Understand Refraction: Key Concepts and Experiments for CBSE 2025-26
Physics Experiment - Tracing of the Path of the Rays of Light through a Glass Prism
A prism is a three-dimensional transparent object with a base and two refractive faces and depending on the type of prism the shape of these faces change, i.e., triangular, square, or n-side polygon. A ray of light, which incident on the one surface of the prism deviates from its original path when emergent from the other side of the faces. This ray of light, passing through the glass prism, encounters the phenomenon of refraction of light, reflection of light, and dispersion. The formation of a rainbow on a rainy day is due to the phenomenon of the dispersion of sunlight because of the water droplets. The path of incident rays can be traced and the angle of deviation of the incident light can be traced by the difference between the original ray and the emergent rays.
Table of Content
Aim
Theory
Procedure
Observations
Result
Aim
Trace the path of the rays of light passing through a glass prism.
Apparatus Required
List of tools required to perform the experiment:
White light source
Glass Prism
Measuring Scale
Paper Pins (8-9)
White Paper
Pencil
Protector
Drawing Board
Theory
A prism can be imagined as a piece of solid geometry; if someone asks that a prism is surrounded by how many surfaces, one can answer that it has one triangular base and three lateral surfaces. This can be used as an optical device to bend, refract, and disperse the white light.
For a Prism, a relation between the prism angle (A), the refractive index of the medium (n), and the deviation of a light beam due to the Prism (D) can be defined as, such that,
\[n = \dfrac{{{\rm{sin}}\left( {\dfrac{{A + D}}{2}} \right)}}{{{\rm{sin}}\dfrac{A}{2}}}\]
After dispersion, a light beam deviates from its original path, and the angle between the incident and emerging rays is called the angle of deviation and can be defined as,
\[D = {i_1} + {i_2} - A\]
Where \[{i_1}\;and\;{i_2}\] are the incident and emergent angles of the beam, respectively.
Procedure
Path of the ray of light through a glass prism.
Please follow these steps one by one:
Fix the white paper on the drawing board with the help of 4 paper pins at the corners.
Now, keep your glass prism on the white paper at its triangular base and mark its outline with the pencil on the white paper.
Draw a normal SPT on the face of the prism OB and make any angle between 30° to 60° (say 45°) with the normal.
Now, fix two paper pins on the line NP at a distance of 5-10 cm and mark these pins as N and X.
View the images of these paper pins (N and X) on the other side of the prism face, i.e., OC, and fix the two paper pins as Y and M so they should appear on the straight line as N and X when seen from the OC face.
Now, remove all the paper pins and the prism.
Now, extend the straight line MY such that it meets the outline of the prism at point Q, so M, Y, and Q are on a straight line/collinear. In the same manner, extend the line NX such that it meets the outline of the prism on the OB face and therefore NXP will be on a straight line.
NXP is the incident ray and MYQ is the emergent ray when the emergent and the incident rays are extended backward and forward respectively to meet at point R.
Now mark the different angles made by the incident and emergent rays as shown in the experimental setup part, i.e., angle of incidence, angle of refraction, and angle of deviation ∠D.
Repeat the experiment for more angles between 30° and 60° (for example 40°, 50°, etc.).
Observations
The incident ray of light (NXP) strikes at the OB surface of the prism and bends towards the normal SPT as it enters from the rarer (air) to the denser (glass) medium.
The ray of light emerges from the OC surface of the prism and bends away from the normal as it enters the rares (air) medium from a denser (glass) medium.
We observe that the incident light deviates from its original path, i.e., when we trace back the emergent ray it strikes at an angle to the incident ray.
Results
The path of the incident ray falling on the OB face of the glass prism is shown with the ray ……..
The path of the emergence ray exiting from the OC face of the prism is shown with the ray ………
The angle of deviation of the ray of the light is measured and has the values of …. (incidence angle = ….), …. (incidence angle = ….), and …. (incidence angle = ….).
Precautions
Please fix the white paper on the board without a tilt.
Draw straight lines very carefully.
Do not move the position of the prism during the measurement.
Mark the position of the paper pins whenever removed.
To watch the alignment of the paper pins, move your head carefully to the left or right to align.
While drawing the arrows for the incident, the reflected and transmitted beams use proper naming and directions.
