Dispersion of Light and Colours

1. Dispersion of White Light

Dispersion is the splitting of white light into its constituent spectral colors when 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 refractive indices for each color. Red light is deviated the least, while violet light is deviated the most.

• The Spectrum: The band of colors produced is: Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV).
• Production of a Pure Spectrum: An impure spectrum has overlapping colors. To produce a pure spectrum, we use a narrow slit, a converging lens to make the light parallel, a prism to disperse the light, and a second converging lens to focus the colors onto a screen.

2. Colour Mixing

Colours can be mixed in two ways: addition (mixing light) and subtraction (mixing pigments).

Colour Mixing by Addition (Light)

• Primary Colours: Red, Blue, and Green.
• Secondary Colours: These are formed by mixing two primary colors.
  • Red + Green = Yellow
  • Red + Blue = Magenta
  • Blue + Green = Cyan
• Complementary Colours: Any two colors that produce white light when mixed (e.g., Yellow + Blue).

Colour Mixing by Subtraction (Pigments/Filters)

This involves absorbing certain wavelengths and reflecting others. The primary pigments are Cyan, Magenta, and Yellow.

3. Colour of Objects and Filters

• Objects: The color of an opaque 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.
• Filters: A filter allows only its own color to pass through and absorbs others. A green filter will only transmit green light.

4. The Rainbow

A rainbow is a natural phenomenon caused by dispersion, refraction, and total internal reflection of sunlight in water droplets. Two refractions and one internal reflection produce a primary rainbow.

5. Electromagnetic Spectrum

The electromagnetic spectrum is a range of all types of EM radiation. They all travel at the speed of light (3 x 10^8 m/s) in a vacuum but differ in frequency and wavelength.

• Radio Waves: Longest wavelength, used in communication.
• Infrared: Sensed as heat, used in remote controls and night vision.
• Visible Light: Detected by the human eye.
• Ultraviolet: Causes tanning, used in sterilization.
• X-rays: High energy, used in medical imaging.
• Gamma Rays: Highest frequency, used in cancer treatment.

6. Newton's Law of Universal Gravitation

Newton's Law states that the gravitational force (F) between two masses (m1 and m2) is directly proportional to the product of their masses and inversely proportional to the square of the distance (r) between them:

F = G(m1 * m2) / r^2

Where G is the universal gravitational constant (6.67 x 10^-11 Nm^2/kg^2).