Vellore Institute of Technology
BPHY101P Engineering Physics Lab Manual
DEPARTMENT OF PHYSICS
SCHOOL OF ADVANCED SCIENCES
VELLORE
INSTITUTE
OF
TECHNOLOGY,
VELLORE
LAB MANUALS
ENGINEERING PHYSICS LAB
(BPHY101P)
WINTER SEMESTER 2024-25
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Engineering Physics Lab Manual: Experiments and Concepts for BPHY101P, Study Guides, Projects, Research of Physics
This lab manual provides a comprehensive guide to various experiments in engineering physics, covering topics like determining the fundamental frequency of a stretched string, planck's constant and work function using the photoelectric effect, refractive index of a glass prism, and wavelength of a laser source using diffraction grating. It includes detailed procedures, theoretical explanations, and analysis questions, making it a valuable resource for students in engineering physics.
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2024/2025
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DEPARTMENT OF PHYSICS
SCHOOL OF ADVANCED SCIENCES
VELLORE INSTITUTE OF TECHNOLOGY, VELLORE
LAB MANUALS
ENGINEERING PHYSICS LAB
(BPHY101P)
WINTER SEMESTER 2024 - 25
Steps to be followed by the students,
while preparing the laboratory report
Prior preparation
1. Objective of the experiment :
2. Apparatus Required :
3. Formulae
4. Schematic diagram(s)
5. Model graph(s)
6. Table
During the lab session
7. Observations
8. Calculations
Before the due date
9. Inferences
Results and discussion
Submission of soft copy of the report in VTOP
Because the wire will be pulled twice in every cycle, at resonance, the wire will vibrate with a frequency twice that of the frequency of the alternating current in the coil. So, the frequency of the alternating current ( f ) in the coil is half of the frequency of the wire in it’s fundamental mode. i.e f =. ……………….(5) From eq (4), 4n^2 l^2 μ = T or l^2 =. T ....................................................................................... (6) If a graph is plotted between l^2 on y-axis and T on x-axis, it will be a straight line with slope equal to. From the values of slope, n can be calculated with the help of equation n = ……………….(7) and hence the frequency of alternating current f = n /. The sources of errors in this measurement are (1) Friction between the pulley and the magnetic wire passing over it. Because of this, the values of tension acting on the wire will be estimated less than that of the actual tension. (2) Instability in the frequency of AC supply. The error in the measurement can be estimated as follows. % error in frequency of alternating current = × 100.
Procedure
- Set up the sonometer by carefully stretching the wire and adding a load on the hanger.
- Adjust the position of electromagnetic coil in such a way it’s pole lies close to the middle of the sonometer wire.
- Switch ON the AC source and adjust the length of the vibrating portion of the wire by varying the positions of the wedges on both sides until the amplitude of the vibrating portion of the string reaches a maximum.
- Measure the vibrating length and make a note of the tension in the string.
- Increase the weight in steps (100 g) and repeat the measurement of vibrating length for every values of change in weight.
- Make a note of the mass per unit length of the sonometer wire used.
- Switch OFF the AC supply and remove the weights from the hanger.
Precautions
✓ Pulley should be as frictionless as possible. ✓ Edges of the wedge should be sharp. ✓ Tip of the electromagnetic coil should be close to the middle of the sonometer wire. ✓ The sonometer wire should not have any bends or kinks.
Inferences/Conclusions
Questions on related concepts (Self-assessment)
- What is the difference between AC and DC?
- How does the magnetic field generated by the electromagnetic coil look like (schematic drawing)?
- What is the force that makes the sonometer wire to vibrate?
- Why do we need two wedges to perform this experiment?
- Why is the electromagnetic coil preferably placed in the middle of the two wedges?
Determination of Planck’s constant and work function of a
metal using Photoelectric Effect
Objective
To determine Planck’s constant and work function of a given metal using the photoelectric effect.
Apparatus to be used
Photoelectric equipment, filters of different colours
Basic theory
It was observed as early as 1905 that most metals emit electrons when their surface is irradiated with radiation. This phenomenon of emission of electrons from the metal surface exposed to the light of suitable frequency is known as the photoelectric emission/photoelectric effect. The electrons emitted in this process are known as photoelectrons, and the current constituted by these electrons is known as photoelectric current. The basic experimental set up explaining the photoelectric effect is given below. The detailed study of this effect has shown:
- That the emission process depends strongly on the frequency of radiation.
- For each metal, a critical frequency exists such that light of lower frequency cannot eject electrons, whilst light of higher frequency always does irrespective of light intensity.
- The emission of electrons occurs within a very short time interval after the arrival of the radiation
- The number of electrons is directly proportional to the intensity of this radiation. The experimental results obtained from this experiment are among the most substantial evidence which prove that the electromagnetic radiation is quantized, and each quanta consisting of packets of energy, where is the frequency of the radiation and is Planck’s constant. These quanta are called photons.
