CDS Physics · Free notes
Light and Optics: CDS Physics
Light and Optics has 42 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 20 EASY, 20 MODERATE and 2 HARD. Only Electricity and Magnetism has more in CDS Physics, and the two 2020 sittings alone asked 11 of these. Most items test one rule: which way a ray bends, where a convex lens puts its image, which radiation has the shorter wavelength. The few sums are one line with the mirror formula, the lens formula or n = c/v. Learn the image table for the convex lens and the order of the spectrum by heart, and keep refraction (twinkling) apart from scattering (the red Sun).
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Reflection and Mirrors
9 PYQsLight travelling in straight lines and the shadows it makes, the laws of reflection and the plane mirror, and how concave and convex mirrors form images.
Refraction and Lenses
17 PYQsWhy light bends between two media, the refractive index, total internal reflection, where a convex lens forms its image, and the lens formula and power.
Dispersion, Scattering and the Spectrum
11 PYQsThe electromagnetic spectrum in order of wavelength, how a prism and a raindrop split white light, and why the sky is blue, the rising Sun is red and stars twinkle.
The Eye and Optical Instruments
5 PYQsHow the eye focuses, the common defects of vision and the lenses that correct them, and which lenses make up a microscope and a telescope.
Formula & revision sheet
5 formulas · 7 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
5 formulas · 7 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Reference tables (1)
Light travels in straight lines: shadows5 rows
| Situation | What you see |
|---|---|
| Light in a uniform transparent medium | A straight pathQ CDS 2016 (II): light keeps a straight line only in a homogeneous, transparent medium. |
| Point source and an opaque object | Umbra only: a sharp shadow |
| Extended source and an opaque object | Umbra with a penumbra round it: a blurred edge |
| Pinhole camera | Inverted real image |
| Standing in the Moon's umbra | Total solar eclipse |
Watch out for (5)
- Straight lines need a uniform medium→ Light travels in straight lines: shadows
- Angles are measured from the normal→ Laws of reflection and the plane mirror
- A plane mirror's image is virtual→ Laws of reflection and the plane mirror
- The focus is at half the radius→ Concave and convex mirrors
- Erect at every distance means plane or convex→ Concave and convex mirrors
Formulas (3)
Reference tables (1)
Images formed by a convex lens5 rows
| Object position | Image (convex lens) |
|---|---|
| Beyond 2F | Between F and 2F; real, inverted, smaller |
| At 2F | At 2F; real, inverted, same size |
| Between F and 2F | Beyond 2F; real, inverted, enlargedQ CDS 2020 (II): an object 40 cm from a lens of focal length 25 cm gives an inverted, magnified image. |
| At F | At infinityQ CDS 2022 (II): an object at the focus of a convex lens has its image at infinity. |
| Between F and the lens | Same side; virtual, erect, enlarged |
Watch out for (8)
- Refractive index is c over v, not v over c→ Refraction and the refractive index
- Slower means towards the normal→ Refraction and the refractive index
- Higher refractive index, smaller critical angle→ Total internal reflection
- Enlarged and real needs F to 2F→ Images formed by a convex lens
- Object at the focus: no image at a finite distance→ Images formed by a convex lens
- Focal length in metres before taking the reciprocal→ Lens formula, magnification and power
- Bending power goes by size, not sign→ Lens formula, magnification and power
- Positive magnification means virtual→ Lens formula, magnification and power
Reference tables (3)
The electromagnetic spectrum6 rows
| Radiation | Wavelength and a use |
|---|---|
| Radio waves and microwaves | Longest, above about 1 mm; broadcasting and satellite communicationQ CDS 2020 (I): satellite communication uses radio waves. |
| Infrared | About 1 mm down to 700 nm; heat, remote controls |
| Visible light | About 700 nm to 400 nm; the only part the eye seesQ CDS 2018 (II): the visible portion of the electromagnetic spectrum is light. |
| Ultraviolet | Below 400 nm; sterilising, causes sunburnQ CDS 2020 (I): of radio, visible, infrared and ultraviolet, ultraviolet has the shortest wavelength. |
| X-rays | Much shorter; imaging bones |
| Gamma rays | Shortest; from radioactive nuclei |
Dispersion by a prism and the rainbow4 rows
| Step inside a raindrop | What happens to the light |
|---|---|
| Light enters the drop | Refraction and dispersion into colours |
| Light reaches the back of the drop | One internal reflection (primary rainbow)Q CDS 2024 (II): a rainbow needs both refraction and internal reflection of sunlight. |
| Light leaves the drop | Refraction again, spreading the colours further |
| Secondary rainbow | Two internal reflections; colours in reverse order |
Scattering and atmospheric refraction5 rows
| What you see | Cause |
|---|---|
| Blue sky | Scattering of short-wavelength light by air molecules |
| Reddish Sun near the horizon | Scattering removes blue over the long path through air |
| Red danger lights | Red is scattered least |
| Twinkling of stars | Atmospheric refractionQ Asked twice: CDS 2022 (I) and 2025 (II). The answer is refraction, never reflection, interference or diffraction. |
| Sun seen 2 minutes before sunrise | Atmospheric refraction |
Watch out for (6)
- Shorter wavelength means more energy, not less→ The electromagnetic spectrum
- Microwaves are part of the radio band→ The electromagnetic spectrum
- A rainbow needs refraction and reflection together→ Dispersion by a prism and the rainbow
- Violet bends most, red least→ Dispersion by a prism and the rainbow
- Twinkling is refraction; the red Sun is scattering→ Scattering and atmospheric refraction
- Blue is scattered most, red least→ Scattering and atmospheric refraction
Reference tables (2)
Defects of vision and their correction4 rows
| Defect | Cause and correction |
|---|---|
| Myopia (near-sightedness) | Lens too curved or eye-ball too long; concave lensQ CDS 2025 (I): myopia is also called near-sightedness. |
| Hypermetropia (far-sightedness) | Focal length too long or eye-ball too short; convex lens |
| Presbyopia | Ageing weakens accommodation; bifocal or progressive lensesQ CDS 2024 (II): presbyopia comes with age and is corrected with bifocal or progressive lenses; it does not affect colour vision. |
| Cataract | Cloudy eye lens; surgery |
Microscopes and telescopes4 rows
| Instrument | Lenses used |
|---|---|
| Simple microscope | One converging (convex) lensQ CDS 2025 (I): a simple microscope uses a converging lens. |
| Compound microscope | Convex objective of very short focal length, convex eyepiece |
| Astronomical telescope | Convex objective of long focal length, convex eyepiece of short focal length |
| Galilean telescope | Convex objective, concave eyepiece |
Watch out for (3)
- Near-sighted means it is the far objects that blur→ Defects of vision and their correction
- Myopia: lens too strong or eye-ball too long→ Defects of vision and their correction
- Telescope objective long, microscope objective short→ Microscopes and telescopes
PYQ weightage by concept
12 concepts · 42 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
12 concepts · 42 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Concave and convex mirrors | 4 | 10% |
| Laws of reflection and the plane mirror | 3 | 7% |
| Light travels in straight lines: shadows | 2 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Refraction and the refractive index | 6 | 14% |
| Images formed by a convex lens | 5 | 12% |
| Lens formula, magnification and power | 5 | 12% |
| Total internal reflection | 1 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| The electromagnetic spectrum | 6 | 14% |
| Scattering and atmospheric refraction | 3 | 7% |
| Dispersion by a prism and the rainbow | 2 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Defects of vision and their correction | 3 | 7% |
| Microscopes and telescopes | 2 | 5% |
Test yourself on Light and Optics
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.