Learning Objectives
- Understand the laws of reflection
- Learn about regular and diffused reflection
- Understand how the human eye works
- Learn about multiple images, kaleidoscope, and dispersion of light
Key Concepts
Reflection of Light
When light falls on a polished or shiny surface, it bounces back. This phenomenon is called reflection of light.
Laws of Reflection:
- The angle of incidence (i) is equal to the angle of reflection (r): ∠i = ∠r
- The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane
These laws apply to all types of reflecting surfaces — plane, curved, rough, or smooth.
Regular and Diffused Reflection
Regular reflection: When parallel rays of light fall on a smooth surface and reflect as parallel rays. Produces clear images (e.g., plane mirror, still water).
Diffused (irregular) reflection: When parallel rays fall on a rough surface and reflect in different directions. No clear image is formed, but the object is still visible (e.g., wall, paper, unpolished surface). Diffused reflection is NOT a failure of the laws of reflection — each individual ray still follows the laws.
Multiple Images
When two plane mirrors are placed at an angle to each other, multiple images are formed. The number of images decreases as the angle between the mirrors increases. When mirrors are parallel (angle = 0°), infinite images are formed (e.g., in a barber shop).
Kaleidoscope and Periscope
Kaleidoscope: Uses three rectangular mirror strips arranged in a triangle. Coloured objects at one end produce beautiful, symmetrical patterns. Used by designers for pattern ideas.
Periscope: Uses two plane mirrors at 45° angles to see around corners or over obstacles. Used in submarines and by soldiers in trenches.
Dispersion of Light
When white light passes through a glass prism, it splits into its component colours. This is called dispersion of light. The band of seven colours is called the spectrum. The colours are: VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red. A rainbow is a natural phenomenon caused by dispersion of sunlight by water droplets.
The Human Eye
The human eye works like a camera. Key parts:
- Cornea: Transparent front covering; refracts light entering the eye
- Iris: Coloured part; controls the size of the pupil
- Pupil: Opening in the iris; regulates the amount of light entering
- Lens: Focuses light on the retina
- Retina: Screen at the back of the eye; contains light-sensitive cells (rods and cones)
- Optic nerve: Carries signals from retina to brain
The eye can adjust the focal length of its lens to see near and far objects. This ability is called accommodation.
Visually Challenged Persons
People who cannot see use other senses (touch, hearing) to navigate the world. Braille is a system of raised dots that visually impaired people can read by touch.
Summary
Light reflects following the laws of reflection. Smooth surfaces cause regular reflection; rough surfaces cause diffused reflection. Mirrors can form multiple images. White light disperses into seven colours (VIBGYOR) through a prism. The human eye uses a lens to focus light on the retina, and the brain interprets the image.
Important Terms
- Reflection: Bouncing back of light from a surface
- Angle of Incidence: Angle between the incident ray and the normal
- Angle of Reflection: Angle between the reflected ray and the normal
- Normal: A line perpendicular to the reflecting surface at the point of incidence
- Dispersion: Splitting of white light into seven colours
- Spectrum: The band of seven colours (VIBGYOR)
- Retina: The light-sensitive screen at the back of the eye
- Braille: Reading and writing system for visually impaired persons
Quick Revision
- Laws of reflection: ∠i = ∠r; incident ray, reflected ray, normal in same plane
- Regular reflection = smooth surface (mirror); Diffused = rough surface (wall)
- Two mirrors at angle: multiple images; parallel mirrors: infinite images
- Prism disperses white light into VIBGYOR
- Eye parts: cornea → iris/pupil → lens → retina → optic nerve → brain
- Periscope: two mirrors at 45°; Kaleidoscope: three mirrors in triangle