Chapter 9 Light

Introduction to Light:

• Light enables us to see objects by reflecting off them and entering our eyes.

• Light generally travels in straight lines, but it can bend around small objects (diffraction) or behave like a wave or particle under certain conditions.

• The chapter focuses on reflection and refraction of light.

Reflection of Light:

• Laws of Reflection:

  • The angle of incidence equals the angle of reflection.

  • The incident ray, reflected ray, and normal all lie in the same plane.

• Plane Mirrors: Produce virtual, erect, and laterally inverted images of the same size as the object.

• Spherical Mirrors:

  • Concave Mirror: Reflecting surface curves inward.

  • Convex Mirror: Reflecting surface curves outward.

  • Key terms: Pole (P), Centre of Curvature (C), Radius of Curvature (R), Principal Axis, Focal Length (f).

  • Relationship: R=2fR=2f.

Image Formation by Spherical Mirrors:

• Concave Mirrors:

  • Can produce real or virtual images depending on the object's position.

  • Real images are inverted; virtual images are erect.

  • Image size varies (diminished, same size, or enlarged).

• Convex Mirrors:

  • Always produce virtual, erect, and diminished images.

  • Used in rear-view mirrors for vehicles due to their wider field of view.

Refraction of Light:

• Light changes direction when passing from one medium to another due to a change in speed.

• Laws of Refraction (Snell’s Law):

  • The incident ray, refracted ray, and normal lie in the same plane.

  • sin⁡isin⁡r=constantsinrsini​=constant (refractive index).

• Refractive Index:

  • n=Speed of light in vacuumSpeed of light in mediumn=Speed of light in mediumSpeed of light in vacuum​.

  • Higher refractive index means the medium is optically denser.

Refraction through a Rectangular Glass Slab:

• Light bends towards the normal when entering a denser medium and away from the normal when exiting.

• The emergent ray is parallel to the incident ray but slightly displaced.

Spherical Lenses:

• Convex Lens: Converges light rays; thicker in the middle.

• Concave Lens: Diverges light rays; thicker at the edges.

• Key terms: Optical Centre (O), Principal Focus (F), Focal Length (f).

• Image Formation by Lenses:

  • Convex lenses can produce real or virtual images depending on the object's position.

  • Concave lenses always produce virtual, erect, and diminished images.

Lens Formula and Magnification:

• Lens Formula: 1v−1u=1fv1​−u1​=f1​.

• Magnification: m=h′h=vum=hh′​=uv​.

• Positive magnification indicates a virtual and erect image; negative indicates a real and inverted image.

Power of a Lens:

• Power (P): P=1fP=f1​ (in dioptres, D).

• Convex lenses have positive power; concave lenses have negative power.

• The power of a combination of lenses is the algebraic sum of their individual powers.

Applications of Mirrors and Lenses:

• Concave Mirrors: Used in torches, searchlights, shaving mirrors, and solar furnaces.

• Convex Mirrors: Used as rear-view mirrors in vehicles.

• Convex Lenses: Used in magnifying glasses, cameras, and corrective lenses.

• Concave Lenses: Used in correcting myopia (nearsightedness).

Sign Conventions:

• For mirrors and lenses, distances are measured from the pole or optical centre.

• Distances measured in the direction of incident light are positive; opposite directions are negative.

Key Phenomena:

• Twinkling of Stars: Due to atmospheric refraction.

• Rainbow Formation: Due to dispersion and refraction of light in water droplets.

• Bending of Light: Occurs when light passes through different media with varying refractive indices.