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Refraction through a prism

For an equilateral prism , find two angle of incidence differing by 20 degrees showing same deviation of 40 degrees?

Asked Atambir Singh


Use the formula \delta =i_{1}+i_{2}-A

Here A = 60 degree (Since equilateral prism)

deviation is 40 degree

This gives A+\delta =i_{1}+i_{2}=100


the values of angles of incidence are 60 degree and 40 degree


Refraction – Disappearing letters

A rectangular block of glass is placed on a printed page lying on a horizontal table. what is the minimum value of u of glass so that the letters on page are not visible from any of the vertical faces of the block.

Asked Kushagr

How light maintain its speed after refraction?

How light maintain its speed after refraction? -(Naveen Saxena asked)


The speed of light in a medium is constant. When light enters from one medium to another, there is a change in the speed of light and the change is almost instantaneous. The speed of light in a medium depends on the electric and magnetic properties of the medium, more specifically, the electric permitivity and magnetic permeability of the medium.

The speed of light in a medium is given by

Wikipedia says

“At the microscale, an electromagnetic wave’s phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the electric susceptibility of the medium. (Similarly, the magnetic field creates a disturbance proportional to the magnetic susceptibility.) As the electromagnetic fields oscillate in the wave, the charges in the material will be “shaken” back and forth at the same frequency. The charges thus radiate their own electromagnetic wave that is at the same frequency, but usually with a phase delay, as the charges may move out of phase with the force driving them . The light wave traveling in the medium is the macroscopic superposition (sum) of all such contributions in the material: The original wave plus the waves radiated by all the moving charges. This wave is typically a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave’s phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions or even at other frequencies .

Depending on the relative phase of the original driving wave and the waves radiated by the charge motion, there are several possibilities:

  • If the electrons emit a light wave which is 90° out of phase with the light wave shaking them, it will cause the total light wave to travel more slowly. This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1.
  • If the electrons emit a light wave which is 270° out of phase with the light wave shaking them, it will cause the total light wave to travel more quickly. This is called “anomalous refraction”, and is observed close to absorption lines, with X-rays, and in some microwave systems. It corresponds to a refractive index less than 1. (Even though the phase velocity of light is greater than the speed of light in vacuum c, the signal velocity is not, as discussed above). If the response is sufficiently strong and out-of-phase, the result is negative refractive index discussed below.
  • If the electrons emit a light wave which is 180° out of phase with the light wave shaking them, it will destructively interfere with the original light to reduce the total light intensity. This is light absorption in opaque materials and corresponds to an imaginary refractive index.
  • If the electrons emit a light wave which is in phase with the light wave shaking them, it will amplify the light wave. This is rare, but occurs in lasers due to stimulated emission. It corresponds to an imaginary index of refraction, with the opposite sign as absorption.

For most materials at visible-light frequencies, the phase is somewhere between 90° and 180°, corresponding to a combination of both refraction and absorption.”


How light maintain its speed after refraction.

Dr. Naveen Saxena

n=c/v,naturally velocity of light decrease in refraction.but even even then it has the velocity in the order of 108m.In refraction,

the frequency of light remains the same ,but the wavelength




What accounts for the acceleration or deceleration of light when it travels from one medium to another?

“When light passes through different mediums speed changes but who provide acceleration to change velocity?”

This question was posted by Jishnu

Answer: Just imagine you are participating in a cross country race. When the road is good you can travel faster; but if the path is a marshy place, your speed will decrease. This is just an analogy. To understand it better, you should know that the way light travels in a medium is different from the way it travels in vacuum.


Particle theory of light and refraction

According to particle theory of light if a light ray bends toward the normal while entering from medium 1 to medium 2 then how the velocity of light in medium 2 is greater than the velocity of light in medium 1 ?


When a medium enters a denser medium from a rarer medium, it bends towards the normal. This is the observation we have in hand and this phenomenon is called refraction.

Sir Isaac Newton tried to explain the phenomenon of refraction using his particle theory. He said that the particles of the denser medium attracts the particles with stronger force towards it which makes it bend towards the normal.

If there is such a force of attraction, then the speed of light would increase inside a denser medium. When the velocity of light in different media was determined by Foucault and other scientists, it was found that the velocity of light in denser medium is less than that in a rarer medium. So, Newton’s explanation of refraction was proved wrong.

Why some substances are transparent while some others are opaque

Why some substance being solid enable light to pass through them (like glass, diamond (the hardest substance etc) but other who have less density do not permit refraction?

(Asked Brahmadutta Mahapatra)

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March 2021

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