Ray Optics and Optical Instruments
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Reflection by Spherical Mirrors
Part of: Mirrors, Refraction and TIR
Concave mirrors converge light and convex ones diverge it. With the Cartesian sign convention, distances in front are negative, and the mirror formula links object distance, image distance and focal length, while magnification is minus image over object distance.
Why it matters. Headlights, shaving mirrors and street mirrors all exploit these rules. Sign slips flip real and virtual answers.
Formulas
- Mirror: 1/v + 1/u = 1/f
- Magnification: m = -v/u = height ratio
- Focal length: f = R/2
Key words
- Focal length
- : Half the radius of curvature, where parallel rays converge or appear to diverge.
- Magnification
- : Image height over object height, equal to minus image distance over object distance.
- Real image
- : Image formed where rays actually meet, catchable on a screen.
- Virtual image
- : Image where rays only appear to meet, seen but not caught on screen.
Try it in two minutes
- Mark object distance negative in front.
- Combine with focal length for image distance.
- Divide for size and check the sign for erect or inverted.
Try at home. Find your image in a spoon's two sides and compare sizes.
Refraction and Total Internal Reflection
Part of: Mirrors, Refraction and TIR
Light bends toward the normal entering denser media by Snell's law, with refractive index equal to c over speed in the medium. A ray inside a denser medium beyond the critical angle cannot escape and reflects fully, which lifts fish visually, creates mirages and traps light in fibres.
Why it matters. Lenses, prisms, fibres and sparkling gems all depend on bending and trapping. Mixing up the bending direction or the TIR condition reverses every prediction.
Formulas
- Snell: n1 sin(i) = n2 sin(r)
- Index: n = c / v
- Critical: sin(C) = 1 / n
- Apparent depth = real depth / n
Key words
- Refractive index
- : Speed of light in vacuum over speed in the medium.
- Snell's law
- : Index times sine of angle matches on both sides of a boundary.
- Critical angle
- : Incidence giving 90-degree refraction, beyond which reflection is total.
- Apparent depth
- : Raised position of an underwater object, real depth over index.
Try it in two minutes
- Multiply sines by indices on the two sides.
- Divide real depth by index for apparent depth.
- Compare incidence with critical angle for escape or trap.
Try at home. Watch a coin rise as water fills a cup and link it to apparent depth.
Lenses: Formula, Power and Combinations
Part of: Lenses, Prism and Instruments
Convex lenses converge and concave ones diverge. The maker's formula sets focal length from index and surface curves, the lens formula links object and image distances with the Cartesian sign rule, and power in dioptres adds directly for lenses in contact.
Why it matters. Spectacles, cameras and microscopes are designed from these numbers. Sign slips swap real and virtual, and adding focal lengths instead of powers ruins combinations.
Formulas
- Lens: 1/v - 1/u = 1/f
- Maker: 1/f = (n - 1)(1/R1 - 1/R2)
- Power: P = 100 / f(cm); contact: P = P1 + P2
Key words
- Lens maker's formula
- : Focal length from index and the two surface curvatures.
- Power of lens
- : One over focal length in metres, measured in dioptres.
- Focal plane
- : Plane through the focus where parallel tilted rays meet.
Try it in two minutes
- Check the signs of both surface curves.
- Combine index and curves for focal length.
- Link object and image distances, then add powers in contact.
Try at home. Focus sunlight with a magnifier and estimate its focal length.
Prism and Optical Instruments
Part of: Lenses, Prism and Instruments
A prism bends light toward its base, most at grazing entry and least at the symmetric minimum-deviation position, splitting colours since index varies. Magnifiers, compound microscopes and telescopes multiply viewing angles, with power set by focal lengths and near-point distance.
Why it matters. Spectrometers identify materials by spectra, and labs choose instruments by magnification and light grasp. Missing the symmetry condition or the angle-dependence misreads every deviation.
Formulas
- Prism index: n = sin((A + dm)/2) / sin(A/2)
- Magnifier: M = 1 + D/f (image at near point)
- Telescope: M = -fo/fe
Key words
- Angle of deviation
- : Bend between incoming and outgoing rays through a prism.
- Minimum deviation
- : Smallest bend, occurring when the ray passes symmetrically.
- Magnifying power
- : Ratio of viewing angle with instrument to that with the naked eye.
- Near point
- : Closest comfortable viewing distance, about 25 cm.
Try it in two minutes
- Trace a ray symmetric through the prism.
- Divide 25 cm by a magnifier's focal length.
- Divide objective by eyepiece focal length for a telescope.
Try at home. Split sunlight with a glass slab edge and order the colours.
What your child will practise
- Lenses: Formula, Power and Combinations: Candy Quick Fire (Quick Fire)
- Prism and Optical Instruments: Desert Quick Fire (Quick Fire)
- Prism and Optical Instruments: Ocean Puzzle (Clue Puzzle)
- Reflection by Spherical Mirrors: Space Quick Fire (Quick Fire)
- Refraction and Total Internal Reflection: Candy Puzzle (Clue Puzzle)
- Quiz: Lenses: Formula, Power and Combinations (8 questions)
- Quiz: Prism and Optical Instruments (8 questions)
- Quiz: Reflection by Spherical Mirrors (8 questions)
- Quiz: Refraction and Total Internal Reflection (8 questions)