Ray optics studies light as straight-line rays that reflect, refract, and form images.
It explains image formation by mirrors, lenses, prisms, microscopes, telescopes, and the human eye.
Light behaves predictably when it meets polished surfaces, transparent media, and curved lenses. Important Questions Class 12 Physics Chapter 9 help students practise mirror formula, refraction, total internal reflection, lens formula, prism deviation, and optical instruments. CBSE 2026 questions often test sign convention, formula substitution, ray diagrams, and magnifying power. The chapter needs careful numerical steps because one wrong sign changes the final image position.
Key Takeaways
- Reflection and Refraction: Ray optics uses straight-line light paths to explain mirrors, lenses, and image formation.
- Lens Formula: The relation between object distance, image distance, and focal length appears often in numerical questions.
- Total Internal Reflection: This occurs when light travels from a denser medium to a rarer medium beyond the critical angle.
- Optical Instruments: Microscopes and telescopes use lenses to form magnified images.
Important Questions Class 12 Physics Chapter 9 Structure 2026
| Concept |
Exam Use |
Key Formula or Rule |
| Spherical Mirrors |
Image formation and mirror numericals |
1/f = 1/v + 1/u |
| Refraction at Spherical Surface |
Lens and surface-based derivations |
n₂/v - n₁/u = (n₂ - n₁)/R |
| Lens Formula |
Convex and concave lens numericals |
1/f = 1/v - 1/u |
Important Questions Class 12 Physics Chapter 9 Overview
Important Questions Class 12 Physics Chapter 9 focus on formula application and concept clarity. The strongest scoring areas include mirrors, lenses, refraction, prism, total internal reflection, and instruments.
Q1. What is Ray Optics in Class 12 Physics?
Ray optics is the study of light using straight-line rays. It explains reflection, refraction, and image formation.
Example: A convex lens forms a real image when the object lies beyond its focal length.
Q2. What is the main assumption of ray optics?
Ray optics assumes that light travels in straight lines in a uniform medium. It ignores wave effects like diffraction.
This assumption works when the obstacle size is much larger than the wavelength of light.
Q3. What are the main topics in Important Questions Class 12 Physics Chapter 9?
The main topics are mirrors, refraction, lenses, prisms, and optical instruments. These topics form most numerical questions.
CBSE 2026 questions often combine sign convention, formula use, and ray diagram interpretation.
Ray Optics Class 12 Important Questions on Reflection by Spherical Mirrors
Ray optics class 12 important questions often begin with spherical mirrors. Students must apply sign convention before substituting values.
Q4. What is the mirror formula?
The mirror formula is 1/f = 1/v + 1/u. Here, f is focal length, v is image distance, and u is object distance.
This formula applies to both concave and convex spherical mirrors.
Q5. What is the relation between focal length and radius of curvature?
The focal length is half the radius of curvature. The relation is f = R/2.
For a mirror with R = 40 cm, the focal length is 20 cm.
Q6. A candle is placed 30 cm before a concave mirror of focal length 20 cm. Find image distance.
The image forms 60 cm in front of the mirror. The image is real and inverted.
- Given Data:
u = -30 cm
f = -20 cm
- Formula Used:
1/f = 1/v + 1/u
- Calculation:
1/(-20) = 1/v + 1/(-30)
1/v = -1/20 + 1/30
1/v = -1/60
- Final Result:
v = -60 cm
Q7. What is magnification by a spherical mirror?
Magnification is the ratio of image height to object height. For mirrors, m = -v/u.
If m is negative, the image is real and inverted.
Q8. Find magnification when u = -30 cm and v = -60 cm.
The magnification is -2. The image is twice the object size and inverted.
- Given Data:
u = -30 cm
v = -60 cm
- Formula Used:
m = -v/u
- Calculation:
m = -(-60)/(-30)
m = -2
- Final Result:
Magnification = -2
Refraction of Light Class 12 Important Questions
Refraction of light class 12 questions test direction change at media boundaries. Students should remember Snell’s law and refractive index.
Q9. What is refraction of light?
Refraction is the bending of light when it passes from one medium to another. It occurs because light speed changes.
Example: A pencil appears bent when placed partly inside water.
Q10. What is Snell’s law?
Snell’s law states that sin i / sin r is constant for two given media. This constant is refractive index.
Formula: n21 = sin i / sin r
Q11. What is refractive index?
Refractive index is the ratio of speed of light in vacuum to speed in a medium. Its formula is n = c/v.
A higher refractive index means light travels slower in that medium.