Procedure
The structure of the experimental set-up and its basic functionalities are demonstrated as:
1. Vacuum Phototube. The sensitive component. 2. The removable forepart is used to install the colour filters and a focus lens fixed in the back end. 3. A scale of 40 cm in length. The centre of the vacuum phototube is used as the zero point. 4. Colour filter Set. Five pieces 5. Light Source, 12V/35W halogen tungsten lamp. 6. To move the light source to adjust the distance between the light Source and the vacuum phototube. 7. Digital Meter. Show current (μA), or voltage (V). 8. Display mode switch. For switching the display between voltage and current. 9. Current Multiplier. 10. Switch to adjust the appropriate intensity of incident light. 11. Accelerate voltage adjustor. Knob for adjusting accelerate voltage. 12. Voltage direction switch. Switch for choosing stopping potential. 13. Power switch. 14. Power indicator.
For determination of Planck’s Constant and work function:
1. Adjust the distance between the Light Source enclosure and the Photodiode enclosure so that the general spacing is between 20.0 cm to 40.0 cm. NOTE: The recommended distance is 25.0 cm. (3 & 6) 2. Turn ON the light source by pressing the power switch (13). Make sure the power indicator (14) turns green LED On. 3. Allow the light source and the apparatus to warm up for 10 minutes. 4. Insert the red colour filter (635 nm) into the port (2), set the light intensity switch (10) at strong light for an appropriate photocurrent, voltage direction switch (12) at ‘+‘, accelerating voltage knob (11) at the minimum position and display mode switch (8) at current display. 5. Set the current multiplier switch (9) for a suitable amount of current on display. 6. Set the voltage direction switch (12) at ‘-‘, then increase the de-accelerating voltage using the knob (11 )to decrease the photocurrent to zero. 7. Measure the de-accelerating voltage/stopping potential (Vs) corresponding to zero current of 635nm wavelength by setting the switch (8) into Voltage display mode. 8. Repeat steps 4-6 for other colour filters of different wavelengths and measure the corresponding stopping potential. 9. Once all measurements are done, remove the colour filters, Put back the blank cap to nozzle (3), Set the voltage direction switch (12) at ‘+‘, the accelerating voltage knob (11) to zero, switch (8) to current display mode, and TURN-OFF the power switch (13). 10. Return the colour filters. 11. Do the calculation and plotting figures from the obtained experimental data.
Observations
Sl. No. Incident Photon Wavelength (Filters) Frequency (Hz) Stopping Potential (Vs in Volts) 1 Red (635 nm) 2 Orange (570 nm) 3 Yellow (540 nm) 4 Green (500 nm) 5 Blue (460 nm)
Model graph
- Plot a graph of Stopping Potential (Vs) versus Frequency ( 1014 Hz).
- Find the slope of the best-fit line through the data points on the graph.
Inferences/Conclusions
- ……………………………………………..
- ……………………………………………..
- ……………………………………………..
Precautions
- This instrument should be operated in a dry, cool indoor space.
- The instrument should be kept in a dust- and moisture-proof environment; if there is dust on the phototube, colour filter, lens, etc., clean it using absorbent cotton with a few drops of alcohol.
- The colour filter should be stored in a dry and dust-proof environment.
- Do not play with the knobs for random movements.
- Do not put scratch marks on colour filters
- While applying the negative potential, move the knob slowly and wait 2 secs after each move.
- After finishing the experiment, remember to switch off the power (14) and cover the drawtube (2) with the lens blank cover provided. Phototube is a light-sensitive device, and its sensitivity decrease with exposure to light and due to aging.
Questions on related concepts (Self-assessment)
Q1. What are the applications of photoelectric effect? Q2. What is the significance of work function? Q3. Are all the metals useful for photoelectric effect? Justify your answer. Q4. Why photoelectric effect cannot be explained by classical physics? Q5. What will be the stopping potential if intensity is tripled? Q6. Explain the relationship between the intensity of radiation and photoelectric current. Q7. What is the difference between photoelectric current and photocurrent? Q8. How does light intensity affect the Stopping Potential? Q9. How does your calculated value of h compare to the accepted value? Q10. What do you think may account for the difference – if any – between your calculated value of h and the accepted value?
Further references
- https://javalab.org/en/photoelectric_effect_2_en/ (Simulation)
- https://applets.kcvs.ca/photoelectricEffect/PhotoElectric.html# (Simulation)
- https://youtu.be/kS4ECdzONfE
- https://youtu.be/5QRR0JIzSX
- https://drive.google.com/file/d/10pespgTuNxCA-186EMShDaMwiFjU57YB/view?usp=share_link (Video Demonstration)
Integrated Optics – Refractive Index of glass prism
Objective
To determine the refractive index of the glass prism using spectrometer for a given colour.
Apparatus to be used
Basic theory
➢ Spectrometer ➢ Spirit level ➢ Magnifying glass ➢ Glass prism ➢ Mercury vapour lamp The spectrometer is an instrument for analysing the spectra of radiations. The glass-prism spectrometer is suitable for measuring ray deviations and refractive indices. When a beam of light
(iii) Adjustment of the collimator
The telescope is brought along the axial line with the collimator. The slit of the collimator is illuminated by a source of light. The distance between the slit and the lens of the collimator is adjusted until a clear image of the slit (Slit thickness should be as narrow as possible) is seen at the cross wires of the telescope. Since the telescope is already adjusted for parallel rays, a well-defined image of the slit can be formed, only when the light rays emerging from the collimator are parallel.
