Refraction of Light

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Characteristic Properties of Light:

  • Light is an electromagnetic radiation. It consists of transverse electromagnetic waves which travel at a speed of 3 × 108 m/s in vacuum.
  • When a monochromatic light travels from one medium to another, its wavelength changes but its frequency remains constant.
  • In the electromagnetic waves, the angle between electric field vector and magnetic field vector is 90°.
  • In the propagation of electromagnetic waves, the angle between the direction of propagation and the plane of polarization is zero.
  • In the propagation of electromagnetic waves, the angle between the plane of vibration and the plane of polarization is 90°.
  • The oscillating electric and magnetic vector of an electromagnetic wave is oriented along a mutually perpendicular direction and are in phase.
  • Electromagnetic waves transport energy, momentum, information.
  • Electromagnetic waves do not carry any charge. The energy of visible light is low (few eV).
  • Light travels in a straight line unless it is reflected by a polished surface or the medium of propagation is changed.

Visible Spectra:

  • The part of the spectrum of light which is visible to the human eye is called visible spectra.
  • The frequency of visible light varies from  3.8 x 1014 Hz to 7.8 x 1014 Hz. The corresponding wavelengths are 360 nm and 750 nm.
  • The colour sensation of the human eye is related to the wavelength of light. The light close to 780 nm appears red and light close to 380 nm appears violet. The human eye is more sensitive to yellow and green light.
  • Colour Wavelengths:  Red (620 – 780 nm) Orange (590 – 620 nm) Yellow (570 – 590 nm) Green (500 – 570 nm) Blue (450 – 500 nm) Violet (380 – 450 nm).

Monochromatic light:

  • A light having one single frequency or wavelength is called as monochromatic light (Mono means one and chroma mean colour).
  • e.g. LASER, Sodium vapour lamp.

Reflection of Light:

  • When light rays are incident on a polished surface, they are sent back in the same medium such that the angle of incidence is equal to the angle of reflection and the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
  • Laws  of Reflection:

    • Angle of incidence is equal to angle of reflection
    • The incident ray and the reflected ray lie on either side of the normal at the point of incidence
    • The incident ray, reflected ray and the normal at the point of incidence lie in the same plane.

Huygenes principle Reflection

Refraction of Light:

  • When light ray travelling in one optically active medium enters another optically active medium, then the light ray deviates from its path. This phenomenon is known as the refraction of light.
  • Laws  of Refraction:

    • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant and is equal to the refractive index of the medium.  This law is known as Snell’s law.
    • The incident ray and the refracted ray lie on the opposite side of the normal at the point of incidence.
    • The incident ray, refracted ray and the normal at the point of incidence lie in the same plane.

Important Terms Used:

Refraction of Light 01



Optical Medium:

  • The medium capable of transmitting light is called as an optical medium.

Isotropic medium:

  • The homogeneous medium, which has same properties in all the directions is called as an isotropic medium.

Incident ray:

  • The light ray, which is falling on reflecting or refracting surface is called as the incident ray.

Point of incidence:

  • The point at which the incident ray cuts the reflecting or refracting surface is called the point of incidence.

Normal:

  • A perpendicular drawn to the surface at the point of incidence is called the normal.

Angle of incidence:

  • The angle made by the incident ray with the normal at the point of incidence is called the angle of incidence.
  • It is denoted by ‘i’.

Angle of Refraction:

  • The angle made by the refracted ray with the normal at the point of incidence is called the angle of refraction.
  • It is denoted by ‘r’

Angle of Deviation:

  • The angle between the direction of incident ray to the direction of the refracted ray is called angle of deviation.
  • it is denoted by ‘δ’.

Glancing Angle:

  • The angle made by incident ray with refracting surface or interface is called glancing angle.

Refractive Index  of a Medium:

  • According to Huygens, the cause of refraction of light is that the velocity of light is different in different media.
  • The velocity of light is maximum in vacuum and air. The velocity of light in vacuum or air is denoted by letter ‘c’.
  • The relative refractive index of the second medium with respect to the first medium (1μ2)is defined as the ratio of the velocity of light in the first medium (v1) to the velocity of light in the second medium (v2). Mathematically,

Refraction of Light 02

  • When the first medium is vacuum or air the relative refractive index is known as the absolute refractive index.

Snell’s Law:

  • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is always constant and is equal to the refractive index of the medium.  This law is known as Snell’s law.
  • Mathematically,

Refraction of Light 04



Principle of Reversibility of Light:

  • If, after undergoing a number of reflections and refractions, the direction of a ray of light is reversed, the ray traces its original path in the reverse direction.
  • This principle is applicable to prisms and lens also.

Refraction of Light 06

Property:

  • If  1μis the  refractive index of the second medium with respect to the first medium  and  2μis  the refractive index of the first medium with respect to the second medium, then
  • Refraction of Light 03

Proof:

Refraction of Light 07

To show that 1μ2 = μ21

  • The absolute refractive index of the medium is defined as the ratio of the velocity of light in the vacuum to the velocity of light in the medium.

Absolute refractive index of the first medium = μ1  =  v/ c  ……………………… (1)

Absolute refractive index of the second medium = μ2  =  v/ c  …………………. (2)



Dividing equation (2)  by equation (1) we have

μ2 =   v2/v1    …………………. (3)

  • The relative refractive index of the second medium with respect to the first medium (1μ2)is defined as the ratio of the velocity of light in the first medium (v1) to the velocity of light in the second medium (v2). Mathematically,

1μ2 = v2/v1     …………………. (4)

From equation (3) and (4) we have

1μ2 = μ21      (Proved as required)



Example – 1:

  • A ray of light in air is incident on a glass surface making an angle of 30° with the surface. the angle of refraction in a glass is 35°16′. Find the angle of deviation of the ray.
  • Solution:
  • Given: Glancing angle = ig = 30°. Hence angle of incidence  = 90° – 30°  = 60°
  • To find : Angle of deviation = δ = ?

Angle of deviation = Angle of incidence – Angle of refraction

Angle of deviation = δ = 60°  –   35°16′ = 24°44′

Ans:  The angle of deviation of the ray is 24°44′

Example -2:

  • Find refractive index of water. Given velocities of light in air and water as 3 × 108 m/s and 2.25 × 108 m/s respectively.
  • Solution:
  • Given: Velocity of light in air = c = 3 × 108 m/s and velocity of light in water = v = 2.25 × 108 m/s
  • Fo find: Refractive index of water = μ = ?

μ = c /ν  = 3 × 10/2.25 × 108 = 1,333

Ans:  The refractive index of water is 1.33



Example – 3:

  • The refractive index of glass with respect to air is 1.5. Calculate the velocity of light in glass. Given velocity of light in air as 3 × 108 m/s.
  • Solution:
  • Given: Velocity of light in air = c = 3 × 108 m/s and  refractive index of glass = μ = 1.5
  • Fo find: velocity of light in glass = v = ?

μ = c /v  

∴   v = c/μ

∴   v = 3 × 10/1.5 = 2 × 108 m/s

Ans:  The velocity of light in glass is 2 × 108 m/s



Example – 4:

  • The refractive index of water w.r.t. air is 4/3. Find the relative refractive index in case when light travels from water into air.
  • Solution:
  • Given: refractive index of water w.r.t. air =  aμ= 4/3
  • Fo find: refractive index of air w.r.t. water =  wμ= ?

wμ=  =  1/aμw = 3/4 = 0.75



Ans:  The refractive index of air w.r.t. water is 0.75

Science > Physics > Refraction of LightYou are Here
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