ICSE Class 10 Physics: Spectrum and Colour — Notes 2026

Visible Spectrum and Dispersion

• White light: mixture of 7 colours (VIBGYOR); different wavelengths; violet (380 nm) to red (700 nm)
• Dispersion: splitting of white light into spectrum by prism; different colours refract at different angles
• Prism: glass or water; violet bends most (shortest wavelength, highest refractive index); red least

How a Prism Creates Spectrum

• Incident ray enters prism and refracts at first surface; exits at second surface and refracts again
• Net effect: each colour travels in slightly different direction; separated into spectrum
• Isaac Newton (1666): first scientific analysis of white light; showed colours can be recombined to white

Rainbow Formation

• Sunlight enters raindrop; refracts, reflects internally, refracts again on exit; dispersion occurs
• Primary rainbow: one internal reflection; red on outside, violet on inside; 40–42° from anti-solar point
• Secondary rainbow: two internal reflections; colours reversed (violet outside, red inside); fainter

Addition and Subtraction of Colours

• Primary colours of light (additive): Red, Green, Blue (RGB); Red + Green + Blue = White
• Complementary colours: each primary has complement; Red + Cyan = White; Green + Magenta = White
• Subtraction: pigments absorb certain wavelengths; red pigment absorbs all except red wavelengths; reflects red

Electromagnetic Spectrum

• Visible light: small part of EM spectrum; from radio waves (longest) to gamma rays (shortest)
• Infrared: beyond red; thermal radiation; remote controls; thermal imaging; night vision
• Ultraviolet: beyond violet; sunburn; disinfection (UV lamps); fluorescence; blocked by ozone layer

Scattering of Light

• Rayleigh scattering: light scattered by gas molecules; short wavelengths scatter more
• Blue sky: sunlight scatters; violet and blue scatter most; violet absorbed by atmosphere; blue reaches eye
• Red sunset: long path through atmosphere at sunset/sunrise; blue scattered away; red and orange reach eye

ICSE Exam Tips

• Spectrum topics tested: dispersion by prism, rainbow formation, addition of colours, blue sky explanation
• Draw diagram of prism dispersion: incident ray, refracted rays, spectrum with VIBGYOR label
• Calculation: not typically required for ICSE spectrum topic; focus on explanation quality