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The glass slab experiment looks simple: four pins, a block of glass, a protractor. Yet marks slip away through a tilted pin, a misplaced normal or a table with no interpretation. This post follows the practical as NCERT's lab manual sets it out, shows a filled table with sample readings, explains the results, and lists the viva questions.
Is this practical on the 2026-27 list?
Yes. The CBSE Class X Science curriculum for 2026-27 lists practical 11: "Tracing the path of a ray of light passing through a rectangular glass slab for different angles of incidence. Measure the angle of incidence, angle of refraction, angle of emergence and interpret the result." The method is Experiment 40 in NCERT's Laboratory Manual Science for Class X, and the theory is section 9.3.1 of the 2026-27 NCERT Class 10 chapter Light – Reflection and Refraction, where it appears as Activity 9.10. Check your school's practical list for the current year.
What do you need, and what are you measuring?
You need a rectangular glass slab, a drawing board, white paper, a protractor, a scale and pins.
Three angles, each measured from the normal (the line at 90° to the glass surface), not from the surface:
Angle of incidence, i: between the incoming ray and the normal at the first face.
Angle of refraction, r: between the ray inside the glass and the same normal.
Angle of emergence, e: between the outgoing ray and the normal at the second face.
How do you do it, step by step?
Fix the paper on the board. Place the slab in the middle and trace its outline, ABCD.
Remove the slab. Mark a point O near the middle of face AD, draw the normal there, and draw a line XO at 30° to 60° to the normal. Put the slab back exactly on its outline.
Fix two pins, P1 and P2, on line XO, about 5 cm apart.
Look through the opposite face, BC. Fix pins P3 and P4 so that they and the images of P1 and P2 all appear in one straight line. Look at the feet of the pins, not the heads.
Remove the slab and pins. Join P3 and P4 and extend the line to meet BC at O′. Join O to O′: that is the ray inside the glass.
Draw the normal at O′. Measure i, r and e with the protractor.
Repeat for two more angles of incidence between 30° and 60°, on fresh lines or a fresh sheet.
What does a filled observation table look like?
Here are sample readings of the kind a careful student might record. They are illustrative, not from a real experiment.
Reading 1: i = 30°, r = 19°, e = 30°
Reading 2: i = 45°, r = 28°, e = 44°
Reading 3: i = 60°, r = 35°, e = 60°
The lab manual's table also compares i with e. Here they agree to within 1°, about as close as a school protractor reads.
How do you interpret the results?
The lab manual asks you to report three things, and the sample readings show all of them.
r < i every time. The ray bends towards the normal as it goes from air into glass, a denser medium.
e = i (within reading error). The ray comes out parallel to the incoming ray. NCERT explains why: the bending at the two parallel faces is "equal and opposite". The emergent ray is shifted sideways, though. That sideways step is the lateral shift, which we work out with numbers in our post on lateral shift through a glass slab.
r increases as i increases. Here r goes from 19° to 28° to 35°.
Going further: the refractive index. Experiment 41 in the same lab manual uses these angles to find the refractive index, n = sin i / sin r. From the sample readings:
sin 30°/sin 19° = 0.500/0.326 ≈ 1.54
sin 45°/sin 28° = 0.707/0.469 ≈ 1.51
sin 60°/sin 35° = 0.866/0.574 ≈ 1.51
The mean is about 1.52, the value NCERT's Table 9.3 gives for crown glass. The ratio stays nearly constant while i and r both change: that is Snell's law.
What errors spoil this experiment?
From the lab manual's precautions:
Pins not upright or too close together. Keep them about 5 cm apart so the direction of the ray is clear.
Angles outside 30° to 60°. The lab manual recommends this range. At small angles the bending is small compared with your protractor's error; at large angles more light is reflected and the images are fainter.
Thick lines. Use a sharp pencil, and read the protractor with your eye above the mark.
A chipped or uneven slab. The faces must be smooth and the slab truly rectangular.
Measuring from the surface. All three angles are from the normal.
Which viva questions should you prepare?
From the lab manual:
Why are the incident and emergent rays parallel?
Why does light bend towards the normal on entering glass and away on leaving?
What happens to a ray that enters perpendicular to the surface? (It goes straight through.)
Why take i between 30° and 60°?
How does the sideways shift depend on the slab's thickness and the angle of incidence?
For the theory around this practical, see our guide to light, reflection and refraction for Class 10. If you want to see how the ray moves as you change the angle before you go into the lab, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
A practice routine
Before the lab: Draw the slab, both normals and all three angles from memory, and label i, r and e.
In the lab: Take three readings, check that e is close to i each time, and redo any reading where they differ by more than about 2°.
After the lab: Answer the viva questions in two lines each.
Before your practical, draw one full ray diagram at i = 40° and mark which angles you would measure.