Make sure that the source of light is a white light source.
Lab Manual Questions
1. Define angle of deviation, when light passes through a glass prism?
Ans: The angle between the original path of the ray of light and the emergence path of the light ray is called the angle of the deviation, and defined as
∠D = ∠i1+∠i2-∠A
2. Give the reason behind the bending of the ray of light when it passes through a glass prism?
Ans: The ray of light bends towards the normal while passing through the prism as it enters from the optically rarer medium (air) to the optically denser medium (glass) following the Snell’s law, and vice versa. When it exits from the glass prim and moves away from the normal, and therefore we observe a bend of the ray of light towards the base.
3. What are the factors that control the angle of deviation of the glass prism?
Ans: The angle of deviation of the glass prism depends on the angle of incidence (∠i1), angle of emergence (∠i2), and prism angle (∠A).
4. Why does the white light split into the colours when it passes through a glass prism?
Ans: The white light splits into the different colours while passing through the glass prism since each wavelength of the light in the visible spectrum bends at a different angle, and therefore we observe a spectrum of the visible light ranging from the violet to red.
Viva Questions
1. What is dispersion?
Ans: Dispersion is the distribution of white light into different colours according to their wavelengths.
2. What is the refractive index of a medium?
Ans: The refractive index of a medium is defined as the speed of light in the medium divided by the speed of light in the vacuum.
3. Why do different wavelengths of light bend at different angles?
Ans: Since the refractive index of light is different for the different wavelengths.
4. Name some materials, which can be used to make a Prism.
Ans: Glass, Acrylic, Fluorite, and Quartz.
5. What is the meaning of the angle of deviation of the Prism?
Ans: The Angle of deviation of the prism is defined as the angle between the original beam path and the outgoing light.
6. Define the formula of the angle of deviation?
Ans: The angle of the deviation can be given as \[D = {i_1} + {i_2} - A\].
7. What colours do you observe after dispersion?
Ans: VIBGYOR, where V (Violet), I (Indigo), B (Blue), G (Green), Y (Yellow), O(Orange), and R (Red).
8. What will happen if you combine two prisms and then observe the dispersion of white light?
Ans: Outgoing light will be again white light. If we join two prisms then the emergent rays of the first prism (which are dispersed) will work as the incident beams of the second prism and therefore they will combine to give a white light to the emergent ray.
9. Does light ray travel in a curved path or a straight path? How do you utilise this in your experiment?
Ans: Light ray travels in a straight path, therefore, we have connected all the points on the incident and emergent ray in our experiment with the straight line.
10. What will happen if you use a monochromatic light instead of white light?
Ans: In the case of monochromatic light there will be no dispersion, only a ray of light will deviate from its path.
Practical-Based Questions
1. Which tool can be used instead of a prism for dispersion?
A. Lens
B. Mirror
C. Grating
D. Convex Lens
Ans: C. Grating
2. Which colour has the largest wavelength?
A. Red
B. Yellow
C. Violet
D. Green
Ans: A. Red
3. What is the range of the visible spectrum?
A. 300-700 nm
B. 100-800 nm
C. 500-1000 nm
D. 100-500 nm
Ans: A. 300-700 nm
4. What is monochromatic light?
A. Light of single wavelength
B. Light of multiple wavelengths
C. White light
D. Light from a Torch
Ans: A. Light of a single wavelength
5. What can be a source of monochromatic light?
A. Torch
B. Laser
C. Sun
D. Candle
Ans: B. Laser
6. Define the unit of the refractive index?
A. m/s
B. unitless
C. Å
D. nm
Ans: B. unitless
7. What will happen to the angle of bending of light if we increase the refractive index of the prism?
A. Increase
B. Decrease
C. No change
D. Depends on the wavelength
Ans: B. Decrease
8. Phenomenon happens because of a Prism, is
A. Dispersion
B. Reflection
C. Total Internal Reflection
D. All of the above
Ans: D. All of the above
9. Factor which governs the phenomenon of dispersion in a Prism?
A. Material of Prism
B. Wavelength of light
C. Medium
D. All of the above
Ans: D. All of the above
10. Which colour is observed near the bottom of the prism after dispersion?
A. Green
B. Red
C. Violet
D. Yellow
Ans: C. Violet
Conclusion
From the experiment with prism, one can conclude that the ray of light deviates from its path when passing through a glass prism, and this deviation depends on the angle of incidence and prism angle. Therefore, the ray of light with different angles will feel the different angles of deviation and always travel in straight lines.
