Chap 14 - Light

• Light is part of the electromagnetic spectrum and energy can be transferred by the propagation of light waves
• A light ray always points from the light source to the eyes, and a beam of light is made up of many light rays
• A parallel beam of light
• For luminous objects (those that give off light on their own), a converging beam is produced
• To see an object, light travels from the source (e.g., a phone) to the eyes of the observer
• A diverging beam of light
• Reflection is the bouncing of light rays off a surface
• Ray diagram for reflection:

  • Incident ray
  • Normal
  • Angle of incidence
  • Angle of reflection
• Laws of reflection:

  • Incident ray: Light hitting the surface
  • Reflected ray: Light reflected from the surface
  • Angle of incidence: Angle between incident ray and normal
  • Angle of reflection: Angle between reflected ray and normal
  • Normal: Imaginary line perpendicular to the surface at the point of incidence
• First law of reflection: Incident ray, reflected ray, and the normal all lie in the same plane
• Second law of reflection: Angle of incidence is equal to the angle of reflection
• Types of reflection:

  • Regular reflection:
  • Smooth and flat surface
  • Clear image
  • Diffused reflection:
  • Rough surface
  • Diffused (not clear) image
• Characteristics of a plane mirror image:

  • Image is upright
  • Image is of the same size as the object
  • Distance from image to mirror = distance from mirror to object
  • Laterally inverted
  • Image is virtual
• Refraction is the bending of light as it passes from one optical medium to another
• Speed of light:

  • Fastest in vacuum (or air) at 3.0 × 10^8 m/s
• Comparing speed of light in different mediums:

  • Least optically dense: Gas (e.g., air)
  • Most optically dense: Solid (e.g., glass)
• Laws of refraction:

  • Incident ray, refracted ray, and the normal all lie in the same plane
  • Ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant (Snell's law)
• Refractive index is the ratio of the speed of light in vacuum to the speed of light in a medium
• Total internal reflection:

  • Complete reflection of a light ray inside an optically denser medium at its boundary with an optically less dense medium
• Conditions for total internal reflection:

  • Light ray must travel from an optically denser medium to an optically less dense medium
  • Angle of incidence must be more than the critical angle of the optically denser medium
• Critical angle, c, is the angle of incidence in the optically denser medium for which the angle of refraction in the optically less dense medium is 90°
• Total internal reflection occurs when the angle of incidence is greater than the critical angle
• Optical fibres are long, thin, and flexible, enabling data transmission over long distances
• The core of an optical fibre is usually made from glass or plastic, while the outer cladding has a lower refractive index to enable total internal reflection
• Converging lenses (convex lenses) focus light rays to a point and are often used as magnifying glasses
• Diverging lenses (concave lenses) spread out light rays and are often used to correct short-sightedness
• A converging lens has an optical centre, principal axis, principal focal point, focal length, and focal plane
• When parallel light passes through a converging lens not parallel to the principal axis, the rays are refracted to a point on the focal plane
• Three light rays can be drawn to locate the position of an image formed by a converging lens
• Types of images formed by a converging lens include inverted real, diminished inverted real, same size inverted real, and magnified upright virtual images
• Object distance (u) and image distance (v) relationships determine the characteristics and types of images formed by a converging lens