Atomic Physics & Optics
Atomic models, the Bohr model, light reflection, refraction, lenses, vision defects, and optical instruments.
A. Atomic Models (Historical)
Evolution of Atomic Models
| Scientist | Model | Key Feature |
|---|---|---|
| John Dalton | Solid sphere | Atoms are indivisible solid balls |
| J.J. Thomson | Plum pudding | Electrons embedded in a positive sphere |
| Ernest Rutherford | Nuclear model | Small dense nucleus; electrons orbit outside |
| Niels Bohr | Planetary / Bohr model | Electrons in fixed circular orbits (energy levels/shells) |
| Quantum Mechanical | Electron cloud | Electrons in probability clouds (orbitals) |
B. Bohr's Model of the Atom
Bohr's Postulates & Energy Levels
- Electrons orbit the nucleus in fixed energy levels (shells)
- Electrons absorb energy to jump to a higher level (excitation)
- Electrons emit energy as photons of light when falling to a lower level
- Each element produces a unique line spectrum — used to identify elements
Photon Energy (Planck's Relation)
E = h × f
h = Planck's constant = 6.63 × 10⁻³⁴ J·s · f = frequency of emitted photon
n=1
Ground State (K shell)
−13.6 eV
n=2
1st Excited State (L shell)
−3.4 eV
n=3
2nd Excited State (M shell)
−1.5 eV
n=∞
Ionisation level
0 eV
C. Light & Optics
Reflection & Refraction
Law of Reflection:
- Angle of incidence = Angle of reflection (measured from normal)
- Incident ray, reflected ray, and normal are all in the same plane
Refraction (Snell's Law):
- Refraction: bending of light as it passes from one medium to another
- Light bends towards normal when entering a denser medium (speed decreases)
- Light bends away from normal when entering a less dense medium
Snell's Law
n₁ sin θ₁ = n₂ sin θ₂
Refractive Index
n = c / v
c = speed of light in vacuum · v = speed in medium
D. Lenses
Convex & Concave Lenses
| Lens Type | Shape | Effect | Use |
|---|---|---|---|
| Convex (Converging) | Thicker in middle | Converges rays to focal point | Magnifying glass, camera, eye lens, projector |
| Concave (Diverging) | Thinner in middle | Diverges (spreads) parallel rays | Correcting short-sightedness (myopia) |
Convex (Converging)
ShapeThicker in middle
EffectConverges rays to focal point
UseMagnifying glass, camera, eye lens, projector
Concave (Diverging)
ShapeThinner in middle
EffectDiverges (spreads) parallel rays
UseCorrecting short-sightedness (myopia)
Lens Formula
1/f = 1/v − 1/u
f = focal length · v = image distance · u = object distance
Magnification
m = image height / object height = v / u
E. Defects of Vision
Common Vision Defects
| Defect | Problem | Cause | Correction |
|---|---|---|---|
| Myopia (short-sightedness) | Cannot see distant objects clearly | Eyeball too long; image forms in front of retina | Concave (diverging) lens |
| Hypermetropia (long-sightedness) | Cannot see near objects clearly | Eyeball too short; image forms behind retina | Convex (converging) lens |
| Astigmatism | Blurred vision at all distances | Irregular cornea curvature | Cylindrical lens |
| Presbyopia | Loss of near vision with age | Lens loses flexibility with age | Bifocal lenses |
Myopia (short-sightedness)
ProblemCannot see distant objects clearly
CauseEyeball too long; image forms in front of retina
CorrectionConcave (diverging) lens
Hypermetropia (long-sightedness)
ProblemCannot see near objects clearly
CauseEyeball too short; image forms behind retina
CorrectionConvex (converging) lens
Astigmatism
ProblemBlurred vision at all distances
CauseIrregular cornea curvature
CorrectionCylindrical lens
Presbyopia
ProblemLoss of near vision with age
CauseLens loses flexibility with age
CorrectionBifocal lenses
F. Optical Instruments
Microscopes & Telescopes
| Instrument | Function | Key Feature |
|---|---|---|
| Compound Microscope | Magnifies very small objects (e.g. cells) | Two convex lenses: objective (short f) + eyepiece |
| Astronomical Telescope | Views distant objects (stars, planets) | Two convex lenses: objective (long f) + eyepiece |
| Simple Microscope | Magnifying glass for small nearby objects | Single convex lens |
| Periscope | Views over obstacles | Two plane mirrors at 45° |
⚡ MCQ Tip
Convex lens = converging = corrects long-sightedness (hypermetropia).
Concave lens = diverging = corrects short-sightedness (myopia).
Snell's Law: n₁sinθ₁ = n₂sinθ₂. Bohr: electrons in fixed orbits emit photon when dropping down.
Live Animation: Bohr Model — Electron Orbit Jumps
Hydrogen Atom — Photon Absorption & Emission
Absorb photon → electron jumps up · Falls back → emits coloured photon
Electron shell: n = 1 (Ground)
Photon energy: —
Photon colour: —
State: Ground state
Quick MCQ Revision
| Formula / Fact | Meaning |
|---|---|
| E = hf | Photon energy = Planck's constant × frequency |
| n₁sinθ₁ = n₂sinθ₂ | Snell's Law of Refraction |
| n = c/v | Refractive index = c ÷ speed in medium |
| 1/f = 1/v − 1/u | Lens formula (f = focal length) |
| m = v/u | Magnification = image distance ÷ object distance |
| Convex lens | Converging — corrects hypermetropia (long-sightedness) |
| Concave lens | Diverging — corrects myopia (short-sightedness) |
| Bohr model | Electrons in fixed energy levels; emit photon when dropping down |
| h | Planck's constant = 6.63 × 10⁻³⁴ J·s |
| c | Speed of light = 3 × 10⁸ m/s |