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The practical exam is the part of Class 12 physics that students prepare for least. They copy the record, hope for an easy experiment and treat the viva as luck. Yet practicals carry nearly a third of the subject's marks, and most of those marks reward preparation you can do at home. This post covers how the marks are split, a worked experiment and the questions examiners tend to ask.
How are the 30 practical marks split?
The CBSE Physics curriculum for 2026-27 sets a 3-hour practical exam for 30 marks:
Two experiments, one from each section: 7 + 7 marks
Practical record (experiments and activities): 5 marks
One activity from any section: 3 marks
Investigatory project: 3 marks
Viva on experiments, activities and project: 5 marks
The record must include at least 8 experiments, 4 from each section, and at least 6 activities, 3 from each section, plus the project report. Section A covers electricity (resistivity, metre bridge, galvanometer, sonometer) and Section B covers optics and the p-n junction diode (mirrors, lenses, prism, refractive index). Check the curriculum for your own year, since the lists can change.
Why does lab work often teach less than it should?
Abrahams and Millar (2008) watched 25 ordinary science practical lessons in English secondary schools. Practical work was generally effective at getting students to do the intended things with the equipment, but much less effective at getting them to use the scientific ideas to guide what they did and to make sense of their data.
That gap is exactly what the viva probes. You can set up the optical bench without knowing why the image must be sharp, or why you take several readings. Preparing for practicals means preparing the ideas behind the actions.
Worked example: focal length of a convex lens
The curriculum lists finding the focal length of a convex lens by plotting graphs between u and v, or between 1/u and 1/v.
Readings (Cartesian sign convention, distances in cm, light travelling left to right):
Reading 1: u = −30.0, v = +60.0
Reading 2: u = −40.0, v = +40.0
Reading 3: u = −25.0, v = +100.0
Calculation with the lens formula 1/f = 1/v − 1/u:
Reading 1: 1/f = 1/60 + 1/30 = 1/60 + 2/60 = 3/60, so f = 20.0 cm.
Reading 2: 1/f = 1/40 + 1/40 = 2/40, so f = 20.0 cm.
Reading 3: 1/f = 1/100 + 1/25 = 1/100 + 4/100 = 5/100, so f = 20.0 cm.
Mean f = 20.0 cm.
Graph checks. On a 1/v against 1/u graph the points lie on a straight line of slope +1, with intercepts of size 1/f on both axes. When u = −2f = −40 cm, v = +2f = +40 cm, which is reading 2. Spotting that point is a quick check that your readings make sense.
Common mistakes. Dropping the minus sign on u, which gives 1/f = 1/60 − 1/30 and a negative focal length; recording v when the image is not at its sharpest; and not correcting for index error, the difference between the bench scale and the actual distance between the lens and the pins or screen.
What does the viva actually ask?
In its guidelines for the practical exam, the curriculum describes viva questions as covering basic theory, principle or concept, the apparatus, the procedure, precautions and sources of error. Those guidelines are written for visually impaired candidates, but the list is a good map for everyone. For each experiment in your record, prepare a short answer to each of these:
Principle: "What formula does this experiment use, and where does it come from?"
Apparatus: "What is the least count of this instrument?"
Procedure: "Why do you take readings at several positions instead of one?"
Precautions: "Why should the null point in a metre bridge be near the middle of the wire?" (End errors and percentage errors in length are smallest there.)
Sources of error: "What could make your value too high or too low?"
Project: "What was your question, what did you measure, and what would you change?"
A two-week plan before the practical exam
Days 1 to 3: For each recorded experiment, write the aim, formula, diagram and one precaution from memory on a single card.
Days 4 to 7: Redo the calculations in your record and plot one graph by hand per experiment. Check signs and units.
Days 8 to 10: Pair up and ask each other the six viva questions above for every experiment. Answer aloud, in full sentences.
Days 11 to 14: Practise any experiment you have only done once, and rehearse a two-minute explanation of your project.
Ray diagrams and circuit behaviour are easier to understand when you can change something and watch the result. The Fermi Simulation Lab has interactive simulations for ideas easier to see than read, useful for checking what you expect to see before you are in the lab. For how virtual labs compare with hands-on work, see our post on virtual chemistry labs.
This week, make the one-card summary for your first experiment and ask a friend to quiz you on it.
