Tracing the Path of the Rays of Light through a Glass Prism

Prism is a triangular, transparent, optical object used primarily to study the movement and behaviour of a white light and its separation into a spectrum of light. When a light passes through a glass prism, due to some scientific phenomenon, it changes its direction and produces a colourful band of light emerging from the other side of the prism.This phenomenon is known as the refraction and dispersion of light. The incident ray, refracted ray and the emergent ray are the important particulars of this experiment.

Read Also: Difference Between Reflection and Refraction

Key Terms: Glass Prism, Refraction, Dispersion, Wavelengths, Incident Ray, Refracted Ray, Emergent Ray.

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What is a Prism?

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A Prism is a transparent optical object usually made of glass. A prism is in the shape of a pyramid deriving its name from the geometric prism as shown in the figure with two triangular bases and three rectangular lateral surfaces, inclined at an angle which is called the angle of the prism. 

Angle of the prism, also known as the refracting angle, is the angle made by any two refracting surfaces of the prism with each other. For an equilateral prism, the refracting angle is an acute angle of sixty degrees. When light is made to pass through a prism, these refracting surfaces separate this white light into a spectrum of colours. This process is known as Dispersion of Light.

What is Refraction of Light?

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What is Refraction of Light?

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Refraction is the phenomenon in which there is a change in the direction of a wave or a propagation of light when passing from one medium to another due to its change in speed.

To put simply, when light is made to travel from one medium to another, its speed changes due to factors such as density of the mediums, causing it to slow down and thus change its direction. Therefore, when light travels from air and passes through the prism glass, its speed changes due to the difference between the refraction index of air and glass causing the light to bend. 

Fun Fact: The different colours of the spectrum of light have different wavelengths respectively. Therefore, each colour or band of light refract differently. The colour with longer wavelength refracts or bends lesser, whereas, the colour with a shorter wavelength bends more. Thereby, when light passes through a glass prism, the colour violet having the shortest wavelength bends the most and the colour red bends the least as it has the longest wavelength.

Examples- Refraction is a common phenomenon that we witness in our daily lives. Some examples are twinkling of stars due to light refracting as it passes through various layers of the atmosphere, occurrence of rainbows, experiences of mirage, etc.

Read Also: Advanced Sunrise and Delayed Sunset 

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Difference Types of Angles formed in Prism

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According to Snell’s Law, when light travels from a rarer medium to a denser medium it bends towards the normal, and vice versa. Thus, when light passes from air to the prism glass it bends towards the normal, glass being denser than air. In the above figure, the incident ray PE is falling on the surface AB of the prism glass at the point E and is getting refracted towards the normal NE.

Similarly, after travelling through the glass the refracted ray travels from glass to air and due to change in density, the emergent ray FS bends away from the normal.

1. The bending of the light depends upon the angle of incidence, normal and refractive indices.
2. The angles that are formed during the experiment in the prism are the angle of incidence at point ‘P’, angle of emergence at point ‘Q’, the angle of prism at point ‘A’ and the angle of deviation at point ‘D’.
3. The angle formed between the incident ray and the emergent ray is called the angle of deviation.
4. Angle of deviation depends on the angle of the prism, angle of incidence, and material of the prism.

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Experiment

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Objective - To trace the path of the rays of light through a glass prism.

Materials Required

• A white sheet
• Soft Board
• Thumb pins
• 4-6 all pins
• Prism
• Pencil
• Scale
• Protractor
• Drawing board

Procedure

1. Paste a white sheet on top of the soft board using the thumb pins.
2. Place the glass prism on top of the paper with its triangular base resting on the paper and draw the outline of the prism using a pencil. Mark the boundary points as A, B and C. Remove the prism.
3. Draw a normal or a perpendicular line NEN’ at E on the surface AB of the prism. Then, draw a line PE making the angle PEB of 30 degrees to the normal. PE is the incident ray on surface AB.
4. Now, on the line PE mark two points P and Q at a distance of 5 cm from each other and fix them with pins.
5. Place the prism back on the paper on the assigned points ABC and try to see the points P and Q from the other opposite side of the prism on the surface AC.
6. Now, with that view, fix the points R and S in such a way that it appears to be vertically on the same straight line as P and Q from the surface AC side of the prism.
7. Remove the prism and all the pins and mark the points of P, Q, R and S.
8. Now, join the points R and S and then extend the line RS upto the refracting side of the prism AC to meet at the point F. The line FS now represents the emergent ray.
9. Now join the points E and F, making the line EF the path of the refracted ray.
10. Now we can repeat the experiment with different degrees of incident ray ranging from 30 degrees to 60 degrees.

Read Also: Refraction At Spherical Surfaces and By Lenses

Observation

We observe that:

1. On the surface AB, the ray of light passes from air to glass resulting in a bend towards the normal after refraction.
2. And on the surface AC, the ray of light travels from glass to air and hence bends away from the normal due to difference in density of the mediums
3. We can see that the emergent ray bends at an angle towards the incident ray forming the angle of deviation.

Result

The path PEFS represents the path of the light passing through the prism.

Conclusion

1. The incident ray bends towards the normal when entering the glass prism, and conversely it bends away from the normal during exit from the prism.
2. When angle of incidence increases, simultaneously the angle of deviation decreases upto the minimum value. After attaining the minimum value, it increases with an increase in incidence angle.

Read Also: Interference of light waves and young's experiment

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Precautions

1. Make sure that the pins are fixed at the correct point and in a straight line.
2. Use a good and sharp pencil to draw the boundary of the prism.
3. Hold the prism firmly while doing the experiment for accurate results.
4. Maintain a minimum distance of 5 cm between the points.
5. Draw and mark arrows properly to indicate the directions of incident ray, refracted ray and the emergent ray.
6. The angle of incidence should be within 30 degrees and 60 degrees and must not exceed above that.
7. The surface of the glass prism should be flat, smooth and transparent for proper experiment. 

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Things to Remember

1. Prism is a transparent optical instrument used to study behaviour of white light.
2. Prism consists of two triangular bases and three rectangular surfaces.
3. Dispersion of Light is the process of white light separating into a spectrum of colours when passed through a prism.
4. Refraction is the phenomenon of change in direction of a wave of light due to change in speed and density. 
5. Refraction through a prism forms various angles- angle of incidence, angle of prism, ang of emergence and angle of deviation.
6. The rays that pass through the prism are known as the incident ray, refracted ray and emergence ray.

Read Also: Difference Between Echo and Reverberation

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