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In the Class 12 physics paper, a ray diagram is not decoration. CBSE sample papers award marks for the diagram itself, and a diagram that disagrees with your calculation can cost you twice. This post covers which diagrams the syllabus implies, the rules examiners look for, and a quick numerical check so your diagram and your answer agree.
Which ray diagrams can come in the board paper?
The CBSE Physics curriculum for 2026-27 lists, under Ray Optics and Optical Instruments: spherical mirrors and the mirror formula, refraction, total internal reflection, lenses and the thin lens formula, refraction through a prism, and "microscopes and astronomical telescopes (reflecting and refracting) and their magnifying powers". Optics carries 18 marks in the Class XII unit table.
Past sample papers show how this turns into questions. The 2025-26 sample paper Q25 asked for the angular magnification of a compound microscope and then "Draw the ray diagram of compound microscope in normal adjustment", worth 1 mark in the marking scheme. The 2024-25 sample paper Q24 asked for "a ray diagram for the formation of image by a Cassegrain telescope", worth 2 marks in its marking scheme.
So your list to practise is: concave and convex mirror images, convex and concave lens images, refraction through a prism, total internal reflection, compound microscope, refracting telescope and reflecting (Cassegrain) telescope.
Which rays should you draw?
You only need two rays to fix an image. For mirrors and lenses, choose two of these:
A ray parallel to the principal axis, which after reflection or refraction passes through (or appears to come from) the principal focus.
A ray through the focus (or heading towards it), which comes out parallel to the axis.
For a mirror, a ray through the centre of curvature, which returns along itself. For a thin lens, a ray through the optical centre, which goes straight on.
Where the two rays meet (or appear to meet) is the image. A third ray is a useful check, not a requirement.
Worked example: check a diagram with the mirror formula
An object stands 15 cm in front of a concave mirror of focal length 10 cm. Draw the image and check it.
Using the NCERT Class 12 Ray Optics chapter sign convention (distances measured from the pole, negative in front of a mirror): u = −15 cm, f = −10 cm.
1/v = 1/f − 1/u = −1/10 + 1/15 = (−3 + 2)/30 = −1/30, so v = −30 cm.
Magnification m = −v/u = −(−30)/(−15) = −2.
So the image is real (in front of the mirror), inverted (m negative) and twice the size, 30 cm from the mirror. On your diagram the object sits between F and C, and the image must form beyond C, upside down and larger. If your rays cross between F and C, a ray has gone wrong.
A lens check works the same way. For a convex lens of focal length 20 cm with the object 30 cm away: 1/v = 1/f + 1/u = 1/20 − 1/30 = 1/60, so v = +60 cm and m = v/u = −2. The image is real, inverted and twice the size on the far side.
What about the microscope and telescope?
For a compound microscope, the object sits just beyond the focus of the objective. The objective forms a real, inverted, magnified image at or just inside the focus of the eyepiece, and the eyepiece, acting as a simple magnifier, forms the final virtual image. In the 2025-26 sample answer (objective focal length 0.82 cm, object at 0.91 cm), the first image is at about 8.3 cm, and the stated magnification is about 87.7.
For a refracting telescope in normal adjustment, parallel rays from a distant object form an image at the common focus of objective and eyepiece, and the final image is at infinity. For a Cassegrain telescope, draw a large concave primary mirror and a small convex secondary that sends the light back through a hole in the primary.
What mistakes cost marks?
Missing arrows, or arrows pointing the wrong way.
Drawing virtual rays as solid lines, or real rays as dashed.
Bending the ray at the curved face of a thin lens instead of at its central plane.
An image position that contradicts your own calculation.
In the microscope, placing the first image beyond the eyepiece's focus, which would not give a magnified virtual final image.
These are not small slips. In a study of university students, Goldberg and McDermott (1987) found that many who had just finished geometrical optics could not relate ray tracing to a real lens, mirror and screen, and that difficulties with ray diagrams pointed to a weak grasp of what a ray represents. For more on the sign-convention slips, see our post on common ray optics mistakes.
A practice routine
Draw each diagram on the list once a week with a pencil and ruler, in under three minutes.
For every mirror or lens diagram, pick numbers and check the image position with the formula.
Redraw the microscope and telescope from memory, then compare with the NCERT figures.
If you struggle to picture where rays go, Fermi's AI tutor writes on a board, draws the figure and checks understanding as it goes, which suits ray diagrams. Check the current year's CBSE sample paper for the latest question pattern.
Start now: draw the concave mirror case above, then confirm the image lands 30 cm from the mirror.
