Physics Notes on Light - Reflection, Refraction, Lenses

Reflection of Light

• Definition of Reflection:

  • Change in direction of a light wave upon hitting a surface.
  • Light waves bounce off surfaces, enabling us to see objects.

• Important Terms in Reflection:

  • Normal: Perpendicular line to the surface where light hits.
  • Angle of Incidence (i): Angle between the incident ray and the normal.
  • Angle of Reflection (r): Angle between the reflected ray and the normal.
  • Law of Reflection: Angle of incidence equals angle of reflection (90°).

• Common Mistakes:

  • Pay attention to the actual angles; always measure from the normal, not the surface.

• Creating Images through Reflection:

  • When light reflects off a surface (like a mirror), it produces a virtual image, which our brain perceives as coming from a location behind the mirror.
  • Properties of virtual images:
  • Same size as the object.
  • Same distance from the mirror as the object.
  • Laterally inverted (left and right reversed).

Ray Diagrams in Reflection

• Steps to Draw a Ray Diagram:

  1. Draw the object as a dot.
  2. Select two points on the object and draw incident rays hitting the mirror.
  3. Measure angles and draw reflected rays. Extend these rays backward to find where they intersect (this is the image).

• Quick Drawing Tips (Egyptian Method):

  • Draw one ray straight up to establish distance from the mirror.
  • Extend a second ray at any angle, applying the laws of reflection (measure normal).

• Example Reflection Questions:

  • If a person at point X can’t see certain parts of a wall in mirror reflection, identify limits by drawing incident and reflected rays from the person's perspective.
  • State properties of images formed by mirrors as virtual and inverted.

Refraction of Light

• Definition of Refraction:

  • Change in the speed and direction of light when it travels between two media (like air and glass).
  • Denser media slow down light.

• Speed of Light:

  • Speed of light in air: 3\times10^8 m/s
  • Speed in glass: 2\times10^8 m/s.
  • Refractive Index (n):
  • Defined as n = \frac{c{air}}{c{medium}}
  • Example: For glass, n = \frac{3\times10^8}{2\times10^8} = 1.5 (indicates light travels faster in air).

• Snell's Law:

  • Relation of angles of incidence and refraction: n = \frac{\sin i}{\sin r}
  • Always keep air (lower index) on top of the equation, ensuring results don’t vary with direction.

Critical Angle and Total Internal Reflection

• Critical Angle:

  • Angle of incidence when light refracts at 90^ ext{°}, denoted by C.
  • Calculated by rearranging Snell's law: n = \frac{1}{\sin C}
  • Example for glass (index 1.5): C \approx 42^ ext{°}.

• Total Internal Reflection: Occurs when:

  1. Light travels from a denser to a less dense medium.
  2. Angle of incidence exceeds critical angle.

Lenses

• Types of Lenses:

  • Convex (Converging): Thickest in the middle; rays converge at a focal point.
  • Image properties: Can be real (projected on screen) and inverted or virtual (visible through lenses).
  • Concave (Diverging): Thinner in the middle; rays appear to diverge from a focal point.
  • Image properties: Always virtual, upright, enlarged.

• Ray Diagrams for Lenses:

  • Convex Lens Drawing:
  1. Ray 1: Horizontal ray hitting lens, bends through focal point.
  2. Ray 2: Ray through the lens center (no bending). Intersect where image is formed (usually inverted).

Applications in Vision Correction

• Shortsightedness:

  • Light fails to focus on the retina (images are blurry).
  • Corrected with diverging lenses to spread light rays before reaching the eye.

• Longsightedness:

  • Light focuses behind the retina.
  • Corrected with converging lenses to focus light onto the retina.

Dispersion of Light

• Definition: Splitting of white light into its constituent colors (Roy G. Biv) when passing through a prism.
  • Different colors have different wavelengths and bend by varying amounts.
  • Red refracts the least, while violet refracts the most.