Spectrum ICSE Class 10 Physics: Dispersion, Electromagnetic Waves and Applications

The spectrum is not just a band of colours; it is the fingerprint of light. This note explains how a prism splits white light, why different colours deviate differently, and how the same physics extends to the invisible electromagnetic spectrum. Focus on understanding the cause, not memorising the list.

What is a Spectrum?

A spectrum is the ordered arrangement of light or other electromagnetic radiation according to wavelength or frequency. When a narrow beam of white light passes through a glass prism, it spreads into a continuous band of colours: red, orange, yellow, green, blue, indigo and violet. This band is called the visible spectrum.

The word spectrum means an appearance or image, but in physics it means the separation of light into its constituent colours. The prism does not create colours; it simply separates the colours that are already mixed together in white light. This is why a spectrum is such a powerful tool: it reveals the hidden composition of light.

Deviation and Dispersion by a Prism

When light enters a prism, it bends towards the normal on entering the glass and away from the normal on leaving. The net change in direction is called deviation. The amount of deviation depends on the angle of incidence, the angle of the prism, the refractive index of the glass, and the colour or wavelength of the light.

For a thin prism, the deviation is approximately δ = (μ − 1)A, where μ is the refractive index and A is the prism angle. This formula is useful because it shows that deviation increases when the refractive index increases.

Dispersion is the splitting of white light into its constituent colours. It happens because each colour has a different wavelength and therefore a different refractive index in glass. Red light is deviated least, while violet light is deviated most. Thus, in the spectrum formed by a prism, red appears at the top and violet at the bottom, with the other colours in between.

Why Violet Deviates More Than Red

In glass, red light has a longer wavelength and travels slightly faster, while violet light has a shorter wavelength and travels slower. The refractive index is given by μ = speed in vacuum ÷ speed in medium. Since violet slows down more in glass, μ for violet is greater than μ for red.

Take a numerical example. In vacuum, both red and violet travel at c = 3 × 10^8 m/s. Suppose red travels through glass at 2.0 × 10^8 m/s, so μ_red = 3.0 ÷ 2.0 = 1.5. Suppose violet travels through glass at 1.9 × 10^8 m/s, so μ_violet = 3.0 ÷ 1.9 ≈ 1.58. Since μ_violet is larger, Snell's law, μ = sin i ÷ sin r, tells us that violet bends more sharply for the same angle of incidence.

Also, the wave relation c = f λ links wavelength and frequency. In vacuum, c is constant, so a shorter wavelength means a higher frequency. Violet has a shorter wavelength and higher frequency than red, which is why it sits at the high-frequency end of the visible spectrum.

Recombination: Proving White Light is a Mixture

If a second identical prism is placed inverted in the path of the dispersed beam, the colours recombine and form white light again. This experiment, first performed by Newton, proves that dispersion is reversible and that white light is a mixture of colours.

Another important observation is that if a single colour from the spectrum is passed through a prism again, it does not split further. This shows that each spectral colour is monochromatic, meaning it contains only one wavelength. The prism only separates what is already present; it does not add anything to the light.

The familiar order VIBGYOR is a convenient memory aid, but the visible spectrum is actually continuous. The colours blend smoothly into one another, and indigo is not a scientifically sharp boundary but a useful label for the region between blue and violet.

The Electromagnetic Spectrum

Visible light is only a tiny part of the complete electromagnetic spectrum. In order of increasing frequency, the spectrum is: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. In order of increasing wavelength, the order is reversed.

All electromagnetic waves travel at the same speed in vacuum, c = 3 × 10^8 m/s, and obey the relation c = f λ. They do not need a material medium to travel, so they can pass through empty space. The difference between one type and another is only in frequency and wavelength, but that difference produces very different properties and uses.

Infrared radiation is felt as heat and is used in remote controls and night-vision devices. Ultraviolet radiation is used for sterilisation and detecting forged currency notes, but overexposure can damage skin. X-rays are used to image bones and teeth, while gamma rays are used in cancer treatment. Because ultraviolet, X-rays and gamma rays carry high energy, they can harm living cells, so proper shielding is essential.

Key takeaways

• White light is a mixture of colours; a prism separates them by dispersion.
• Red deviates least and violet deviates most because the refractive index of glass is higher for violet.
• A second inverted prism recombines the dispersed colours into white light, proving dispersion is reversible.
• Deviation depends on the angle of incidence, the angle of the prism, the refractive index, and the wavelength of light.
• Visible light is part of the electromagnetic spectrum; all electromagnetic waves travel at 3 × 10^8 m/s in vacuum and obey c = f λ.
• Infrared, ultraviolet, X-rays and gamma rays have important uses but can be harmful in excess.

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