FAQs on Stepwise Guide: How Light Travels and Bends Through a Glass Prism (CBSE Class 10)
1. What is a glass prism? For a 1-mark question, how is the 'angle of the prism' defined?
A glass prism is a solid, transparent, and homogeneous optical object with at least two plane surfaces inclined at an acute angle. The angle of the prism (A) is the angle between its two lateral, non-parallel refracting surfaces. This is a fundamental definition frequently asked in exams.
2. With a neat diagram, explain the path of a light ray passing through a glass prism. What are the 4 important angles to label for full marks?
When a ray of light enters a glass prism, it bends towards the normal as it goes from a rarer medium (air) to a denser medium (glass). As it exits the prism, it bends away from the normal, moving from a denser to a rarer medium. To secure full marks in a board exam, you must label:
- Angle of incidence (i): Angle between the incident ray and the normal.
- Angle of refraction (r): Angle between the refracted ray and the normal inside the prism.
- Angle of emergence (e): Angle between the emergent ray and the normal on the exit face.
- Angle of deviation (D): The angle between the extended incident ray and the emergent ray.
3. Why does a glass prism split white light into a spectrum of colours? Explain the underlying reason.
A glass prism splits white light because the material of the prism (glass) has a slightly different refractive index for different colours (or wavelengths) of light. This phenomenon is called dispersion. When white light enters the prism, each colour bends at a slightly different angle. Since violet light has the shortest wavelength, it slows down the most and bends the most. Red light, with the longest wavelength, slows down the least and therefore bends the least. This difference in bending angles separates the colours into a visible spectrum (VIBGYOR).
4. What is the phenomenon of dispersion of light? List the colours of the spectrum in the order of increasing wavelength.
Dispersion is the splitting of white light into its constituent colours when it passes through a refracting medium like a glass prism. The band of seven colours obtained is called a spectrum. In order of increasing wavelength (and decreasing deviation), the colours are: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR).
5. If a ray of red light and a ray of blue light are passed through a glass prism under identical conditions, which will show a greater angle of deviation and why?
The blue light ray will show a greater angle of deviation. This is a high-order thinking question based on the principle of dispersion. The reason is that the refractive index of glass for blue light is higher than that for red light. According to Snell's law, a higher refractive index results in more bending of light. Therefore, blue light bends more than red light at both refracting surfaces of the prism, leading to a larger overall deviation.
6. What is the significance of Newton's recombination experiment using two identical prisms for the CBSE Class 10 syllabus?
Newton's experiment is highly significant as it proves that a prism does not create colours but merely separates the colours already present in white light. In this experiment, the first prism disperses white light into its seven colours. When a second, identical but inverted prism is placed in the path, it recombines these seven colours to produce a beam of white light again. This is a frequently discussed concept in exams to test the understanding of dispersion.
7. What is the mathematical relationship between the angle of incidence (i), angle of emergence (e), angle of the prism (A), and the angle of deviation (D)?
For a ray of light passing through a prism, the relationship connecting these four important angles is given by the formula: i + e = A + D. This equation is crucial for solving numerical problems related to the refraction of light through a prism in the Class 10 board exams for the 2025-26 session.
8. A glass slab and a glass prism are both transparent. Why does a prism produce a clear spectrum while a glass slab does not?
A glass slab does not produce a spectrum because its two refracting surfaces are parallel to each other. While dispersion occurs at the first surface, the emergent rays from the second, parallel surface recombine. The dispersion produced at the first surface is equal and opposite to that produced at the second surface, cancelling the effect. In a prism, the refracting surfaces are inclined at an angle, which causes the deviation of each colour to be permanent and in the same direction, resulting in a visible spectrum.
9. State two important applications of prisms that are relevant for board exam questions.
Two important applications of prisms frequently cited in exams are:
- In Periscopes: Right-angled prisms are used to reflect light at 90 degrees, allowing vision over obstacles.
- In Spectrometers/Spectroscopes: Prisms are used to disperse light from a source into its constituent spectrum, which can then be analysed.
