Ray Optics: Mirror, Lens, Prism & Optical Instruments
Study reflection, refraction, mirror and lens formula, Snell's law, prism deviation, total internal reflection, optical instruments and magnification for JEE Main Physics.
The central idea of this chapter
Light travels in straight lines and changes direction at interfaces. This chapter connects reflection, refraction, image formation and optical instruments through geometry.
What should you understand first?
Ray optics is highly scoring in JEE Main. Once you understand sign convention, mirror/lens formula and prism relations, most problems become direct formula applications.
Core concepts
- Reflection ∠i = ∠r
- Mirror formula 1/f = 1/v + 1/u
- Lens formula 1/f = 1/v − 1/u
- Snell's law n₁ sin i = n₂ sin r
- Telescope (normal adjustment) M = fo/fe
Useful building blocks
- Mirror magnification m = −v/u
- Lens magnification m = v/u
- Lens maker formula 1/f = (n−1)(1/R₁ − 1/R₂)
- Thin prism deviation δ = (n−1)A
- Critical angle sin c = n₂/n₁
Ray Optics formula sheet
Use sign convention consistently. Apply mirror/lens formula with proper signs for u, v, f and R.
| Topic | Formula / Relation | Meaning or use |
|---|---|---|
| Reflection | ∠i = ∠r | Angle of incidence equals angle of reflection |
| Mirror formula | 1/f = 1/v + 1/u | Relates focal length, image and object distance |
| Mirror magnification | m = −v/u = h'/h | Sign indicates image orientation |
| Lens formula | 1/f = 1/v − 1/u | For thin lenses with sign convention |
| Lens magnification | m = v/u = h'/h | Positive for erect, negative for inverted |
| Lens maker formula | 1/f = (n−1)(1/R₁ − 1/R₂) | Focal length from radii and refractive index |
| Snell's law | n₁ sin i = n₂ sin r | Refraction at plane interface |
| Critical angle | sin c = n₂/n₁ (n₁ > n₂) | For total internal reflection |
| Prism deviation | δ = i + e − A | Angle of deviation through prism |
| Minimum deviation | n = sin[(A+δm)/2] / sin(A/2) | Refractive index from prism |
| Thin prism | δm = (n−1)A | For small angle prism |
| Power of lens | P = 1/f (f in metres) | Measured in dioptres (D) |
| Compound microscope | M = −(L/fo)(1 + D/fe) | L = tube length, D = least distance |
| Telescope | M = fo/fe | Angular magnification in normal adjustment |
How to approach Ray Optics problems
First identify element (mirror, lens, prism), then apply appropriate formula with sign convention. Use ray diagrams for image nature and position.
Mirrors & lenses
- Concave mirror? 1/f = 1/v + 1/u
- Convex lens? 1/f = 1/v − 1/u
- Magnification? m = −v/u or v/u
- Lens in liquid? Use nrel
- Image nature? Sign of m & v
Prism & instruments
- Prism deviation? δ = i + e − A
- Minimum δ? n formula
- Microscope? M = −(L/fo)(1+D/fe)
- Telescope (normal adjustment)? M = fo/fe
- TIR? i > c
How to prepare Ray Optics
Start with reflection, refraction and sign convention, then master mirror and lens formula. Finish with prism, optical instruments and TIR applications.
What to do
- Learn sign convention and ray diagrams for mirrors and lenses
- Practise mirror and lens formula with magnification
- Master lens maker formula and combination of lenses
- Understand prism deviation, minimum deviation and TIR
- Revise compound microscope and telescope formulas
Common mistakes
- Wrong sign convention in mirror/lens formula
- Confusing magnification sign for mirror and lens
- Using wrong radii signs in lens maker formula
- Forgetting condition for TIR (denser to rarer, i > c)
- Mixing microscope and telescope magnification formulas
Ready to test Ray Optics?
Revise the formula sheet, then solve mixed JEE Main problems on mirrors, lenses, prism and optical instruments.
Ray Optics FAQ
Short answers to frequently tested ideas in this chapter.
Mirror formula relates object distance (u), image distance (v) and focal length (f) as 1/f = 1/v + 1/u. It is valid for both concave and convex mirrors with proper sign convention.
Lens formula relates object distance (u), image distance (v) and focal length (f) as 1/f = 1/v − 1/u. It is used for thin lenses with proper sign convention.
Total internal reflection occurs when light travels from denser to rarer medium and angle of incidence exceeds critical angle. It is used in optical fibres and prisms.