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Refraction of light through a glass prism

Tutormate > CBSE Syllabus-Class 10th Physics > Refraction of light through a glass prism

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  • The direction of light changes as it passes from one medium to another. In other words, light bends as it travels from one medium to another. This phenomenon is known as refraction.
  • When light waves travel from one optical medium to another, they get refracted due to difference in optical density of the two media.
  • The degree of refraction varies with the wavelength of light.
  • The speed of light in different media varies inversely with the refractive index of a medium.
  • Angle of Prism: A glass prism has two triangular bases and three rectangular lateral surfaces that are inclined at an angle, called the angle of the prism.
  • A glass prism PQR has been kept on its base QR. A ray of light AB is incident on the faces PQ of the prism. Here AB is an incident ray.
  • When a ray of light passes through a prism, it bends towards the base of the prism: The incident ray AB is incident is going from air(rare medium) into glass (denser medium), so it bends towards the normal BN’ and goes along the direction BC inside the glass prism. Thus BC is the refracted ray of light which bends towards the base QR of the prism.
  • When the ray of light BC travelling in the glass prism comes out into air at point C, refraction takes place again.
  • Since the ray BC is going from the glass (denser medium) into air(rarer medium), the ray bends away from the normal MC and goes along the direction CD in the form of emergent ray.


  • Spectrum of white light: The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is called spectrum of white light.
  • White light is a mixture of seven colours of different wavelengths (colors) viz. violet, indigo, blue, green, yellow and red
  • Red colour has the highest wavelength while the violet colourhas the lowest wavelength.
  • As the ray of white light passes through a prism, the various constituents of the white light deviate through different angles and we see the incident ray splitting into 7 different rays on coming out of the prism.
  • Prism only acts as a medium for the separation of the seven colors. When light falls on the glass prism, refraction takes place.

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