Q12. What is real and apparent depth relation?
Real depth divided by apparent depth equals refractive index. The relation is n = real depth / apparent depth.
This explains why a swimming pool appears shallower than its actual depth.
Q13. A tank appears 80 cm deep. Its refractive index is 4/3. Find real depth.
The real depth is 106.67 cm. Apparent depth is smaller in denser media.
- Given Data:
Apparent depth = 80 cm
n = 4/3
- Formula Used:
n = Real depth / Apparent depth
- Calculation:
Real depth = n × Apparent depth
Real depth = (4/3) × 80
Real depth = 320/3
- Final Result:
Real depth = 106.67 cm
Total Internal Reflection Class 12 Important Questions
Total internal reflection class 12 questions usually test critical angle. The ray must travel from denser to rarer medium.
Q14. What is total internal reflection?
Total internal reflection occurs when light reflects completely inside a denser medium. It happens after the angle of incidence exceeds critical angle.
It occurs only when light travels from denser medium to rarer medium.
Q15. What are the conditions for total internal reflection?
Total internal reflection needs two conditions. Light must travel from denser to rarer medium.
The angle of incidence in the denser medium must be greater than critical angle.
Q16. What is critical angle?
Critical angle is the angle of incidence for which refraction angle becomes 90°. It is denoted by C.
For a denser medium to air, sin C = 1/n.
Q17. Calculate critical angle for glass of refractive index √2.
The critical angle is 45°. This is because sin 45° = 1/√2.
- Given Data:
n = √2
- Formula Used:
sin C = 1/n
- Calculation:
sin C = 1/√2
C = 45°
- Final Result:
Critical angle = 45°
Q18. Why does a diamond sparkle?
A diamond sparkles because total internal reflection traps and redirects light inside it. Diamond has a high refractive index.
Its small critical angle allows many internal reflections before light emerges.
Lens Formula Class 12 Important Questions
Lens formula class 12 questions need correct sign convention. Convex lenses usually have positive focal length in Cartesian convention.
Q19. What is the lens formula?
The lens formula is 1/f = 1/v - 1/u. Here, f is focal length, v is image distance, and u is object distance.
This formula applies to thin lenses.
Q20. A convex lens has focal length 20 cm. An object is placed 30 cm before it. Find image distance.
The image forms 60 cm on the other side of the lens. The image is real and inverted.
- Given Data:
f = +20 cm
u = -30 cm
- Formula Used:
1/f = 1/v - 1/u
- Calculation:
1/20 = 1/v - 1/(-30)
1/v = 1/20 - 1/30
1/v = 1/60
- Final Result:
v = +60 cm
Q21. What is power of a lens?
Power of a lens is the reciprocal of focal length in metres. Its unit is dioptre.
Formula: P = 1/f
Q22. Find the power of a convex lens of focal length 25 cm.
The power of the lens is +4 D. A convex lens has positive power.
- Given Data:
f = 25 cm = 0.25 m
- Formula Used:
P = 1/f
- Calculation:
P = 1/0.25
P = 4 D
- Final Result:
Power = +4 D
Q23. What is the power of lenses in contact?
The equivalent power equals the sum of individual powers. The relation is P = P1 + P2.
If two lenses have powers +3 D and -1 D, their equivalent power is +2 D.
Ray Optics and Optical Instruments Class 12 Important Questions on Prism
Ray optics and optical instruments class 12 important questions often include prism deviation. Learn the minimum deviation formula clearly.
Q24. What is angle of deviation in a prism?
Angle of deviation is the angle between incident ray direction and emergent ray direction. It is denoted by δ.
Deviation depends on angle of incidence, prism angle, and refractive index.
Q25. What is the prism formula at minimum deviation?
The prism formula is n = sin[(A + Dm)/2] / sin(A/2). Here, A is prism angle.
Dm represents the angle of minimum deviation.
Q26. Why does dispersion occur in a prism?
Dispersion occurs because different colours have different refractive indices in glass. Violet deviates more than red.
White light splits into its component colours after refraction through a prism.
Q27. What is minimum deviation?
Minimum deviation is the least deviation produced by a prism. At this position, the light path is symmetrical.
The angle of incidence equals the angle of emergence at minimum deviation.
Optical Instruments Class 12 Important Questions
Optical instruments class 12 important questions test magnifying power. Students should know simple microscope, compound microscope, and telescope formulae.
Q28. What is a simple microscope?
A simple microscope is a convex lens used to produce a magnified virtual image. It works when the object lies within focal length.
