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Light Waves

Reflection of Light

  • Plane (flat) mirrors produce a reflection.
  • Rays from an object reflect off the mirror into our eyes, but we see them behind the mirror.
  • The image has these properties:
    • The image is the same size as the object.
    • The image is the same distance from the mirror as the object.
    • A line joining equivalent to the points of the image and object meet the mirror at a right angle.
    • The image is virtual; no rays pass through the image and the image cannot be formed on a screen
    • Laws of Reflection: Angle of incidence = Angle of reflection
      • The incident ray, reflected ray and normal are always on the same place (side of the mirror)
Refraction of Light
  • Refraction is the bending of light when light travels from one medium to another.
Experimental Demonstration
  • Using the ray box, pass a ray through a glass slab on a white sheet of paper.
  • Mark two points on the incident ray, refracted ray, emergent ray, and draw an outline of the glass slab with a pencil on the paper.
  • Then, by connecting the dots, you can produce a diagram like the one below, a protractor is used to find the angles.
  • When a ray passes through a transparent parallel-sided material, its passage will look like this:
  • NOTE: The emergent ray is parallel to the incident ray.
Critical Angle
  • The angle at which the refracted ray is parallel to the surface of the material.
  • If the angle of incidence is greater than the critical angle, there is no refracted ray, there is total internal reflection.
  • If the angle of incidence is less than the critical angle, the incident ray will split into a refracted ray and a weaker reflected ray.
  • Refractive Index, n = Speed of light in vacuum/Speed of light in a medium
  • Refractive index = sin i/sin r
  • Critical angle = sin-1 1/n
Optical Fibers
  • Light put in at one end is totally internally reflected until it comes out the other end.
  • It is used in communications: Signals are coded and sent along the fiber as pulses of laser light.
  • It is used in medicine:  An endoscope, an instrument used by surgeons to look inside the body, contains a long bundle of optic fibers.
Thin Converging Lens
  • Principal Focus is the point where rays parallel to the principal axis converge with a converging lens.
  • Focal Length is the distance from the principal focus and the optical center.
  • Principal Axis is the line that goes through the optical center, and the two foci.
  • Optical Center is the center of the lens.
  • A Real Image can be caught on a screen.
  • A Virtual Image cannot be caught on a screen.

Real Image

  • When the object is further away from the optical center than F‘ is

    1. A ray through the center of the lens passes straight through the lens.
    2. A ray parallel to the principal axis passes through the focus on the other side of the lens
    3. A ray through F’ will leave the lens parallel to the principal axis.

Virtual Image

  • When the object is closer to the optical center than F‘ is
    • Magnifying Glass
      • When a convex lens is used like this – an object is closer to a convex (converging) lens than the principal focus (like the diagram above), the rays never converge. Instead, the appear to come from a position behind the lens.
      • The image is upright, and magnified, it is a virtual image.
Dispersion of Light

  • Refraction by a Prism
    • When light is refracted by a prism, the incidence ray is not parallel to the emergent ray, since the prism’s sides are not parallel.
    • If a beam of white light is passed through a prism, it is dispersed into a spectrum.
    • White light is a mixture of colors, and the prism refracts each color by a different amount – red is deviated least and violet most.
Light Spectrum

Electromagnetic Spectrum

  • All electromagnetic waves:
    • travel at the speed of light; 3 × 108 m/s
    • don’t need a medium to travel through (they travel through a vacuum)
    • can transfer energy
    • are produced by particles oscillating or losing energy in some way
    • are transverse waves


  • Radio Waves: In radio and television communications
  • Microwaves: Satellite television and telephones
    • Safety Issue: They cause internal heating of body tissues
  • Infrared: Electrical appliances (radiant heaters and grills), remote controllers for televisions and intruder alarms
  • X-rays: Medicine (x-ray photography and killing cancerous cells) and security
    • Safety Issue: It is a mutagen, it can cause cancer (mutations)
  • Monochromatic: Light of a single wavelength and color (used in lasers)