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Ray Optics carries more JEE Main marks than most chapters in this half of the syllabus, and almost all of them come from three places: the mirror and lens formula, the prism, and the two instruments. The sign convention is where the marks actually go missing, so be strict with it from the first line of every solution. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 2 to 3 questions every year, which is 8 to 12 marks.
- Mirror and lens numericals and lens combinations are asked every year.
- The prism, total internal reflection and the two instruments fill the rest.
What the syllabus says
In the syllabus (Unit 16, Optics): Reflection of light, spherical mirrors and the mirror formula, refraction of light at plane and spherical surfaces, the thin lens formula and the lens maker’s formula, total internal reflection and its applications, magnification, power of a lens, thin lenses in contact, refraction of light through a prism, and the microscope and the astronomical telescope, both reflecting and refracting, with their magnifying powers.
Taken out in 2024: deviation and dispersion of light by a prism, so dispersive power and angular dispersion go with it.
Never in the JEE Main syllabus: the human eye and its defects and scattering of light and the blue colour of the sky. They are in NCERT, so they still turn up in old question banks.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Mirror formula | 1/v + 1/u = 1/f, with f = R/2 |
| Magnification | m = −v/u for a mirror, v/u for a lens |
| Lens formula | 1/v − 1/u = 1/f |
| Lens maker’s formula | 1/f = (μ − 1)(1/R1 − 1/R2) |
| Power of a lens | P = 1/f in metre, measured in dioptre |
| Lenses in contact | P = P1 + P2 |
| Critical angle | sin C = 1/μ |
| Prism at minimum deviation | μ = sin((A + D)/2) / sin(A/2) |
| Simple microscope | m = 1 + D/f |
| Telescope, normal adjustment | m = fo/fe |
| Telescope, tube length | L = fₒ + fₑ in normal adjustment |
The lens maker’s formula explains why a glass lens stops working in a denser liquid. The factor (μ − 1) is really (μlens/μmedium − 1), and when the medium is denser that bracket turns negative and a converging lens starts to diverge.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. An object is placed 30 cm in front of a concave mirror of focal length 10 cm. The image forms at
- (a) 15 cm in front of the mirror
- (b) 15 cm behind the mirror
- (c) 7.5 cm in front
- (d) 30 cm in front
Q2. For that same mirror and object, the magnification is
- (a) −0.5
- (b) +0.5
- (c) −2
- (d) +2
Q3. The critical angle for a medium of refractive index 1.5 is about
- (a) 30°
- (b) 41.8°
- (c) 48.6°
- (d) 60°
Q4. A thin lens has a focal length of 20 cm. Its power is
- (a) 20 D
- (b) 5 D
- (c) 0.05 D
- (d) 2 D
Q5. Two thin lenses of powers +10 D and −4 D are placed in contact. The power of the combination is
- (a) 14 D
- (b) 6 D
- (c) −6 D
- (d) 40 D
Q6. A prism of angle 60° gives a minimum deviation of 30°. Its refractive index is
- (a) 1.33
- (b) 1.414
- (c) 1.50
- (d) 1.60
Q7. The magnifying power of a simple microscope, with the image at the near point, is
- (a) D/f
- (b) 1 + D/f
- (c) f/D
- (d) Df
Q8. The magnifying power of an astronomical telescope in normal adjustment is
- (a) fo/fe
- (b) fe/fo
- (c) fofe
- (d) fo + fe
Q9. A convex glass lens behaves as a diverging lens when it is immersed in a liquid whose refractive index is
- (a) less than that of glass
- (b) greater than that of glass
- (c) exactly equal to that of glass
- (d) it never happens
Q10. An astronomical telescope in normal adjustment has an objective of focal length 100 cm and an eyepiece of focal length 5 cm. Its magnifying power is
- (a) 20
- (b) 500
- (c) 95
- (d) 105
Answer key, with a hint for each
| Q1 | (a) | With u = −30 and f = −10, 1/v = −1/10 + 1/30, giving v = −15 cm, so it is real and in front. |
| Q2 | (a) | m = −v/u = −(−15)/(−30). The minus sign says the image is inverted and the half says it is smaller. |
| Q3 | (b) | sin C = 1/1.5 = 0.667, and the angle whose sine is 0.667 is about 41.8°. |
| Q4 | (b) | P = 1/0.20 m. Convert the centimetres to metres first, every time. |
| Q5 | (b) | Powers simply add when the lenses touch. |
| Q6 | (b) | μ = sin45°/sin30° = 0.707/0.5. |
| Q7 | (b) | The 1 appears because the image is formed at the near point rather than at infinity. |
| Q8 | (a) | A long objective and a short eyepiece give the most magnification. |
| Q9 | (b) | The bracket (μlens/μmedium − 1) turns negative, so the focal length changes sign. |
| Q10 | (a) | In normal adjustment M = fₒ / fₑ = 100 / 5 = 20. The tube length is fₒ + fₑ = 105 cm, which is the usual trap. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
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