A magnifying glass is a common simple microscope.
Q29. What is magnifying power of a simple microscope for final image at least distance?
The magnifying power is M = 1 + D/f. Here, D is the least distance of distinct vision.
For normal human vision, D = 25 cm.
Q30. Find magnifying power of a simple microscope with focal length 5 cm.
The magnifying power is 6. The final image lies at least distance of distinct vision.
- Given Data:
D = 25 cm
f = 5 cm
- Formula Used:
M = 1 + D/f
- Calculation:
M = 1 + 25/5
M = 1 + 5
- Final Result:
M = 6
Q31. What is a compound microscope?
A compound microscope uses objective and eyepiece lenses to produce high magnification. The objective forms a real enlarged image.
The eyepiece then acts as a simple microscope.
Q32. What is an astronomical telescope?
An astronomical telescope forms magnified images of distant objects. It uses an objective lens and an eyepiece.
The objective has a large focal length and large aperture.
Q33. What is magnifying power of a telescope in normal adjustment?
The magnifying power is M = fo/fe. Here, fo is objective focal length and fe is eyepiece focal length.
This formula applies when the final image forms at infinity.
Q34. A telescope has objective focal length 100 cm and eyepiece focal length 5 cm. Find magnifying power.
The magnifying power is 20. The objective focal length is much larger than eyepiece focal length.
- Given Data:
fo = 100 cm
fe = 5 cm
- Formula Used:
M = fo/fe
- Calculation:
M = 100/5
M = 20
- Final Result:
Magnifying power = 20
Ray Optics Pyq CBSE Class 12 and Board Exam Pattern Questions
Ray optics pyq cbse class 12 questions usually combine formulae with sign convention. The safest method is to write given data first.
Q35. A concave mirror has radius of curvature 40 cm. Find its focal length.
The focal length is 20 cm with negative sign. A concave mirror has negative focal length.
- Given Data:
R = -40 cm
- Formula Used:
f = R/2
- Calculation:
f = -40/2
f = -20 cm
- Final Result:
f = -20 cm
Q36. A lens has power -2 D. Find its focal length.
The focal length is -0.5 m. Negative power means the lens is concave.
- Given Data:
P = -2 D
- Formula Used:
P = 1/f
- Calculation:
f = 1/P
f = 1/(-2)
f = -0.5 m
- Final Result:
f = -0.5 m
Q37. What happens to focal length when a lens goes from air to water?
The focal length increases when a glass lens goes from air to water. Its effective refractive contrast decreases.
Lower refractive contrast reduces the converging or diverging power.
Q38. Why is sign convention important in ray optics class 12 questions?
Sign convention decides whether distances are positive or negative. It directly affects final numerical answers.
For mirrors, object distance usually remains negative for real objects.
Ray Optics Class 12 Important Topics
Ray optics class 12 important topics include both conceptual and numerical parts. Formula clarity matters more than memorising long explanations.
Q39. Which topics are most important in ray optics class 12?
The most important topics are mirror formula, lens formula, refraction, TIR, prism, and optical instruments. These areas cover most board-style numericals.
Students should also revise sign convention and magnification formulae.
Q40. Which formulae are most important for ray optics class 12 important questions?
The most important formulae are mirror formula, lens formula, Snell’s law, and power formula. Prism and magnifying power formulae also matter.
Key formulae include 1/f = 1/v + 1/u, 1/f = 1/v - 1/u, and P = 1/f.
Q41. Where do students lose marks in ray optics class 12 pyq?
Students lose marks in signs, units, and final image nature. They also skip ray diagram logic.
Common errors include using cm in power formula and ignoring negative focal length.
Q42. How should students solve ray optics questions class 12?
Students should write given values, formula, substitution, calculation, and final result. This format prevents sign mistakes.
Numericals on mirrors and lenses need correct Cartesian sign convention.
Q43. Why are class 12 physics ray optics important questions scoring?
They are scoring because most answers follow fixed formulae and ray rules. Numericals often use direct substitution.
Conceptual questions usually test definitions, conditions, and image nature.
Q44. What is the difference between real and virtual image?
A real image forms when rays actually meet. A virtual image forms when rays appear to meet.
Real images can form on a screen, while virtual images cannot form on a screen.
Q45. What is the difference between microscope and telescope?
A microscope magnifies nearby tiny objects, while a telescope magnifies distant objects. Both use lens combinations.
A compound microscope has short focal length objective. A telescope has long focal length objective.
Class 12 Physics Chapter List