The refractive index of glass is 1.5. What is the speed of light in glass? (Speed of light in vacuum is 3.0 × 108 m s−1) - Physics


The refractive index of glass is 1.5. What is the speed of light in glass? (Speed of light in vacuum is 3.0 × 108 m s−1)


Solution 1

Refractive index of glass, μ = 1.5

Speed of light, c = 3 × 108 m/s

Speed of light in glass is given by the relation

`"v" = "c"/μ`

= `(3 xx 10^8)/1.5`

= 2 × 108 m/s

Hence, the speed of light in glass is 2 × 108 m/s.

Solution 2


Refractive index of glass = 1.5

Speed of light in air = 3.0 × 108 m/s

Speed of light in glass = ?

Applying the formula for refractive index, we get:

Refractive index of a medium = (Speed of light in air)/(Speed of light in medium)

For glass:

Refractive index of glass = (Speed of light in air)/(Speed of light in glass)

(Speed of light in glass) = (Speed of light in air)/(Refractive index of glass)

= (3.0 × 108)/1.5

= 2.0 × 108 m/s

Concept: Reflection and Refraction of Plane Wave at Plane Surface Using Huygens' Principle - Refraction of a Plane Wave
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Chapter 10: Wave Optics - Exercise [Page 383]


NCERT Physics Class 12
Chapter 10 Wave Optics
Exercise | Q 10.3 (a) | Page 383
NCERT Physics Class 12
Chapter 10 Wave Optics
Exercise | Q 3.1 | Page 383


Is the speed of light in glass independent of the colour of light? If not, which of the two colours red and violet travels slower in a glass prism?

Explain how Corpuscular theory predicts the speed of light in a medium, say, water, to be greater than the speed of light in vacuum. Is the prediction confirmed by experimental determination of the speed of light in water? If not, which alternative picture of light is consistent with the experiment?

Let us list some of the factors, which could possibly influence the speed of wave propagation:

(i) Nature of the source

(ii) Direction of propagation

(iii) Motion of the source and/or observer

(iv) Wavelength

(v) Intensity of the wave

On which of these factors, if any, does the speed of light in vacuum?

A plane wavefront propagating in a medium of refractive index 'μ1' is incident on a plane surface making the angle of incidence 'i' as shown in the figure. It enters into a medium of refractive index 'μ2' (μ2 > μ1). Use Huygens' construction of secondary wavelets to trace the propagation of the refracted wavefront. Hence verify Snell's law of refraction.

The wavefronts of a light wave travelling in vacuum are given by x + y + z = c. The angle made by the direction of propagation of light with the X-axis is _________ .

The wavefronts of light coming from a distant source of unknown shape are nearly _________ .

Light waves travel from optically rarer medium to optically denser medium. Its velocity decreases bec;ause of change in ______.

The optical properties of a medium are governed by the relative permittivity (εr) and relative permeability (µr). The refractive index is defined as `sqrt(µ_r ε_r) = n`. For ordinary material εr > 0 and µr > 0 and the positive sign is taken for the square root. In 1964, a Russian scientist V. Veselago postulated the existence of material with εr < 0 and µr < 0. Since then such ‘metamaterials’ have been produced in the laboratories and their optical properties studied. For such materials `n = - sqrt(µ_r ε_r)`. As light enters a medium of such refractive index the phases travel away from the direction of propagation.

  1. According to the description above show that if rays of light enter such a medium from air (refractive index = 1) at an angle θ in 2nd quadrant, them the refracted beam is in the 3rd quadrant.
  2. Prove that Snell’s law holds for such a medium.

In the figure, the optical fibre is l = 2 m long and has a diameter of d = 20 µm. If a ray of light is incident on one end of the fiber at angle θ1 = 40°, the number of reflections it makes before emerging from the other end is close to ______.

(Refractive index of fiber is 1.31 and sin40° = 0.64)

A glass slab of thickness 4 cm contains the same number of waves as 5 cm of water. When both are traversed by the same monochromatic light. If the refractive index of water is `(4/3)`. What is that of glass?

In the given figure, the face AC of the equilateral prism is immersed in a liquid of refractive index 'n'. For incident angle 60° at the side AC, the refracted light just grazes along face AC. The refractive index of the liquid n = `sqrtx/4`. The value of x is ______.

(Given refractive index of glass = 1.5)

White light is an incident at 20° on a material of silicate flint glass slab as shown. `μ_"violet"` = 1.66 and µr = 1.6. For what value of d will the separation be 1 mm in red and violet rays?


A light ray in a medium (RI = 5/3) enters another medium at an angle of 30°. The angle in the other medium is sin-1 (5/6). The incident angle must be increased such that the ray is completely reflected at minimum degrees is ______.

Using Huygens’s construction of secondary wavelets draw a diagram showing the passage of a plane wavefront from a denser to a rarer medium. Using it verifies Snell’s law.


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