Ray Optics and Optical Instruments Class 12 Notes describes how light travels in straight lines and how mirrors, lenses, and prisms form images. You’ll use the mirror/lens formulae, magnification, Snell’s law, total internal reflection (TIR), and lens maker’s formula to solve board-style numericals. This guide is aligned to the CBSE Class 12 Physics syllabus and NCERT textbook, focusing on exam patterns for 2025–26.
Ray Optics and Optical Instruments Class 12 Notes
Why ray optics? At the Class 12 level, light can be understood as rays that travel in straight lines and bend/reflect at boundaries. This “geometrical” approach lets us design and analyze devices like microscopes, telescopes, cameras, spectacles, and optical fibres.
- Reflection: Light bounces off a surface (plane/spherical mirrors). You use the law of reflection and mirror formula to locate images and compute magnification.
- Refraction: Light bends when moving across media (air → glass → water). You apply Snell’s law, refractive index, and lens formula to predict image position, size, and nature.
- TIR: At high incidence angles in a denser medium, light reflects 100% internally. This is the core of optical fibre communication, mirages, and diamond sparkle.
- Prism & dispersion: Prisms deviate light; white light splits into VIBGYOR due to wavelength-dependent refractive index.
- Instruments: Combination of lenses/mirrors gives high magnification and resolution in microscopes and telescopes.
Topic Map (What You Will Master)
| Section | Concept Focus | Exam Skills You Gain |
|---|---|---|
| Reflection from Plane & Spherical Mirrors | Laws of reflection, sign convention, mirror formula | Locate images, handle sign conventions, draw neat ray diagrams |
| Refraction & Snell’s Law | Refractive index, ray bending rules | Compute angles, identify denser/rarer media, use critical angle |
| Total Internal Reflection (TIR) | Condition for TIR, critical angle, optical fibre | Explain mirage/diamond sparkle, fibre design basics |
| Prisms & Dispersion | Minimum deviation, angular dispersion, dispersive power | Compare deviation for colours, solve minimum-deviation numericals |
| Lenses (Convex/Concave) | Thin lens formula, magnification, power (dioptre) | Calculate image position/size, nature, and m; add powers |
| Lens Maker’s Formula | Dependence on radii & refractive index | Design lenses for air/other media, predict focal length shifts |
| Optical Instruments | Simple microscope, compound microscope, telescope | Derive magnifications, understand near point & tube length effects |
Core Laws & Principles
- Law of Reflection: Angle of incidence = Angle of reflection; all rays and normal lie in the same plane.
- Snell’s Law: \( n=\dfrac{\sin i}{\sin r} \) for refraction across two media; also \( n=\dfrac{c}{v} \).
- TIR Condition: Occurs when light goes from denser → rarer medium and \( i > i_c \); here \( \sin i_c=\dfrac{n_2}{n_1} \).
- Prism at Minimum Deviation: \( D_{\min}+A=i+e,\ i=e \) and \( n=\dfrac{\sin\left(\tfrac{A+D_{\min}}{2}\right)}{\sin\left(\tfrac{A}{2}\right)} \).
- Sign Convention (Mirrors/Lenses): Distances measured from pole/optical centre; direction of incident light is positive (mirrors often use Cartesian convention with care).
Important Formulas (Class 12 Physics)
\( \frac{1}{f}=\frac{1}{v}+\frac{1}{u} \) (Mirror), \( m=\frac{h_i}{h_o}=\frac{-v}{u} \)
\( \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \) (Thin Lens), \( m=\frac{h_i}{h_o}=\frac{v}{u} \)
\( P=\frac{1}{f(\text{m})} \) dioptre, \( P_{\text{eq}}=P_1+P_2+P_3+\cdots \) (lenses in contact)
\( n=\frac{c}{v}=\frac{\sin i}{\sin r}, \quad \sin i_c=\frac{n_2}{n_1} \ (n_1>n_2) \)
\( n=\frac{\sin\left(\frac{A+D_{\min}}{2}\right)}{\sin\left(\frac{A}{2}\right)} \) (prism at minimum deviation)
Lens Maker’s Formula: \( \frac{1}{f}=(n-1)\!\left(\frac{1}{R_1}-\frac{1}{R_2}\right) \)
Worked Idea & Analogy
Analogy: Think of light rays as disciplined cyclists. On a straight road (uniform medium) they go straight. A mirror is a barricade—cyclists turn back with the same angle (reflection). A glass surface is like switching to a slower lane—cyclists bend their path because speed changes (refraction). If they try to exit a slow lane to a fast lane at too steep an angle, they hit the divider and bounce back (TIR) — exactly how optical fibre traps light.
Mini worked idea (lens): An object at \( u=-20\ \text{cm} \) in front of a thin convex lens \( f=+10\ \text{cm} \). Using \( \tfrac{1}{f}=\tfrac{1}{v}-\tfrac{1}{u} \Rightarrow \tfrac{1}{10}=\tfrac{1}{v}-\left(-\tfrac{1}{20}\right) \Rightarrow \tfrac{1}{v}=\tfrac{1}{10}-\tfrac{1}{20}=\tfrac{1}{20} \Rightarrow v=+20\ \text{cm} \). So a real, inverted image forms at 20 cm on the other side; \( m=\tfrac{v}{u}=\tfrac{+20}{-20}=-1 \) (same size, inverted).
Real-life Applications (Exam-ready points)
- Optical Fibre: Telecommunication backbone uses TIR to guide light with minimal loss.
- Mirage in deserts: Hot air near ground lowers refractive index upward; rays bend, creating “water-like” illusions.
- Diamond sparkle: Very low critical angle → repeated TIR enhances brilliance.
- Prisms: Deviation/dispersion, periscopes (totally reflecting prisms) provide bright, inversion-free images.
- Microscopes & Telescopes: High magnification from lens combinations; resolving power depends on aperture and wavelength.
Quick Quiz (2 Questions) + Answers
Answer: \( \sin i_c=\dfrac{n_{\text{air}}}{n_{\text{glass}}}=\dfrac{1}{1.5}=\tfrac{2}{3} \Rightarrow i_c\approx 41.8^\circ \). TIR occurs when light goes from denser to rarer medium and \( i>i_c \).
Answer: Using \( \frac{1}{f}=\frac{1}{v}+\frac{1}{u} \) (take \( u=-30\ \text{cm} \), \( f=+20\ \text{cm} \) by sign rule for concave):
\( \frac{1}{20}=\frac{1}{v}+\left(-\frac{1}{30}\right) \Rightarrow \frac{1}{v}=\frac{1}{20}+\frac{1}{30}=\frac{1}{12} \Rightarrow v=+12\ \text{cm} \)
Real, inverted image at 12 cm; \( m=\frac{-v}{u}=\frac{-12}{-30}=0.4 \) (reduced).