What is Refraction of Light?
What is Refraction of Light: If light passes from one medium to another through the interface between two media, then, in general, the direction of the light ray changes at the interface. This phenomenon is called refraction of light.
In short, refraction of light is the phenomenon of light passing from one medium to another.
Let CD be the difference between air medium and glass medium. Because light is refracted on this surface, it is called a refracting surface. A ray AO passes through air and is incident diagonally at point O on the plane of divergence of the two media. After refraction at the diffraction surface CD, the ray changes direction and goes along the straight-line OB. At point O, NON’ is drawn perpendicular to the plane of divergence. NON’ is called perpendicular to the plane of incidence at the point of incidence. AO is called the incident ray, and OB is the refracted ray. The angle (i) that the incident ray AO makes with the normal NO is called the angle of incidence. The angle (r) which the refracted ray OB makes with the normal ON’ is called the angle of refraction.
Refraction from a Thinner Medium to a Thicker Medium
When a light ray passes diagonally from a lighter medium to a denser medium (e.g. air to glass), the refracted ray moves perpendicularly. In this case, the angle of incidence i is greater than the angle of refraction r. That is, i > r.
Refraction from Thicker Medium to Thinner Medium
If a light ray passes obliquely from a denser medium to a lighter medium (e.g. glass to air), the refracted ray deviates from the perpendicular. In this case the angle of refraction r is greater than the angle of incidence i. That is, r > i.
Laws of Refraction
Refraction of light obeys the following two laws. They are called the law of refraction of light.
- The incident ray, the refracted ray, and the perpendicular drawn on the refractor plane at the point of incidence lie in the same plane.
- The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. The value of this constant depends on the nature of the medium and the colour of the incident light.
The second law of refraction is called Snell’s law after the scientist Snell (1951–1626).
Refractive Index
If angle of incidence = i and angle of refraction = r then by second law of refraction,
This constant μ is called the refractive index of the second medium with respect to the first medium.
The value of the refractive index depends on 1. the nature of the medium involved and 2. the colour of the incident light. Regardless of the value of the angle of incidence, the colour (i.e., frequency) of the incident light and the refractive index remain constant if the two mediums are unchanged.
Special Case: Normal Incidence
If a ray of light is incident perpendicularly on the interface from one medium to another, then i = 0. In that case, according to Snell’s Law,
μsinr = μsin0 = 0 or, r = 0
Therefore, if a ray of light is incident perpendicularly on the plane of separation of two mediums, it enters the second medium without changing its direction.
Relative Refractive Index
When a ray of light is refracted from one medium a to another medium b, the ratio of the sine of the angle of incidence to the law of the angle of refraction is called the ‘refractive index of medium a with respect to medium b or relative refractive index. This refractive index is expressed by aμb.
We know that the path of light is reversible, that is, if light is deflected 180° from a point on the path, the light can return to the same path. Hence, a ray of light coming from medium b in path BO and incident at an angle r, at point O on the diffraction surface will, after refraction, travel along path OA in medium a and the angle of refraction through a will be i.
For example, if the refractive index of water relative to air is 4/3, the refractive index of air relative to water will be 3/4.
Absolute Refractive Index
When a ray of light is refracted from a vacuum to another medium, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is called the absolute refractive index of the medium,
If angle of incidence = i and angle of refraction = r, then the absolute refractive index of the medium,
Hence, the relative refractive index of a medium with respect to vacuum is the absolute refractive index of that medium. Obviously, the refractive index is 1 in vacuum.
The refractive index of a medium usually refers to the refractive index of that medium relative to air. It is not the absolute refractive index of the medium. But experiments show that the difference between the values of ‘refractive index relative to air’ and ‘absolute refractive index’ of a medium is very small. This is because the refractive index of air at STP is 1.00029, which is approximately equal to 1. So, the refractive index of any medium with respect to air is considered as the absolute refractive index of that medium. For example, if glass has a refractive index of 1.5, it means that the glass has a refractive index of 1.5 relative to air.
