Class 10 – Light: Reflection and Refraction

Class 10 Science Chapter 9: Concepts

Chapter 9: Light – Reflection and Refraction

Introduction

Light is a form of energy that enables us to see objects. It travels in a straight line and exhibits properties like reflection and refraction. These properties help us understand the formation of images by mirrors and lenses.

Reflection of Light

Reflection is the phenomenon in which light bounces back after striking a smooth surface.

A. Laws of Reflection

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the reflected ray, and the normal to the surface lie in the same plane.
Diagram: Reflection of Light on a Plane Mirror

B. Types of Reflection

  • Regular Reflection: Occurs on smooth surfaces, producing clear images.
  • Diffuse Reflection: Occurs on rough surfaces, scattering light in multiple directions.

Spherical Mirrors

Spherical mirrors are curved mirrors that can be concave or convex.

A. Types of Spherical Mirrors

  1. Concave Mirror: Reflecting surface is curved inward.
  2. Convex Mirror: Reflecting surface is curved outward.

B. Important Terms

  • Pole (P): The center of the mirror’s surface.
  • Center of Curvature (C): The center of the sphere of which the mirror is a part.
  • Principal Axis: The straight line passing through P and C.
  • Focus (F): The point where parallel rays converge or appear to diverge.
Diagram: Concave and Convex Mirrors with Ray Diagrams

C. Image Formation by Spherical Mirrors

  • Concave Mirror: Forms real and inverted images, except when the object is between the focus and the mirror (then it forms a virtual and enlarged image).
  • Convex Mirror: Always forms virtual, erect, and diminished images.
Diagram: Ray Diagrams for Image Formation by Spherical Mirrors

D. Mirror Formula and Magnification

where,

  • f = Focal length
  • v = Image distance
  • u = Object distance

where, m is magnification.

Refraction of Light

Refraction is the bending of light when it passes from one medium to another due to a change in speed.

A. Laws of Refraction

  1. The incident ray, the refracted ray, and the normal to the surface lie in the same plane.
  2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant (Snell’s Law):where is the refractive index of the medium.
Diagram: Refraction of Light through a Glass Slab

B. Refractive Index

The refractive index determines how much light bends in a medium. It is given by:

where,

  • c = Speed of light in vacuum
  • v = Speed of light in the medium

Spherical Lenses

Lenses are transparent objects that refract light to form images. They can be convex or concave.

A. Types of Lenses

  1. Convex Lens (Converging Lens): Thicker in the middle, used in magnifying glasses and cameras.
  2. Concave Lens (Diverging Lens): Thinner in the middle, used in spectacles for myopia.
Diagram: Convex and Concave Lenses with Ray Diagrams

B. Image Formation by Lenses

  • Convex Lens: Forms both real and virtual images depending on object placement.
  • Concave Lens: Always forms virtual, erect, and diminished images.
Diagram: Ray Diagrams for Image Formation by Lenses

C. Lens Formula and Magnification

 

Power of a Lens

The power of a lens is the ability to converge or diverge light and is given by: where, P is power in diopters (D) and f is focal length in meters.

  • Convex Lens: Positive power
  • Concave Lens: Negative power
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