Refraction and Dispersion of Light through a Prism

Edited By Vishal kumar | Updated on Jul 02, 2025 04:42 PM IST

What is prism in physics? Refraction of light:

When a ray of light travels obliquely from one clear medium to another, it deviates from its original course. Refraction is the term for the deviation of a light ray. The variation in the velocity of light in the different mediums causes refraction. In a rarer media, light travels faster, while in a denser medium, it travels slower. There are two laws of refraction that govern light refraction.

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What is prism in physics? Refraction of light:
What is meant by dispersion of light by prism? What is dispersion of light class 10?
Relation between refraction and dispersion:

$Refraction and Dispersion of Light through a Prism$

Refraction and Dispersion of Light through a Prism

First law of refraction
Second law of refraction

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First law of refraction:

In the same plane as the incident ray, the refracted beam of light, and the normal to the refracting surface.

Second law of refraction:

The ratio of the refractive indices of the two media is equal to the sine of angle of incidence as well as sine of angle of refraction. Snell's law is another name for this law.

sin i sin r =21

The refractive index tells us how quickly or slowly light moves through a medium. The ratio of speed of light in vacuum (c) to speed of light in that medium is the refractive index of that medium (v).

=cv

In general, 12=speed of light in medium 1speed of light in medium 2

If i is the incidence angle and r is the refraction angle, then

sin i sin r =constant

The refractive index of the second media in relation to the first medium is called this constant. It is generally represented by the Greek letter 12

If the refractive index of a medium is high, speed of light in that medium is low, as well as if refractive index of medium is low, the speed of light in that medium is high.

The refracted beam is bent away from the normal drawn to the interface as light moves from denser medium into rarer medium.

The refracted ray is bending towards normal drawn to interface when light moves from a rarer medium into a denser material.

$Second law of refraction$

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What is meant by dispersion of light by prism? What is dispersion of light class 10?

Dispersion of light definition: The phenomenon of dispersion happens when a wave's phase velocity varies with its frequency. Dispersion media are media that have this attribute in common. The term "chromatic dispersion" is frequently used for clearness. Although the term is most commonly associated with light and other electromagnetic waves, it can also refer to any type of wave motion, such as acoustic dispersion in the case of sound and seismic waves, gravity waves, and telecommunication signals travelling along transmission lines or optical fibre. Dispersion results in a loss of kinetic energy due to absorption on a physical level.

Refraction of light through prism:

Refraction is the curving of light as it travels from one medium to another. When a beam of light passes through a glass prism, for example, it refracts both entering and exiting the prism.

Dispersion of light through prism:

The degree to which light refracts is determined by the wavelength of the light. Because each light wave has a different wavelength, it will deviate in different ways. White light is made up of several wavelengths of light (colours). Violet, indigo, blue, green, yellow, and red are dispersion of light examples of these colours. The maximum wavelength is red, while the lowest is violet. The variation in the path of light is dependent on the wavelength. The least deviated light is red, whereas the most deviated light is violet. When white light passes through a prism spectrum of seven hues appears, indicating that white light is made up of seven different colours. Prism just serves as a conduit for the dispersion of light in the seven primary hues. When light strikes the prism, refraction occurs. These deviated colours have distinct wavelengths and frequencies, and they deviate differently at different angles due to the prism's velocity differential. As a result, the colour red deviates the least because it has the longest wavelength, while the colour violet deviates the most because it has the shortest wavelength.

Dispersion of light through prism

Relation between refraction and dispersion:

Any bending of waves caused by a change in speed is referred to as refraction. A wave is said to be refracted when it travels through different depths. The frequency dependence of refraction is referred to as dispersion. When light is refracted by a prism, dispersion refers to how much the higher frequency light bends.

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Refraction of a composite light-Dispersion of light:

We know that the sun is the fundamental and natural source of light. A monochromatic source is one that produces only one colour of light. A composite light source, on the other hand, creates white light that incorporates light of various colours. Sunlight is a composite light made up of a variety of colours and wavelengths. A mercury vapour lamp is another example of a composite source. When a white light is refracted through a glass prism, what do you see?

When a beam of white or composite light passes through a transparent medium like glass or water, it splits into its component colours. Dispersion of light is the term for this phenomena.

Spectrum refers to the colour bands that make up a spectrum. Violet, Indigo, Blue, Green, Yellow, Orange, and Red are the hues that make up this spectrum. The acronym “VIBGYOR” stands for these colours. When white light is refracted by a transparent medium, why do we receive the spectrum? This is due to the fact that different coloured lights are bent at different angles. That means, the angle of refraction varies depending on the colour.