Absolute refractive index of air at STP = 1.0002918 and refractive index of glass relative to air is 1.5. Hence, the absolute refractive index of the tree is 1.5/1.0002918 = 1.49956 and taking this value as 1.5 will obviously not cause any significant error. The absolute refractive index of a medium is denoted by μ.
Relation of Refractive Index with Different Physical Quantities
Relation of Speed of Light with Refractive Index
According to the wave theory of light, the speed of light is different in different mediums. According to this theorem it can be proved that if the absolute refractive index of a medium is μ,
Now, the speed of light in vacuum (c) is maximum; In no medium can the speed of light (v) be greater than c. Hence, in any medium the value of μ is greater than one.
If the refractive index of medium b with respect to medium a is aμb, it can be proved from wave theory that,
If medium b is denser than medium a, then aμb > 1 and in this case va > vb. Hence, speed of light in denser medium is less than in lighter medium. In fact, the speed of light decreases as the refractive index of the medium increases.
Relationship between Relative Refractive Index and Absolute Refractive Index
If absolute refractive index of medium a and b are μa and μb respectively,
Hence the relative refractive index of medium b with respect to medium a,
aμb = μb / μa
Relation of Refractive Index and Wavelength of Light
The refractive index of one medium relative to another depends on the wavelength of light. Scientist Cauchy revealed an acceptable relationship between refractive index and wavelength. The relationship is:
Here A and B are two constants; They have different values in different mediums. Obviously, as the wavelength of light increases, the refractive index of a medium decreases. Figure shows the variation of refractive index μ of BK7 glass with wavelength λ of light.
Deviation of a Ray
The change in direction during reflection or refraction of light ray is called its deviation. During refraction, light rays change direction from the point of incidence on the interface between the two media. The angle formed between the direction of the incident ray and the direction of the refracted ray is the measure of the ray’s deviation.
Let AO be the incident ray, OB the refracted ray, ∠AON = angle of incidence (i) and ∠BON’ = angle of refraction (r) for refraction from a thinner medium a to a thicker medium b. If the incident ray had not been refracted, the rope would have moved straight along the path OC. Refraction causes the ray to follow the path OB instead of the path OC. Hence deviation of ray due to refraction, δ = ∠BOC.
Now, δ = ∠BOC
= ∠N’OC – ∠N’OB = ∠AON – ∠N’OB = i – r
As the value of angle of incidence increases, the value of deviation increases.
- In the case of perpendicular incidence, i.e., if i = 0, then r = 0. Hence the value of δ is minimum, i.e. δ = 0.
- For maximum values of i, i.e., when i = 90°, the value of δ is maximum.
In case of refraction from a denser medium to a thinner medium, the angle of refraction is greater than the angle of incidence, i.e. r > i.
So, in this case, δ = ∠BOC = ∠N’OB – ∠N’OC = ∠N’OB – ∠AON = r – i.
FAQs on Refraction of Light
What is the refraction of light?
- Refraction of light is the bending or change in direction of light as it passes from one medium to another (e.g., from air to water) due to a change in its speed.
What causes refraction?
- Refraction occurs because light changes speed when it enters a medium with a different density. This change in speed causes the light to bend.
What are some examples of refraction?
- Common examples include the apparent bending of a straw in water, the sparkle of diamonds, mirages in the desert, and the formation of rainbows.
What is Snell’s Law?
Snell’s Law describes the relationship between the angles of incidence and refraction. It states that n1sin(θ1) = n2sin(θ2), where n represents the refractive index of each medium, and θ represents the angle of the light ray in each medium.
What is the refractive index?
- The refractive index is a measure of how much a material slows down light. A higher refractive index means light travels more slowly through that medium.
Why does a pencil look bent in water?
- A pencil appears bent in water because light rays change direction as they move from the water to the air, altering the perceived position of the submerged part of the pencil.
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