Law of reflection & plane mirrors - Physics

• Light


  • Travels in a straight line
  • Radiates in all directions from a luminous object

  
• Light ray


  A line and arrow representing the direction and straight line path of light

  
• Types of matter based on how much light passes through

  • Transparent
  • Translucent
  • Opaque

  
• Fermat's Principle


  • Predicts the path light takes after reflecting from a surface or passing through a medium
  • Light follows the path that will take the least time

  
• Incident ray


  A ray of light that travels from a light source toward a surface

  
• Angle of incidence


  The angle between the incident ray and the normal in a ray diagram

  
• Normal


  A line that is perpendicular to a surface where a ray of light meets the surface

  
• Reflected ray


  Ray that begins at point where incident ray and normal

  
• Angle of reflection


  The angle between the reflected ray and the normal in the ray diagram

  
• If an incident ray hit at exactly 90 degrees to the mirror, the angle of incidence would be 0 degrees

  
• Plane Mirror Symbol


  Front part are mirrors consist of glass, back part thin layer of reflective silver or aluminum

  
• The symbol that is used in physics to represent a mirror refers only to the reflective thin film

  
• In this lesson, we will be looking at plane (flat) mirrors
• If you have a series of parallel incident rays that strike a plane mirror, the angles of incidence would all be identical

  
• Due to the law of reflection, the angles of reflection will also be identical and thus the reflected rays are also parallel to each other

  
• Specular reflection


  Reflection of light off smooth surface

  
• Diffuse reflection


  Reflection off rough or dull surfaces

  
• The angles of incidence are not parallel and thus the angles are not the same in diffuse reflection

  
• People with dyslexia have difficulty reading print due to too much glare off the white paper - too much reflected light
• Glasses that reduce reflected light can help some people with dyslexia
• Object


  The object in front of mirror
• Image


  The one in the mirror
• Virtual image


  An image in which light does not arrive at or come from the image location
• Light travels in a straight line and this belief is strong that when your eyes see reflected light from a plane mirror, it projects these rays backwards in a straight line

  
• This results in your brain thinking there is light source behind the mirror and this apparent light source behind the mirror that results in seeing an image behind the mirror
• Cannot have a real light source behind the mirror as the mirror is opaque
• Characteristics of images


  • Size (same size, larger, or smaller)
  • Altitude (upright or inverted)
  • Location (closer, farther, or same distance as object to mirror / in front or behind mirror)
  • Type (real or virtual)

  
• Locating image in plane mirror

  1. Draw line to represent mirror and dashed lines behind

  2. Draw incident ray from point A directly to mirror at 90° angle

  3. Draw another incident ray from point A to the mirror (any angle)

  4. Use dashed line to extend the reflected rays

  
• In plane mirrors, the distance between object and mirror and image and mirror are the same

  
• Size is also the same
• Since image is flipped, words appear backward
• Can use the fact that image distance and object distance are the same to draw the image