Refractive Index: The ratio of the speed of light in a vacuum to the speed of light in the second medium of larger density is used to determine the Refractive Index (Index of Refraction). It is most usually represented by the letter n or n'.

Atmospheric Refraction

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As light travels from outer space into the atmosphere, the air density rises, and hence the refractive index changes. As a result, the degree of refraction rises as light passes through the atmosphere, causing the light to take a “curved” route rather than a straight path as it would in a homogenous medium like glass.

$Atmospheric Refraction$

Atmospheric Refraction

The diagram depicts rays of light from the stars traveling through the atmosphere at various elevations above the horizon. Observe that:

The light from star A, which is straight overhead, is not refracted, and it reaches the viewer without deviating from its original course.
The larger the length of the distance of air a star must pass through, the lower its height.
The more obliquely a star's light strikes the atmosphere, the lower its altitude.

As an object approaches the horizon, these final two points combine to cause the amount of refraction to rapidly grow. As a result, star B is moved to position B', and star C is moved to position C'.

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Advanced Sunrise Explanation

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When the Sun's rays strike the atmosphere, they are refracted. As a result, the Sun appears to be in a position that indicates it has already risen.

The sun rises above the horizon during sunrise. As they travel from more thick air to less dense air, the sun's rays are refracted. Furthermore, the human eye perceives the solar rays as a straight line, giving the impression that the sun has risen. However, in reality, it has not risen yet.

Advanced Sunrise Explanation

Advanced Sunrise Explanation

Delayed Sunset Explanation

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As the sun is disappearing behind the horizon after dusk, we can see the sun's apparent position rather than its true position. Because of atmospheric refraction sun rays coming from denser air towards rarer air get refracted and appear to come from above the horizon. As human eyes perceive light to travel in a straight line, it creates an illusion as if the sun has not set.

Delayed Sunset Explanation

Delayed Sunset Explanation

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Things to Remember

The bending of a light wave's path as it passes through the border separating two mediums is known as refraction.
Delayed Sunset and Advanced Sunrise are caused due to the phenomenon of atmospheric refraction.
The Sun appears to rise 2 minutes earlier and set 2 minutes later.
The human eye perceives the solar rays as a straight line.
The ratio of the speed of light in a vacuum to the speed of light in the second medium of larger density is used to determine the Refractive Index (Index of Refraction).
As light travels from outer space into the atmosphere, the air density rises, and hence the refractive index changes. As a result, the degree of refraction rises to cause the light to take a “curved” route.

Read More: Ozone Layer Depletion

Sample Questions

Ques. What is the cause of atmospheric refraction? (1 mark)

Ans. The bending of light rays as they pass through the layers of the earth's atmosphere with varied optical densities causes atmospheric refraction.

Ques. Define Refractive Index? (2 marks)

Ans. The ratio of the speed of light in a vacuum to the speed of light in the second medium of higher density is used to determine the Refractive Index (Index of Refraction). It is represented by the letter n or n'.

Ques. Why the delay and advance in sunset and sunrise is 2 minutes? (3 marks)

Ans. Atmospheric refraction occurs when light rays pass through layers of varying densities and refractive indices in the atmosphere. When the sun is above the horizon, the sunrise occurs. When the sun is just below the horizon, the light rays it emits are refracted by a rarer to denser medium as they enter the Earth's atmosphere. As a result, during each refraction, they bend towards the normal. It follows a curved course and reaches the observer's eye due to the constant refraction of light rays at each layer. As a result, in the morning, we can see the sun 2 minutes before it rises beyond the horizon.

Ques. What is the phenomenon of refraction? (3 marks)

Ans. The bending of a light wave's path as it passes through the border separating two mediums is known as refraction.

When a light wave moves from a medium where it travels slowly to a medium where it travels quickly, it refracts away from the normal.
When a light wave moves from a media in which it travels quickly to a medium in which it travels slowly, the light wave refracts back to the normal.

Ques. Why is the sun dim at sunset? (3 marks)

Ans. At noon, sunlight shines practically straight above you and travels 2 miles (4 kilometers) through a moderately dense atmosphere to reach you (Atmospheric pressure at 2 miles is about one-half of sea level pressure). Because the sun travels mostly parallel to the surface at sunset (and sunrise), it travels through the dense atmosphere for much longer. When light travels through a dense atmosphere, it is prone to additional scattering, which reduces the light's apparent intensity.

Ques. Why do stars produce a twinkling effect? (3 marks)

Ans. When light from the stars enters the atmosphere, it is refracted due to the different optical densities of air at different altitudes. Because the atmosphere is always changing, the optical densities of air at various heights in the atmosphere also changes.

From one minute to the next, the constantly shifting atmosphere refracts the light from the stars in diverse ways. The star appears brighter when the atmosphere refracts more light towards us, and dimmer when the atmosphere refracts less light. As a result, a twinkling effect is created.

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Advanced Sunrise and Delayed Sunset: Atmospheric Refraction

What is Refraction?

Human Eye One Shot Detailed Video Explanation:

Refraction Terminology

Atmospheric Refraction

Advanced Sunrise Explanation

Delayed Sunset Explanation

Things to Remember

Sample Questions

CBSE CLASS XII Related Questions

1.
Write any two features of nuclear forces.

2.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

3.
If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.

4.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.
Suppose a pure Si crystal has \( 5 \times 10^{28} \) atoms per \( \text{m}^3 \). It is doped with \( 5 \times 10^{22} \) atoms per \( \text{m}^3 \) of Arsenic. Calculate majority and minority carrier concentration in the doped silicon. (Given: \( n_i = 1.5 \times 10^{16} \, \text{m}^{-3} \))

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Advanced Sunrise and Delayed Sunset: Atmospheric Refraction

What is Refraction?

Human Eye One Shot Detailed Video Explanation:

Refraction Terminology

Atmospheric Refraction

Advanced Sunrise Explanation

Delayed Sunset Explanation

Things to Remember

Sample Questions

CBSE CLASS XII Related Questions

1. Write any two features of nuclear forces.

2.Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

3. If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.

4.Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.Suppose a pure Si crystal has \( 5 \times 10^{28} \) atoms per \( \text{m}^3 \). It is doped with \( 5 \times 10^{22} \) atoms per \( \text{m}^3 \) of Arsenic. Calculate majority and minority carrier concentration in the doped silicon. (Given: \( n_i = 1.5 \times 10^{16} \, \text{m}^{-3} \))

Comments

SUBSCRIBE TO OUR NEWS LETTER

1.
Write any two features of nuclear forces.

2.
Two thin lenses of focal length \( f_1 \) and \( f_2 \) are placed in contact with each other coaxially. Prove that the focal length \( f \) of the combination is given by \[ f = \frac{f_1 f_2}{f_1 + f_2}. \]

3.
If Bohr’s quantization postulate (angular momentum \( = \frac{nh}{2\pi} \)) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why, then, do we never speak of quantization of orbits of planets around the Sun? Explain.

4.
Two small identical metallic balls having charges \( q \) and \( -2q \) are kept far at a separation \( r \). They are brought in contact and then separated at distance \( \frac{r}{2} \). Compared to the initial force \( F \), they will now:

5.
Suppose a pure Si crystal has \( 5 \times 10^{28} \) atoms per \( \text{m}^3 \). It is doped with \( 5 \times 10^{22} \) atoms per \( \text{m}^3 \) of Arsenic. Calculate majority and minority carrier concentration in the doped silicon. (Given: \( n_i = 1.5 \times 10^{16} \, \text{m}^{-3} \))