Refraction and Dispersion of Light

Define Refraction

Refraction is defined as the bending of light when it travels from one medium to another medium. Here, the degree at which refraction occurs relies on the light's wavelength. Each light wave has a range or set of wavelengths and will so deviate in a different direction. 


What is Dispersion of Light?

Dispersion of light can be defined as the splitting of the light beam into its seven constituent colors when it is passed through any transparent media. This phenomenon was given by Sir Issac Newton in 1666 A.D. He discovered that white light consists of seven types of distinguished colors when sunlight passes through a glass prism. 

Newton conducted an experiment where he took a glass prism and passed light rays through it. Here, the glass prism splits the sunlight or rays into a band of several colors, mostly identified as seven colors on the wall. The band of colorful lights is known as "spectrum". As such, the bank of the spectrum consists of seven colors obtained by splitting the white light by passing through a glass prism. 


The Order of Colors Found out from the Lower End of the Spectrum is:

  1. Violet (V)

  2. Indigo (I)

  3. Blue (B)

  4. Green (G)

  5. Yellow (Y)

  6. Orange (O)

  7. Red (R)

The sequence of these seven colors that are obtained can be remembered by using the acronym "VIBGYOR".


Cause of Dispersion of Light

The dispersion of light diagram is shown below:

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The cause of dispersion of light through prism is that white light has a range array of seven colors, and each of that has a subsequent angle of deviation. As such, when light passes through a prism, different colors deviate from different angles. Therefore, those colors get separated and form a series of bands called a spectrum. Out of those seven colors, the red one deviates the least and has got the position on top of the spectrum. Whereas the violet color deviates most, that is why it has got the position at the bottom of the spectrum.


Here, the sole cause of dispersion of light is refraction.


Because of refraction, every color of light takes a different path after polychromatic light enters from the less dense medium to a large dense medium. This happens as per Snell's law. It states that sini()/sin(r) is different for a different color of light and medium where it travels. Therefore, the split light represents the component of the original incident light.


The above-mentioned explanation shows how dispersion occurs. One thing to be noted here is that in case of normal incidence, dispersion and refraction doesn't occur.

Fun facts- Have you ever seen the rainbow and got mesmerized by its natural beauty? They are the perfect phenomenon that occurs and is the best example to bring light for "dispersion of light" alongside refraction. This is the reason you can see rainbow-like occurrences in both crystals and prisms. 


What is Dispersion of White Light?

Dispersion of White Light by a Prism is shown here below:

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Wavelength is inversely proportional to the deviation where the light travels. Here, the prism only acts as a medium for dispersion made of seven different colors. Further, refraction occurs when light rays fall on it, and depending on that, the frequency and wavelength deviate differently at a different angle because of the difference in their velocities. The color deviates the least because it has a maximum wavelength, and the violet color deviates the most because of its lesser wavelength.

FAQ (Frequently Asked Questions)

1. Explain why Planets don't Twinkle but the Star does.

Stars twinkle because of the refraction of starlight in the atmospheric region. When the light from the star enters the atmosphere of earth, it undergoes continuous refraction before it reaches the earth's surface. The atmospheric refraction is because of the change in the refractive index at a range of atmospheric levels. Here, the starlight bends towards the normal, where the apparent point of stars is different from that of the actual one.


Since the atmospheric region doesn't remain constant, it changes. Since the traveling orbit of light rays coming from the star varies slightly, then the virtual position also varies and so the amount of light entering our eyes does flicker. Sometimes, it remains bright, and sometimes blurs. Because of the same reason, stars twinkle and planets do not as they are close to the earth and are said to be extended sources. If a planet is taken as a collection of point sources, then the total light that enters our eye on average is zero. This makes the twinkling effect of the planet to be null and void.

2. Give some Examples of Dispersion of Light.

Given below is the dispersion of light examples

  1. Dewdrops in morning during winter

  2. Formation of rainbow

  3. The Compact Disc

  4. Diamond

3. Explain the Formation of the Rainbow.

Ans- We can explain the formation of the rainbow by considering the process of dispersion of light. When it comes to the rainbow, we see its seven colors in the sky just after the rain when the sun shines. Here, the crucial condition to see a rainbow is that one should stand with his back facing sun in order to see the rainbow. 


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Normally, after rainfall, a huge number of water droplets get suspended in the atmosphere. These droplets of water function as small prisms. Therefore, when the white light emitted by the sun falls on the droplets, then the light gets split into several different colors and the rainbow gets formed.

4. What is the Difference Between Dispersion and Refraction of Light?

The main difference between refraction and dispersion of light is given here below:

As mentioned earlier, refraction is any bending of waves due to the change in speed. When water waves move through a range of depths and heights, there is the formation of refraction. 


On the other hand, dispersion depends on frequency and when we say that light is refracted by a prism, dispersion here means there will be a more high frequency of bending of light.


As such, refraction is the bending of light, and dispersion is the frequency dependency of the characteristics of light.