Laws of Reflection

Laws of Reflection & Formation of Image by a Plane Mirror

Reflection of Light

• Reflection occurs when light strikes a surface and returns to the same medium.

• Reflected light is essential for seeing objects: luminous objects emit light, while non-luminous objects reflect light.

• Different surfaces reflect light differently; polished surfaces (like mirrors) reflect most of the light.

• Plane mirrors consist of a polished surface and a silvered backing, with light entering through the polished side.

Kinds of Reflection

• Regular Reflection: Occurs on smooth surfaces (e.g., mirrors); reflected light rays remain parallel and follow a fixed direction.

• Irregular Reflection: Occurs on rough surfaces; light is reflected in multiple directions, allowing visibility from various angles.

Reflection Terms

• Incident Ray: Light ray striking a surface.

• Point of Incidence: Where the incident ray hits the surface.

• Reflected Ray: Ray after reflection.

• Normal: Perpendicular to the surface at the point of incidence.

• Angle of Incidence (i): Angle between the incident ray and the normal.

• Angle of Reflection (r): Angle between the reflected ray and the normal.

• Laws of Reflection: 1) Angle of incidence equals the angle of reflection ($i = r$) 2) Incident ray, reflected ray, and normal are in the same plane.

Image Formation by Reflection

• Light rays from an object reflect off a mirror; to locate an image, at least two light rays are necessary.

• Types of Images:

  • Real Image: Can be projected on a screen; inverted.

  • Virtual Image: Cannot be projected; erect.

Characteristics of Plane Mirror Images

1. Erect,

2. Virtual,

3. Same size as the object,

4. Laterally inverted (left-right swapped).

Uses of Plane Mirrors

• Common in household mirrors, in periscopes, and in vehicles for visibility.

Spherical Mirrors

• Types:

  • Concave Mirrors: Curved inward; converge light rays.

  • Convex Mirrors: Curved outward; diverge light rays.

• Focal Length (f): Distance from the mirror's pole to its focus. For concave, f < 0; for convex, f > 0.

Key Formulas

• Relationship between object distance (u), image distance (v), and focal length (f):
  $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$

• Magnification: $m = \frac{I}{O} = \frac{v}{u}$.

Sign Convention for Spherical Mirrors

• Distances measured from the mirror's pole as the origin; incident light from the left is positive.

Summary of Image Properties in Concave Mirrors

• As position changes:

  • At Infinity: Image at focus (real, inverted).

  • Beyond C: Between focus and C (real, inverted).

  • At C: Same size (real, inverted).

  • Between C and F: Beyond C (real, inverted, magnified).

  • At F: At infinity (real, inverted, highly magnified).

  • Between F and P: Behind mirror (virtual, upright, magnified).

Uses of Concave Mirrors

• Used in shaving mirrors, reflectors for headlights, and as doctor's head mirrors for focused light.

Uses of Convex Mirrors

• Used as rear-view mirrors for vehicles to provide a wider field of view and as reflectors in street lamps.