Light

Rectilinear Propagation, Reflection, Refraction and Image Formation by Mirrors/Lenses

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Overview

Light is a fundamental topic in the UPTET Paper II Science section, bridging everyday observations with core physics principles. Questions typically test your understanding of how light travels, how it behaves when it strikes surfaces or passes through different media, and how mirrors and lenses form images. This topic connects directly to practical applications like spectacles, cameras, and periscopes—contexts often used in exam questions.

For UPTET, you must be comfortable with ray diagrams, mirror and lens formulas, and the characteristics of images formed under different conditions. Conceptual clarity matters more than complex calculations; most questions are application-based or require you to predict image properties. Mastering this topic also helps in teaching Class 6–8 students, where light is introduced through hands-on activities and real-life examples.

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Key Concepts

• **Rectilinear propagation**: Light travels in straight lines in a homogeneous medium. This explains shadow formation, eclipses, and the working of pinhole cameras.

• **Luminous vs non-luminous objects**: Luminous objects emit their own light (sun, bulb); non-luminous objects reflect light from other sources (moon, books).

• **Reflection of light**: When light bounces off a surface. The angle of incidence equals the angle of reflection. Both angles are measured from the normal (perpendicular to the surface).

• **Types of reflection**: Regular/specular reflection occurs on smooth surfaces (mirrors); diffuse/irregular reflection occurs on rough surfaces (walls, paper).

• **Refraction of light**: Bending of light when it passes from one medium to another of different optical density. Light bends towards the normal when entering a denser medium and away when entering a rarer medium.

• **Refractive index**: The ratio of speed of light in vacuum to speed of light in a medium. Higher refractive index means denser medium and more bending.

• **Mirrors**: Plane mirrors produce virtual, erect, same-size images. Concave mirrors (converging) and convex mirrors (diverging) produce images depending on object position.

• **Lenses**: Convex lenses (converging) and concave lenses (diverging) form images based on object distance. Convex lenses can form real or virtual images; concave lenses always form virtual, erect, diminished images.

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Formulas / Key Facts

**Laws of Reflection** 1. Angle of incidence (i) = Angle of reflection (r) 2. Incident ray, reflected ray, and normal lie in the same plane