CDS Physics · Light and Optics
Dispersion, Scattering and the Spectrum
The 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.
Why this matters
Eleven CDS questions from 2017 (II) to 2025 (II). Six place a radiation on the electromagnetic spectrum, two ask how a rainbow forms, and three explain a sight in the sky: the red Sun at the horizon and, twice, the twinkling of stars.
Concept 1 of 3: The electromagnetic spectrum
Visible light is only a thin slice of a much wider family of waves, the electromagnetic waves. All of them are transverse, all travel at the speed of light, and none needs a medium. They differ only in wavelength. The shorter the wavelength, the more energy each photon carries, which is why ultraviolet can burn skin and radio waves cannot.
Definition
The family, from longest wavelength to shortest:
- Radio waves (including microwaves) → infrared → visible light → ultraviolet → X-rays → gamma rays.
- Visible light runs from about 700 nm (red) to 400 nm (violet): the only part the eye can see.
- Photon energy : shorter wavelength, more energy. So ultraviolet photons carry more energy than visible ones, and visible more than infrared.
- All are transverse electromagnetic waves and travel through a vacuum at about m/s.
- Uses: radio and microwaves for broadcasting, mobile phones and satellite communication; infrared for remote controls and night vision; ultraviolet for sterilising; X-rays for imaging bones.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (I) 2019 — General Knowledge · Q60Moderate
Example 1 · Light and Optics · Dispersion, Scattering and the Spectrum
If the wavelengths corresponding to ultraviolet, visible and infrared radiations are given as , and respectively, then which one of the following gives the correct relationship among these wavelengths?
Shorter wavelength means more energy, not less
Energy per photon rises as the wavelength falls. Ultraviolet has a shorter wavelength than visible light and so more photon energy; infrared has a longer wavelength and less.
Microwaves are part of the radio band
Satellite links use microwave frequencies, which lie within the radio-wave region of the spectrum. Infrared, ultraviolet and visible light are not used for satellite communication.
Concept 2 of 3: Dispersion by a prism and the rainbow
White light is a mixture of colours. Glass slows each colour by a slightly different amount, so a prism bends each colour by a slightly different angle and they fan out into a band: the spectrum. Violet is bent the most and red the least. A raindrop does the same job as a prism, with one extra step: the light bounces once off the back of the drop before coming out.
Definition
Splitting white light:
- Dispersion: splitting white light into its colours, because the refractive index of glass is different for each colour.
- Order of colours: VIBGYOR (violet, indigo, blue, green, yellow, orange, red). Violet deviates most, red least.
- Newton first showed with two prisms that white light is a mixture of these colours.
- Rainbow: sunlight enters a raindrop and is refracted and dispersed, is reflected once inside the drop, and is refracted again as it leaves. So it needs both refraction and internal reflection.
- A rainbow is seen opposite the Sun, with the Sun behind the observer. A fainter secondary rainbow comes from two internal reflections and has its colours reversed.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (I) 2021 — General Knowledge · Q117Moderate
Example 2 · Light and Optics · Dispersion, Scattering and the Spectrum
How many internal reflections of light take place in the formation of primary rainbow ?
A rainbow needs refraction and reflection together
Refraction alone gives no rainbow, and nor does reflection alone. Light is refracted in, reflected once inside, and refracted out. An option naming only one of these is incomplete.
Violet bends most, red least
Glass has its highest refractive index for violet light, so violet is deviated the most. Red, with the longest wavelength, is deviated the least.
Concept 3 of 3: Scattering and atmospheric refraction
Air molecules are tiny, and they scatter short-wavelength blue light far more strongly than long-wavelength red light. So blue light from the Sun is thrown about the whole sky, and the sky looks blue. Separately, the air is not uniform: layers of different temperature bend light by different amounts, and since they keep moving, a star's light keeps shifting. That is why stars twinkle.
Definition
Two effects of the atmosphere:
- Scattering by small particles is much stronger for shorter wavelengths (blue) than for longer ones (red).
- Blue sky: scattered blue light reaches the eye from every direction. In space, with no air, the sky looks black.
- Red Sun at sunrise and sunset: near the horizon sunlight crosses much more air, most of the blue is scattered out of the beam, and mainly red reaches us.
- Danger signals are red because red is scattered least and can be seen from far away.
- Atmospheric refraction: starlight bends through moving air layers of changing refractive index, so a star's apparent position and brightness flicker: twinkling. Planets do not twinkle, because they look like small discs, not points.
- The same bending lets us see the Sun about 2 minutes before it actually rises and after it sets.
| 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2018 · CDS (I) 2018 — General Knowledge · Q34Easy
Example 3 · Light and Optics · Dispersion, Scattering and the Spectrum
When the Sun is near the horizon during the morning or evening, it appears reddish. The phenomenon that is responsible for this observation is
Twinkling is refraction; the red Sun is scattering
Two sky questions, two different causes. Stars twinkle because of atmospheric refraction. The Sun looks red at the horizon because of scattering. Dispersion and reflection are distractors for both.
Blue is scattered most, red least
Scattering by air molecules grows sharply as the wavelength falls. That one rule gives the blue sky, the red sunset and the red danger light.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
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
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.