Red has the least angle of reflection while violet has the most. The angle of refraction is determined in terms of the medium's refractive index, according to Snell's law. As a result, the refractive index of the medium varies depending on the colour of the light.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. Light travels from rarer medium to denser medium. The angles of incidence as well as refraction are 45oand 30o. Calculate refractive index of second medium with respect to first medium.

The angle of incidence, i=45o

The angle of refraction, r=30o

To find the refractive index of the second medium with respect to the first medium:

Refractive index, 12=sin i sin r

=sin 45o sin 30o =1 212

=2=1.414

2. What is refraction?

3. What is dispersion?

The phenomenon of dispersion happens when a wave's phase velocity varies with its frequency. Dispersion media are media that have this attribute in common. Dispersion results in a loss of kinetic energy due to absorption on a physical level.

4. What is refraction of light through a prism?

The bending of light as it moves from one medium to another is known as refraction. For example, when a ray of light passes through a glass prism, refraction happens both when it enters and exits the prism.

5. What is the Difference between Light Dispersion and Light Refraction?

Any bending of waves caused by a change in speed is referred to as refraction. The production of refraction occurs when water waves travel over a range of depths and heights.

When we state that light is refracted by a prism, we are referring to dispersion, which means that there will be a higher frequency of light bending.

The bending of light is referred to as refraction, while the frequency dependence of light properties is referred to as dispersion.

6. Why does a rainbow form after rain?

A rainbow forms when sunlight enters water droplets in the air, undergoes refraction, reflection, and dispersion. The dispersed light exits the droplet at different angles for different colors, creating the familiar arc of colors we see as a rainbow.

7. Why does a straw appear bent when placed in a glass of water?

The straw appears bent due to refraction. Light rays from the part of the straw underwater bend as they exit the water into the air, making that portion appear displaced from its actual position. This creates the illusion of a bent straw.

8. What is the refractive index of a medium?

The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. It indicates how much light slows down and bends when entering the medium, with higher values indicating greater refraction.

9. What is the difference between refraction and reflection?

Refraction is the bending of light as it passes from one medium to another, while reflection is the bouncing back of light from a surface. Refraction changes the direction of light within a medium, while reflection changes its direction at the surface between media.

10. What is the critical angle in refraction?

The critical angle is the angle of incidence in a denser medium for which the angle of refraction in the less dense medium is 90°. At angles greater than the critical angle, total internal reflection occurs, and no light passes into the less dense medium.

11. How does a prism disperse white light?

A prism disperses white light by separating it into its component colors. This occurs because different colors of light have different wavelengths, which refract at slightly different angles when passing through the prism, causing them to spread out into a spectrum.

12. What is dispersion in optics?

Dispersion in optics refers to the phenomenon where different wavelengths of light are refracted at different angles when passing through a medium. This causes white light to separate into its component colors, as seen in a rainbow or when light passes through a prism.

13. What is the difference between normal dispersion and anomalous dispersion?

Normal dispersion occurs when the refractive index of a medium decreases as the wavelength of light increases, causing red light to bend less than blue light. Anomalous dispersion is the opposite, where the refractive index increases with wavelength in certain regions of the spectrum.

14. Why do diamonds sparkle more than glass?

Diamonds sparkle more than glass because they have a higher refractive index. This causes greater refraction and total internal reflection of light within the diamond, leading to more light being reflected back to the observer and creating a brilliant sparkle.

15. What is chromatic aberration in lenses?

Chromatic aberration is a type of distortion where a lens fails to focus all colors to the same point. It occurs because different colors of light refract at slightly different angles, causing a slight separation of colors and resulting in colored fringes around images.

16. What is refraction of light?

Refraction is the bending of light as it passes from one medium to another with different optical densities. This occurs due to the change in the speed of light as it moves between media, causing a change in its direction.

17. How does a mirage form in a desert?

A mirage forms due to the refraction of light in layers of air with different temperatures. Hot air near the ground bends light rays upward, creating an illusion of a reflective surface or water. This is an example of atmospheric refraction.

18. What is the relationship between the angle of incidence and the angle of refraction?

The relationship between the angle of incidence and the angle of refraction is described by Snell's Law: n1 sin(θ1) = n2 sin(θ2), where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.

19. How does a fiber optic cable work using the principle of refraction?

Fiber optic cables use total internal reflection to transmit light signals. The light enters the fiber at an angle greater than the critical angle, causing it to reflect off the inner walls of the fiber repeatedly, allowing the signal to travel long distances with minimal loss.

20. Why does light slow down when entering a denser medium?

Light slows down in a denser medium because it interacts with the atoms or molecules in the medium. These interactions cause the light to be repeatedly absorbed and re-emitted, resulting in an overall slower propagation speed through the material.