(iv) Levelling the prism table
The horizontal level of prism table is adjusted using a spirit level and levelling screws.
NOTE:
➢ Once the telescope is focused at the distant object it should not be disturbed throughout the experiment. ➢ The verniers (Vernier A and Vernier B) should not be interchanged throughout the experiment. ➢ The Spectrum obtained for the Mercury lamp that was visible with the resolution of the prism is as follows, given from Left to Right as observed: Red (Weak, 623.437nm), Yellow 1 (Weak, 579.065nm), Yellow 2 (Strong,576.959nm), Green (Very Strong, 546.074nm), Blue Green (Very Weak,491.604nm), Blue (Very Strong,435.835nm), Violet (Strong,404.656nm). All the reported wavelength values are information that was gathered from books and articles. ➢ Only figure 3 is to be drawn in the lab note book.
To Determine the Angle of Minimum Deviation (D)
➢ Mount the prism on the prism table, with the refracting edge turned away from the collimator. So that light falling on the refracting face AB emerges out through the face AC. Figure 1 ➢ Now slowly rotate the telescope towards the side BC and obtain the spectrum by placing the telescope at C.
Figure 2 ➢ Observe the spectrum by rotating the prism table while looking through the telescope. As you move the prism table the spectrum will also start to move but at one particular position (Minimum deviation position) the spectrum will retraces its path although the rotation of the table is continued in the same direction. Lock the telescope in this position, coincide the cross wire with the spectral line (particular colour) and note the readings on both the vernier scales (Reading for minimum deviation position). Figure 3 ➢ Release the telescope and remove the prism from the prism table. Rotate the telescope to capture the direct ray (slit image). Note the readings on both the vernier scales (Reading for direct ray). ➢ Figure 4
Precautions
- The telescope and collimator should be individually set for parallel rays.
- Slit should be as narrow as possible.
- While taking observations, the telescope and prism table should be clamped with the help of clamping screws.
- The levelling screws of prism table is adjusted with the help of spirit level to make it horizontal.
Questions on related concepts (Self-assessment)
- Which colour in the spectrum is having more refractive index?
- How does refractive index vary with wavelength?
- What is the principle behind using a glass prism to measure refractive index?
- Which source of light are you using? Is it a monochromatic source of light?
- Can we use sodium lamp instead of mercury lamp?
- How does the angle of minimum deviation help in finding the refractive index?
- What is Snell's law? What is the formula to calculate refractive index using Snell's law?
- Is it necessary to use a specific type of glass prism for the experiment?
- What happens if the incident angle is less than the angle of minimum deviation?
- Can we use this experiment to find the refractive index of other materials?
Further references
- https://drive.google.com/file/d/1uXXDEaAl4CDVc- Jqe7YkOKoF8PKh3ki9/view?usp=sharing
- https://drive.google.com/file/d/1NS3yzvHa-k-aatJnnir7Fjm9dIzP0uiU/view?pli=
Determination of the wavelength of a LASER source using
diffraction grating
Objective
To determine the wavelength of a given laser source using an optical transmission grating element.
Apparatus to be used
He-Ne laser source, transmission grating element, and scale with measurements.
Basic theory
Single slit diffraction:
When light passes through a slit, the width of which is comparable as the wavelength of the incident light, it will spread out in the region of geometrical shadow beyond the slit. This phenomenon is known as the diffraction, a characteristic of wave property of light. Huygens proposed each point along a wave front to be the source of a secondary disturbance, forming secondary wavelets (Fig. 1a). Diffraction is due to the constructive and destructive interference of these secondary wavelets, forming maximum and minimum intensity patterns respectively (Fig. 1b). Secondary circular wavefronts Intensity distribution Slit θ Plane wavefront Screen n = 2 n = 1 n = 0 n = - 1 n = - 2 (a) (b) Figure1. Single slit diffraction. (a) Huygens’s principle, wherein each point of the primary plane wavefront acts as the secondary wavefront. (b) Intensity distribution pattern due to the diffraction. θ: angle of diffraction, n: order of diffraction maxima.
The diffraction grating:
Grating is a repetitive array of diffracting elements, either apertures or obstacles , which has the effect of producing periodic alterations of phase, amplitude, or both of an emergent wave. The simplest example of a grating is a multiple-slit configuration. Mostly used multiple-slit configuration modulate the amplitude of the incident wavefront; and known as transmission amplitude grating. Similarly, depending on design, we can also have transmission phase grating,as well as reflection grating. Figure 2 shows fabricated diffraction grating element. Here, it is optically plane glass plate on which numbers of equidistant parallel slits are drawn using a pointer diamond. The region where the lines are drawn becomes opaque to the light; while the space between the two lines is transparent. Source of secondary wavefront Plane wavefront