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What is refraction of light? Law of refraction of light? Reversibility of light refraction. Refraction of light when traveling from water to air and vice versa.

Lessons about phenomenon of light refraction It is one of the basic but important knowledge in the Physics curriculum. Many parts of knowledge about light refraction are still abstract and difficult to understand. The following article will help you with the basic system of knowledge you need to master about light refraction.

1. What is refraction of light?

Refraction of light is the phenomenon that when passing through two surfaces separating two light transmission mediums, the light beam is suddenly changed direction. It is easier to understand: The phenomenon of light refraction is the phenomenon of light breaking when traveling at an angle between two transparent media. This can also be considered a change due to velocity, the speed leading to the bending of light when traveling from different media.

What is the cause of the refraction of light?

When light hits different media, it will have different speeds, which proves that the medium is a serious influence on the transmission of light. According to scientific research, the phenomenon of light refraction is caused by 2 reasons:

– One is: Speed ​​is changed. When light is refracted (aka bent) more, it has been made to speed up or slow down.

– Second: Angle of incident ray. The amount of light refraction will also be more when entering the larger angle. But when light enters at a 90 degree angle to the normal surface, the light slows down and doesn’t change direction.

Example: Take a glass beaker filled with water then place a drinking straw on its side inside the cup. Observation will see that the reflected light transmitted from the straw is no longer transmitted straight, but has been broken at the interface between the two media, air and liquid. Because of this, when we put the straw in the cup of water and observe, we see that the straw seems to have been tilted partially.

Explanation of the phenomenon:

Our eyes can see everything because of light and light always travels in a straight line. When we look at an object (not a light source) and the light from the emitted light source hits the object, depending on the angle and color of the light source, our eyes will observe different shapes of the object. need to be observed.

2. What is the law of refraction of light?

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In there:

– i is the angle between the light ray traveling from medium 1 to the plane separating the two media and the normal to the plane separating the two media.

– r is the angle between the light ray traveling from the interface to medium 2 and the normal to the plane separating the two media.

– n1 is the refractive index of medium 1.

– n2 will be the refractive index of medium 2.

With the above formula, we can state the law of refraction of light which is explained as follows:

The refracted ray is always in the incident plane and on the other side of the normal to the incident ray. The incident plane is the plane formed by the incident ray and the normal. For a given two transparent media, the ratio between sin i and sin r is a constant. The ratio between the sine of the angle of incidence (sin i) and the sine of the angle of refraction (sin r) is always constant.

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sin i / sin r = n /n = const

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3. Refraction of light when traveling from water to air and vice versa:

When a ray of light travels from air to water:

The refracted ray lies in the incident plane

The angle of refraction is less than the angle of incidence

When a ray of light travels from water to air:

The refracted ray lies in the incident plane

The angle of refraction is larger than the angle of incidence

4. Refractive index:

Because the refractive index of different media is different, the speed of light transmission in different media is different. The refractive index according to the physics definition is the ratio between the speed of light in a vacuum and the phase speed of electromagnetic radiation in a material. This refractive index is usually denoted by n. The speed of light as it propagates through a transparent material such as glass or air is usually less than c. The ratio between c and the velocity v of the light passing through the material will be called the refractive index n of the material.

It can be seen that the refractive index of the medium in the refraction of light has two types, namely the relative refractive index and the absolute refractive index. So what is the relative refractive index? What is absolute refractive index?

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4.1. Reciprocal refractive index:

In the law of light refraction, the constant ratio sini/sinr denoted by n21 is called the relative refractive index of medium 2 (the medium containing the refracted ray) to medium 1 (the medium containing the incident ray). .

Whereby:

– If n21 < 1, then r > i: The refracted ray is deviated further from the normal axis, we say that medium 2 is less refractive than medium 1.

– If n21 > 1, then r < i: The refracted ray is deviated closer to the normal, we say medium 2 is more refractive than medium 1.

Table of relative refractive index of some media:

Solids (20oC) Extracts Solids (20oC) Extracts Diamond
Crao’s glass
Flint glass
Ice 2.419
1.464 1.532
1.603 1,865
1.309 Table salt (NaCl)
Amber
Politiren
Sapphire 1.544
1.546
1,590
1,768 Liquid (20oC) Extracts Liquid (20oC) Extracts Country
Benzene 1.333
1.501 Ethyl alcohol
Glycerol 1.361
1.473 Gas (0°C, 1atm) Extracts Gas (0°C, 1atm) Extracts Atmosphere 1,000293 Carbon dioxide gas 1,00045

4.1. Absolute refraction:

The absolute refractive index of a medium is the relative refractive index of that medium with respect to the vacuum. The refractive index of a vacuum is 1. All other transparent media have a refractive index greater than 1.