21. What is the principle behind anti-reflective coatings on glasses?

Anti-reflective coatings work by using destructive interference. A thin layer with a specific refractive index and thickness is applied to the lens surface. Light reflected from the top and bottom of this layer interferes destructively, reducing overall reflection.

22. What is the principle behind fiber optic communication?

Fiber optic communication uses total internal reflection to transmit data as light pulses through thin glass or plastic fibers. The light signal bounces along the fiber's core, maintaining its intensity over long distances due to the high refractive index difference between the core and cladding.

23. How does the refractive index of a medium change with temperature?

Generally, the refractive index of a medium decreases as temperature increases. This is because higher temperatures cause materials to expand, reducing their density and thus lowering their refractive index. This principle is used in some optical temperature sensors.

24. How does the refractive index of water change when salt is added to it?

Adding salt to water increases its refractive index. This is because the dissolved salt ions increase the density of the solution, slowing down the speed of light more than in pure water. This principle is used in some salinity sensors.

25. What is the cause of the "glory" optical phenomenon sometimes seen around airplane shadows on clouds?

The "glory" is caused by a combination of diffraction and backscattering of light by water droplets in clouds. Light waves interfere constructively at specific angles, creating colored rings around the shadow of the observer (or airplane) projected onto the cloud.

26. What is the principle behind liquid crystal displays (LCDs)?

LCDs use the principle of light polarization and the ability of liquid crystals to change the orientation of polarized light. By applying an electric field, the orientation of the liquid crystals can be controlled, allowing or blocking light passage through polarizing filters, thus creating images.

27. How does the refractive index of air affect astronomical observations?

The varying refractive index of air due to temperature and pressure changes causes atmospheric distortion in astronomical observations. This results in the twinkling of stars and blurring of images. Adaptive optics systems are used to correct for these distortions in large telescopes.

28. What is the principle behind polarizing sunglasses?

Polarizing sunglasses use a special filter that blocks light waves oscillating in specific planes. This filter is particularly effective at blocking horizontally polarized light, which is often reflected from surfaces like water or roads, reducing glare and improving visibility.

29. What is the cause of the "heiligenschein" effect seen around one's shadow on dewy grass?

The heiligenschein effect is caused by retroreflection of light from dew droplets on grass blades. Light is focused by the spherical droplets onto the grass surface, then reflected back along the same path, creating a bright halo around the shadow of the observer's head.

30. How does the refractive index of a medium affect the speed of light?

The refractive index of a medium is inversely proportional to the speed of light in that medium. A higher refractive index means light travels more slowly through the medium, while a lower refractive index indicates faster light propagation.

31. Why does a diamond appear colorless despite dispersing light?

A diamond appears colorless because its high refractive index and cut cause most of the dispersed light to be internally reflected and recombined before exiting. The sparkle we see is actually rapid flashes of different colors that our eyes perceive as white.

32. How does a prism differ from a lens in its effect on light?

A prism primarily disperses light, separating it into its component colors without focusing it. A lens, on the other hand, refracts light to converge or diverge it, forming images. While lenses can cause some dispersion (chromatic aberration), their main purpose is focusing light.

33. What is the cause of the blue color of the sky?

The blue color of the sky is caused by Rayleigh scattering, a type of dispersion where shorter wavelengths (blue light) are scattered more by air molecules than longer wavelengths (red light). This scattered blue light reaches our eyes from all directions in the sky.

34. How does refraction affect the apparent depth of a swimming pool?

Refraction makes a swimming pool appear shallower than it actually is. Light rays from the bottom of the pool bend as they exit the water, making the bottom appear closer to the surface than its true position. This can lead to underestimating the pool's depth.

35. How does a rainbow spectrum differ from the visible spectrum produced by a prism?

A rainbow spectrum is circular or arc-shaped due to the spherical nature of water droplets, while a prism produces a linear spectrum. The rainbow also shows colors in a specific order (ROYGBIV), while a prism's spectrum can vary based on its orientation and the light source.

36. What is the cause of the "green flash" sometimes seen at sunset?

The "green flash" is a rare optical phenomenon caused by atmospheric refraction and dispersion. As the sun sets, its light travels through more atmosphere, causing greater refraction and dispersion. For a brief moment, only the green light is visible above the horizon.

37. How does the refractive index of air change with altitude, and what effect does this have?

The refractive index of air decreases with increasing altitude due to lower air density. This gradual change causes light to bend as it travels through the atmosphere, allowing us to see objects slightly below the horizon and affecting the apparent position of celestial bodies.

38. How does a convex lens form an image?

A convex lens forms an image by refracting light rays from an object. Parallel rays are bent towards the focal point, while rays passing through the center are undeviated. The intersection of these refracted rays forms the image, which can be real or virtual depending on the object's position.

