Dispersion of Light and the Electromagnetic Spectrum

1. Dispersion of White Light

Dispersion is the splitting of white light into its component colors as it passes through a transparent medium like a glass prism. This occurs because different colors of light travel at different speeds in glass, leading to different angles of refraction.

• The Spectrum: The band of colors produced is: Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV).
• Deviation: Red light is deviated the least (longest wavelength, highest speed in glass), while Violet light is deviated the most (shortest wavelength, lowest speed in glass).

2. Pure vs. Impure Spectrum

An impure spectrum occurs when colors overlap. A pure spectrum is one in which the colors are distinct and do not overlap. To produce a pure spectrum, we use:

• A narrow slit to produce a thin beam of light.
• A converging lens (collimator) to make the rays parallel.
• A prism to disperse the light.
• A second converging lens to focus the parallel rays of the same color to a specific point on a screen.

3. Colour Mixing

Colour Addition (Light)

Mixing light colors is additive. The primary colors of light are Red, Blue, and Green.

• Red + Green = Yellow
• Red + Blue = Magenta
• Blue + Green = Cyan
• Red + Blue + Green = White

Colour Subtraction (Pigments/Paints)

Mixing pigments is subtractive. The primary colors of pigments are Cyan, Magenta, and Yellow.

• Cyan + Magenta = Blue
• Magenta + Yellow = Red
• Yellow + Cyan = Green
• Cyan + Magenta + Yellow = Black

4. Colour of Objects and Filters

The color of an object is determined by the color of light it reflects. A red shirt appears red because it reflects red light and absorbs all other colors. If placed under green light, a red shirt appears black because there is no red light to reflect.

Filters: A filter allows only its own color to pass through (transmission) and absorbs others. A blue filter only transmits blue light.

5. The Rainbow

A rainbow is a natural phenomenon caused by refraction, dispersion, and total internal reflection of sunlight in water droplets. In a primary rainbow, light undergoes one total internal reflection inside the droplet.

6. Electromagnetic (EM) Spectrum

The EM spectrum consists of waves that do not require a medium for propagation and travel at the speed of light (3 x 10⁸ m/s).

• Order (Increasing Frequency/Decreasing Wavelength): Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays.
• Formula: v = fλ (where v is the speed of light, f is frequency, and λ is wavelength).

7. Newton's Law of Universal Gravitation

Though usually treated separately, Newton's law states that every particle attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G(m₁m₂)/r².