8.1 Usage of Mirrors
Light travels in straight lines at a speed of approximately $3 \times 10^8\text{ m/s}$ in a vacuum. Light allows us to see objects when light rays from a luminous source reflect off objects into our eyes.
Types of Mirrors and Images
- Plane Mirror: Flat reflecting surface. Produces a virtual, upright, laterally inverted image that is the same size as the object and located at an equal distance behind the mirror.
- Concave Mirror: Curving inwards (like the inside of a spoon). Converges light rays to a focal point. Used to enlarge close-up images (e.g., dental mirrors, makeup mirrors) or focus light (e.g., solar furnaces, flashlights).
- Convex Mirror: Curving outwards (like the back of a spoon). Diverges light rays and provides a wider field of view. Produces upright, virtual, and diminished images (e.g., security mirrors in stores, blind-spot corner mirrors, vehicle side mirrors).
8.2 Properties of Light
- Reflection of Light: Light bounces off opaque surfaces. Governed by the Laws of Reflection:
- The incident ray, the reflected ray, and the normal line all lie on the same plane.
- The angle of incidence ($i$) is equal to the angle of reflection ($r$): $$i = r$$
- Refraction of Light: The bending of light as it passes from one medium to another of different optical density due to a change in the speed of light.
- Dense medium to less dense medium (e.g., glass to air): Light bends away from the normal line.
- Less dense medium to dense medium (e.g., air to water): Light bends towards the normal line.
- Effects of refraction: Apparent depth of swimming pools, bending of a pencil in a glass of water.
- Dispersion of Light: White light is composed of seven distinct spectrum colors (ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet). When white light passes through a glass prism, each color bends at different angles due to different wavelengths (Red bends the least; Violet bends the most).
- Natural phenomenon: Rainbow formation when sunlight refracts and disperses through raindrops.
- Scattering of Light: The redirection of light rays in all directions by particles in the atmosphere (gas molecules, dust, water droplets).
- Blue Sky at Noon: Blue light has shorter wavelengths and is scattered the most by fine atmospheric particles.
- Red Sky at Sunset: Sunlight travels through a thicker atmospheric path; short-wavelength blue light is scattered away, leaving longer-wavelength red and orange light to reach our eyes.
8.3 Addition and Subtraction of Light
Primary and Secondary Colors of Light
- Primary Colors: Red, Green, Blue (RGB). Cannot be created by mixing other light colors.
- Secondary Colors: Formed by mixing two primary colors of light:
- $$\text{Red} + \text{Green} = \text{Yellow}$$
- $$\text{Red} + \text{Blue} = \text{Magenta}$$
- $$\text{Green} + \text{Blue} = \text{Cyan}$$
- $$\text{Red} + \text{Green} + \text{Blue} = \text{White Light}$$
Subtraction of Light (Color Filters and Colored Objects)
An opaque object reflects light of its own color and absorbs (subtracts) all other colors. Color filters allow light of their own color to pass through while absorbing other colors.