Properties of Light and Reflection

Reflection Is the change in direction of a wave when it reaches a surface and bounces off that surface.

Rays of Light Light travels in s straight line as long as it is moving through a medium. Medium is the substance through which light travels. You can use a technique called ray tracing to make ray diagrams. A ray is a straight line with an arrowhead that shows the direction in which light waves are traveling.

Fermat’s Principle Predicts the path that light will take after reflecting from a surface or passing through more than one medium. According to the principle, light follows the path that will take the least time. When light reflects from a surface and stays in the same medium, its speed is constant.

Laws of Reflection A ray of light coming toward a surface is called an incident ray. The angle of incidence is measured between the incident ray and a perpendicular line drawn from the point of contact of the incident ray at the surface. Perpendicular line is called the normal. The reflected ray begins at the point of contact. The angle of reflection is measured between the reflected and the normal.

The angle of reflection and the angle of incidence are the same. The reflected ray always lies on the plane that is defined by the incident ray and the normal. These laws of reflection apply to light and all other forms of waves (sound waves).

Drawing Ray Diagrams Step 1: Draw the incident ray using a ruler Step 2: At the contact point, draw a normal at 90with a protractor Step 3: measure the angle of incidence (i) between incident ray and the normal. Step 4: Draw the reflected ray Step 5: label the reflected ray, the incident ray, the angle of incidence (i), the angle of refraction (r) and the normal.

normal Incident ray Reflected ray i r Reflecting surface

Images in Plane Mirrors If you apply the laws of reflection to rays going from an object, you can predict where the image will be and what the image will look like. A plane mirror is a mirror with a flat, reflective surface. The brain assumes light travels in a straight line, so to find out where the eye sees the image, extend the rays that reach the eye backwards until the meet at a point beyond the mirror.

Image in Plane Mirror

Image in Plane Mirror

Virtual Images An image formed by rays that appear to be coming from a certain position, but are not actually coming from this position; image does not form a visible projection on a screen. If light rays hit a screen and form an image, the image is real and not virtual. If the image is behind a mirror, there is no way that light rays could get there, and it would be virtual.

Ray Diagrams and Plane Mirrors An image has four characteristics: Its location (closer than, farter than, same distance as the object to the mirrow) Orientation (upright or inverted) Size (same size, larger, smaller than the object) Type (real image or virtual image).

The four characteristics of an image in a plane mirror are always: You can predict characteristics by drawing ray diagrams to locate the image of an object. The four characteristics of an image in a plane mirror are always: Same size Same distance Same orientation Virtual image

You Try It! On page 416 Follow the instructions in table 10.1 in order to draw a pencil. P. 418 # 1-4, 6-8. Section 10.2 Review Handout