39. What is the cause of mirages on hot roads?

Mirages on hot roads occur due to temperature gradients in the air above the road surface. The hot air near the road has a lower refractive index than the cooler air above, causing light rays to bend upwards. This creates an illusion of a reflective surface, often mistaken for water.

40. What is the difference between dispersion and scattering of light?

Dispersion is the separation of light into its component colors due to different wavelengths refracting at different angles. Scattering, on the other hand, is the redirection of light in multiple directions when it interacts with particles or molecules in a medium.

41. How does a diffraction grating differ from a prism in dispersing light?

A diffraction grating disperses light through interference of waves from multiple slits or grooves, while a prism disperses light through refraction. Gratings can produce higher spectral resolution and multiple spectra (orders), whereas prisms produce a single spectrum with less separation between wavelengths.

42. What is the principle behind the formation of a mirage?

Mirages form due to the refraction of light in layers of air with different temperatures and densities. Light rays bend as they pass through these layers, creating an optical illusion where objects appear displaced from their actual position or inverted.

43. What is the cause of the twinkling of stars?

The twinkling of stars, also known as stellar scintillation, is caused by atmospheric refraction. As starlight passes through turbulent layers of air with varying temperature and density, it refracts differently, causing rapid changes in the star's apparent brightness and position.

44. How does a rainbow's position change with the observer's location?

A rainbow's position is always relative to the observer and the light source (usually the sun). It appears as a circle centered on the antisolar point (the point directly opposite the sun from the observer's perspective). Moving the observer changes this point, shifting the rainbow's position.

45. What is the principle behind gradient-index (GRIN) lenses?

Gradient-index lenses have a varying refractive index throughout their material, typically decreasing from the center outwards. This gradual change in refractive index bends light rays continuously through the lens, allowing for unique optical properties and potentially thinner lens designs.

46. How does the refractive index of a medium affect the wavelength of light passing through it?

When light enters a medium with a higher refractive index, its wavelength decreases while its frequency remains constant. This is because the speed of light decreases in the medium, and since frequency is unchanged, the wavelength must decrease to maintain the same number of wave crests per second.

47. How does atmospheric refraction affect the apparent position of the sun at sunset?

Atmospheric refraction causes the sun to appear higher in the sky than its actual position at sunset. This effect is most pronounced near the horizon, where light travels through more atmosphere. As a result, we can still see the sun for a few minutes after it has geometrically set below the horizon.

48. How does the shape of a water droplet affect the formation of a rainbow?

The spherical shape of water droplets is crucial for rainbow formation. As light enters the droplet, it undergoes refraction, reflection, and dispersion. The spherical shape ensures that light exits the droplet at specific angles for each color, creating the familiar rainbow pattern.

49. What is the cause of the "green ray" phenomenon sometimes observed at sunset?

The "green ray" is a rare optical phenomenon caused by atmospheric refraction and dispersion. As the sun sets, its light is refracted and dispersed by the atmosphere. For a brief moment, when only a small portion of the sun is visible, the green light component may be the last to disappear, creating a flash of green.

50. How does the refractive index of a gemstone affect its brilliance?

A higher refractive index in a gemstone increases its brilliance by causing greater total internal reflection of light within the stone. This results in more light being reflected back to the observer's eye, creating a brighter and more sparkling appearance.

51. How does the refractive index of a medium affect the critical angle for total internal reflection?

The critical angle for total internal reflection decreases as the difference in refractive indices between two media increases. A higher refractive index in the first medium relative to the second results in a smaller critical angle, making total internal reflection more likely to occur.

52. What is the principle behind the operation of optical fibers in endoscopes?

Optical fibers in endoscopes use total internal reflection to transmit light and images. Light enters the fiber at an angle greater than the critical angle, causing it to reflect off the inner walls of the fiber repeatedly. This allows the transmission of images from inside the body to an external viewing device.

53. How does the dispersion of light in a prism depend on the prism's material?

The dispersion of light in a prism depends on the variation of the material's refractive index with wavelength. Materials with a greater change in refractive index across the visible spectrum (higher dispersion) will separate colors more effectively, producing a wider spectrum.

54. What is the cause of the "Fata Morgana" mirage?

The Fata Morgana mirage is caused by a temperature inversion, where a layer of warm air sits above a layer of cooler air. Light rays bend as they pass through these layers with different refractive indices, creating complex, often vertically stretched or inverted images of distant objects.

55. How does the refractive index of the eye's lens change to focus on objects at different distances?

The eye's lens changes shape to alter its refractive power, a process called accommodation. When focusing on nearby objects, the ciliary muscles contract, allowing the lens to become more convex and increase its refractive power. For distant objects, the muscles relax, flattening the lens and reducing its refractive power.