ERHS PHYSICS

Chapter 17.1 Notes

LIGHT
 

Reflection: Light, as with water or sound, can be expected to reflect from dense surfaces. Reflection is the change in direction of a light ray as it bounces off such a surface. For extra reading, go to this site at Arizona State University and read about how light travels.

Reflection Definitions:

  • Ray (of light).. The smallest beam of light imaginable. Narrower than a pencil mark. So narrow that it actually has no dimension of width at all. However, such a beam is impossible to draw, so rays are typically shown wider than they really are when we construct them with pencil and paper.
  • Incident Ray. A ray of light which is approaching a reflective surface
  • Reflected Ray: A ray of light which leaves (has bounced off) a reflective surface.
  • Reflector: The surface of any medium which causes incident rays to bounce off.
  • Regular Reflectors: Also, specular reflectors: Reflectors which produce images, such as a mirror, shiny metal. Parallel incident rays are still parallel as reflected rays. In regular reflection, both the incident and reflected rays lie in a single plane.
  • Diffuse Reflectors: Parallel rays are spread out upon reflection so as to not produce an image. Example: painted walls, your skin.
  • Angle of Incidence: The angle between the incident ray and a line drawn normal to a reflective surface at the point of incidence. See diagram.
  • Angle of Reflection: The angle formed by the reflected ray and the normal. See diagram.

Law of Reflection: The angle of reflection is equal to the angle of incidence.

The laws of reflection apply to diffuse reflection. The irregular surface can be considered to be made up of a large number of small planar reflecting surfaces positioned at slightly different angles.

Indirect (or diffuse) lighting produces soft shadows. It produces less eye strain than harsher, direct lighting.

A sharp contrast between highlights and shadows is produced by direct lighting.

Strongly recommended: Go to the Arizona State site and go through the module on Reflection. Read the Objectives there too.

Refraction: As light passes through a media interface, it can be expected to bend, or change direction. The change of direction of a light ray as it passes through the interface between a translucent medium of one density into one of a different density is called refraction. At an interface, an incident ray can undergo partial reflection or, in certain situations, total internal reflection. For extra reading, Arizona State offers this module on Refraction.

No bending of the incident ray occurs if it strikes the boundary along the normal.

Refraction Definitions:

  • Refracted Ray: The ray of light leaving a media interface.
  • Angle of Refraction: The angle formed by the ray leaving a media interface and a line drawn normal to the point of incidence. See diagram
  • Incident medium: The medium containing the incident ray.
  • Refraction medium: the medium containing the refracted ray.

Snell's Law.

There is a relationship between translucent media that allows us to predict the behavior of light rays as they pass through the interface between them. The relationship includes the indices of refraction of each medium, and the sines of the angles of incidence and refraction. This relationship is known as Snell's law, and may be stated as:

  • The product of the index of refraction of the incident medium and the sine of the angle of incidence is equal to the product of the index of refraction of the refractive medium and the sine of the angle of refraction. See diagram.

If light is travelling from a less refractive medium to a more refractive medium (i.e., nr > ni), the refracted ray will be bent towards the normal. If lich is travelling from a more refractive medium into a less refractive one, (i.e., ni > nr ) the refracted ray will be bent away from the normal. Another way to remember this: The medium with the LARGER index of refraction will contain the SMALLER angle, and vice versa. (More is Less!)

Glossary of Terms

Homework: Set #1: Answer questions 1-9, p 410

Set #2: Ch 17 practice problems 1-8 page 400,402.

Set #3: Ch 17 problems31-40, 42,43

activity: PSSC Lab #28. Reflection in a plane mirror, Parallax and Ray Tracing.