Refraction Light Glass Prism


The direction of light immediately changes after it passes from one medium to another. In simple words, light bends while travelling from one medium to another. In the case of a glass prism, the degree of refraction varies with the light's wavelength. It eventually gets segregated into seven different lights while passing through a glass prism in real-time. When the wave of the light travels from one optical medium to another, refraction takes happens. It is mostly caused by the difference in optical density between different mediums.

What is the Refraction of Light in a glass prism?

In a glass prism, when the ray of light directly falls on one side of the prism, the light gets immediately refracted due to its bending towards the normal medium. This process takes place due to the movement between an optically rarer medium towards an optically denser medium (Tutormate, 2022).

  • For example, when light falls on air and then travels towards a glass prism, the process of refraction takes place there as the light bends from a rarer medium to a denser medium.

After reaching the other side of the prism, the light gets refracted again due to bending towards the normal side. This phenomenon happens because light has travelled from an optically denser medium to an optically rarer medium (Patankar, 2021).

  • For example, when the light travels via a prism and then exits on the other side of the prism, it directly touches the air. So, here the process of refraction takes place between a glass prism and air.

Figure 1: Refraction through a glass prism

Within a prism, the light's direction changes, as it bends while it passes from one medium to another. Here, the degree of refraction can vary with the light's wavelength. In addition, the speed of light within different media also inversely varies with the refractive index of a certain medium of light. In a glass prism, if the PQR is kept on the base of QR, the light ray (AB) then incident on the face of the PQ of the prism. So, the AB here appears as an incident ray that allows the prism to refract the light in seven different colour components.

Process of Refraction of Light through a glass prism

A glass prism has three rectangular lateral surfaces and two triangular bases that are inclined towards the angle of the prism. If a prism is observed closely, it can be seen that, from the top, light is entering the prism through a triangular pathway.

As per the law of refraction by Snell, when light travels from a rarer medium (ex: air) to a denser medium (ex: prism), light bends quickly towards the normal medium (Idris & Yusibani, 2020). The same phenomenon can be seen when light travels from a denser medium (ex: prism) towards a rarer medium (ex: air).

Figure 2: Passing white light through a glass prism

Similarly, when a beam of white light passes through a glass prism, it is segregated into seven colours, formed by the spectrum of the white light. When white light passes through a glass prism, the red colour appears with the highest wavelength whereas the violet colour appears with the lowest wavelength (Tutormate, 2022).

Required rays in the process of Refraction in a glass prism

Incident Ray

The incident ray is the first ray of light that falls on the prism at first. This is one of the essential components for instigating the initial stage of refraction within a glass prism.

Refracted ray

The refracted ray is the ray of light that is refracted after entering a glass prism. After entering a glass prism, a refracted ray starts to bend, eventually starting the process of refraction within a glass prism.

Emergent Ray

This ray is the result of the process of refraction of light within a glass prism. After the refracted ray is scattered into seven different types, it finally starts to emerge from the prism into a rarer medium (Haines, 2021). This ray is commonly called the emergent ray, one of the essential components for the process of refraction in a glass prism.


When normal light is passed through a glass prism, the incident ray becomes a refracted ray within the prism and exits as an emergent ray from a glass prism. While moving from the glass prism towards the air, the emergent ray bends from its normal form and deviates according to the laws of refraction. After the ray is refracted, it gets scattered into seven different colours through a prism. At this time, the different colours of light wave experience different degrees of deviation eventually enabling the light to split into different colour components after it is subjected to the laws of refraction of light.


Q1. What causes the bending of light within a glass prism?

While travelling through glass prism different colours of light travel at different speeds. Due to different speeds of lights at the same time, they are bent in different amounts within a glass prism.

Q2. How many surfaces does a glass prism have for the refraction of light?

A glass prism generally has 3 rectangular surfaces and 2 triangular surfaces. The rectangular surfaces are denoted as ABCF, BCDE and AFDE.

Q3. How many times a ray of light is refracted within a glass prism?

A ray of light travels only two times within a glass prism, first when it enters the prism and the second time when it emerges from the prism. The refraction happens two times because the refracting surfaces of a prism are not parallel with one another.

Q4. Why does the bending of light slow down within a glass prism?

Light refracts with a different refractive index whenever it travels through a refractive substance. Due to that, the ray of light experiences a change in its direction which eventually slows down it within a glass prism.



Haines, E. (2021). Reflection and Refraction Formulas. In Ray Tracing Gems II (pp. 105-108). Apress, Berkeley, CA. Retrieved from:

Idris, N., & Yusibani, E. (2020). The effect of the thickness of the glass plate of a hollow prism on the accuracy of measuring the refractive index of edible oil. Optik, 217, 164834. Retrieved from:

Patankar, P. R. (2021). Determination of Pollution of Four River Water by Using Refractive Index Method. International Journal of Recent Advances in Multidisciplinary Topics, 2(7), 11-12. Retrieved from:


Teachoo, (2022). Refraction of light through prism. Retrieved from: [Retrieved on 9th June, 2022]

Tutormate, (2022). Reflection of white light through glass prism. Retrieved from: [Retrieved on 9th June, 2022]

Updated on: 13-Oct-2022

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