Between the relative refractive index n21 of medium 2 with respect to medium 1 and their absolute refractive indices n2 and n1 there will be the relationship: n21 = n2n1

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In addition, it has been shown that: The absolute refractive index of transparent media is inversely proportional to the speed of light in such media: n2n1=v1v2

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Therefore, if medium 1 is a vacuum, then we have: n1 = 1 and v1 = c = 3.108m/s => Result: n2=cv2

Since the speed of light in all media is less than the speed of light in a vacuum, the absolute refractive index of the media is always greater than 1.

The meaning of absolute refractive index: The absolute refractive index of a transparent medium shows how many times the speed of light in that medium is less than the speed of light in a vacuum.

We can set up the formula:

In there:

n2 is the absolute refractive index of medium 2.

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n1 is the absolute refractive index of medium 1.

Relationship between absolute refractive index of a medium and velocity:

In there:

c is the speed of light in a vacuum (c = 3.108 m/s).

v is the speed of light in the medium under consideration.

Note:

The refractive index of vacuum is 1.

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The refractive index of air = 1,000293 and is usually rounded = 1.

All other transparent media have absolute refractive index greater than 1.

Some other relations of the law of refraction of light that you need to keep in mind:

The formula for refraction of light is: n1sini = n2sinr

– If i and r are less than 10 degrees, sini ≈ i; sinr ≈ r → n1i = n2r .

– If i = 0, r = 0, then no refraction occurs.

5. Reversibility of light refraction:

The propagation of light is reversible, which means that whatever path light travels, it also travels back in that direction.

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From this property you will have the formula:

6. Application of the phenomenon of light refraction:

Application of the phenomenon of light refraction: In astronomy, in the early days of telescopes, the observation of distant objects was affected by the phenomenon of light refraction when light passed from space through the atmosphere into the earth. Thanks to the phenomenon of light refraction, astrophysicists know how to edit telescopes to observe stars and planets in outer space without being hindered by light refraction. Astrophysicists were able to fine-tune the telescopes to make the observable images clearer. Previously observations were distorted due to the phenomenon of refraction of light traveling from space through the Earth’s atmosphere.
Today, to completely eliminate the phenomenon of light refraction, scientists have placed a telescope in space.

Besides, thanks to the theory of the phenomenon of light refraction, humanity can clearly understand why when observing in the night sky, we can see twinkling stars. Because at night, when you look up at the dark sky, you will see the light from the stars being refracted many times as it travels from space and through the atmosphere to Earth.

7. Some exercises on the law of refraction of light:

Question 1: Why is it that when a rod or a straight stick is tilted in a glass of water, the rod is no longer straight, but tilted at a different angle? When removing the straw from the cup, or plugging the straw vertically into the cup, we do not observe the above phenomenon anymore?

Explain:

Light is refracted as it passes out of the water, giving the illusion that objects in the water appear to be both distorted and closer than they really are. The wave must first travel through water, then through the glass-water interface, and finally into the air. The light waves coming from the sides (front and back) of the tube are deflected to a greater extent than the waves coming from the very center of the tube, making it appear larger than it really is.

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Question 2: Why do we see the phenomenon of the starry night sky?

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Explain:

At night, when you look up at the sky, you see twinkling “stars”, which is caused by the light from the stars being refracted (broken) many times when traveling from space through the atmosphere. Earth.

Question 3: Why is the basin full of water, when viewed from the side, the water becomes shallower?

Explain:

In the same type of medium, light always travels in a straight line-the shortest path. But it goes from one type of medium to another, for example from air to water, or from water to air, because the speed of light in those two types of media is different, on the interface. of the two media, the light will be bent, following a meandering path. This type of phenomenon of light is called refraction of light. Your water bowl looks shallow because of the refraction of light.

Question 4: Why does a fisherman who uses a harpoon to launch a fish under the water do not launch directly at the fish, but aim a bit further away?

Answer: The image that we see fish in the water is a bent light ray that changes direction. Thus, the naked eye cannot perceive, so the position of the fish is easy to be mistaken. The position of the fish in the water and the visible image is different. Experienced people decide not to rush into the fish because it is just a virtual image of the fish. If you launch into a place slightly farther away, you will definitely hit.

Question 5: A ray of light travels from water with refractive index 4/3 to glass with refractive index 1.5. Calculate the angle of refraction and deviation D formed by the refracted ray and the incident ray, given the angle of incidence i = 30o.

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The answer:

According to the problem we have: n1=4/3, n2=1.5, i=30o

Apply the formula: n1.sini = n2.sinr

<=> 4/3.sin30 = 1.5.sinr

<=>r 26.4o

=> D = i – r = 30o – 26.4o = 3.6o

Question 6: A ray of light travels from water and is refracted into air. The refracted ray and the reflected ray at the water surface are perpendicular to each other. Water has a refractive index of 4/3. What is the angle of incidence of the light ray (round number)?

The answer:

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According to the problem we have: n1=4/3, n2=1, i’ + r = i + r = 90o

Apply the formula: n1.sini = n2.sinr

<=> 4/3.sini = sinr.

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