LIGHT The Ray Model of Light Textbook Section

LIGHT The Ray Model of Light Textbook Section 10. 3

RECAP • Remember: Light is a form of energy that consists of particles (electrons called photons), but which also travels in waves (like a the waves of a ripple in a pond, expanding in all directions).

RECAP: The Wave Model of Light • We’ve looked at the Wave Model of Light • Wavelength, frequency, speed, amplitude • Electromagnetic spectrum • Visible light • Frequencies: from 4. 3 x 1014 to 7. 5 x 1014 • Wavelengths: from 700 nm to 400 nm • Colours created by frequency of light • The colours we see = the light reflected

The Ray Model of Light • Now we will discuss the Ray Model of Light • The Ray Model of Light illustrates the properties that show light rays interacts with matter (objects, a medium, etc. )

The Ray Model of Light In the Ray Model of Light, light is represented in straight lines called rays, which show the direction in which light travels. Ray: Straight line with a single point of origin and extending infinitely in one direction

The Ray Model of Light • In reality, all points on an object (i. e. an infinite number of points) are a source of an infinite number of rays that radiate out in all directions. • In diagram representations, however, we only draw a few of the rays radiating from a given object.

The Ray Model of Light • Ray diagrams show the direction that the light ray takes after it leaves its source. • Rays always end in an arrow to indicate the direction in which the light is traveling.

The Ray Model of Light • The closer a source of light is to the eyes, the more light rays that reach the eye, and the brighter the source of light appears.

Ray Diagrams: Three Main Uses 1) Used to describe how light moves when light rays strike an object. 2) Used to explain whether a shadow will be sharp or have blurred (diffuse) edges 3) Used to explain the size and location of shadows

Ray Diagrams: (1) 1) Can be used to describe how light moves when light rays strike an object.

Ray Diagrams: (1) • When a light ray strikes an object, it can: • Be transmitted fully (the way it does through transparent objects) • Be transmitted but only to an extent (the way it does with translucent objects) • Not be transmitted at all (the way it does with opaque objects)

Ray Diagrams: (1) O B J E C T T Y P E ALL light passes through SOME light passes through NO light passes through

Ray Diagrams: (2) 2) Used to explain whether a shadow will be sharp or have blurred edges

Shadows • Shadows form when an opaque object blocks the light rays directed at the object from a light source.

Ray Diagrams: (2) 2. Whether a shadow will be sharp or have blurred edges depends on the size of the light source and the size of the light-blocking object in relation to one another

Ray Diagrams: (2) 2. 1. A. If the light-blocking object is large relative to the light source (i. e. the light source is small, a point source of light), the shadow will be sharp • Because the object, by virtue of being large, can fully block the rays that the light source emits

Ray Diagrams: (2) 2. 1. B If the object is small in relation to the light source (i. e. the light source is large, a nonpoint source), the shadow will be blurry • Because the object, by virtue of being small, can block only a portion of the rays emitted by the light source, such that the edges are only partially blocked • The wider the light source, the more blurry the shadow will be

Ray Diagrams: (2) • The part of the shadow in which ALL light rays are blocked by the object is called the umbra • The part of the shadow in which some, but not all, light rays are blocked by the object is called the penumbra

Ray Diagrams: (2) • Large sources of light form an umbra as well as a penumbra because not all of their rays are blocked (i. e. the penumbra is the blurred outline) • Small sources of light are fully blocked, so they only create an umbra (no penumbra, i. e. no blurred parts)

Ray Diagrams: (3) 3. Used to explain the size and location of shadows The size of a shadow depends on the distance between the light-blocking object and the light source • The closer the object is to the light source (i. e. the shorter the distance between the two), the larger the shadow will be.

Light Reflection • We can see the world around us because light reflects off surfaces. • Incoming rays travel parallel to one another. • Angle of incidence is always equal to angle of reflection. • The angle of incidence and the angle of reflection are formed on the same plane.

Light Reflection: Regular Reflection • In regular reflection, when light rays hit a surface, all rays are reflected in the same direction as each other • This is because the surface is flat, so the angle of reflection is the same for each ray. • Creates an image on surface.

Light Reflection: Regular Reflection

Light Reflection: Diffuse Reflection • In diffuse reflection, when light rays hit a surface, the rays are reflected still at the same angle of incidence, but they are no longer reflected in the same direction as each other • This is because the surface is not flat, so each point on the surface has its own unique slope, with its own unique “normal” (the line perpendicular to that slope).

Light Reflection: Diffuse Reflection Note the “normals” are perpendicular to the unique slope of each specific point on the surface.

Light Reflection: Diffuse Reflection Since every normal is perpendicular to its own unique slope on the surface, every normal will have its own unique orientation. As such, the rays, although still exhibiting the “Angle of Incidence = Angle of Reflection” rule, will not be reflected in the same direction as one another.

Light Reflection: Diffuse Reflection • Objects that exhibit diffuse reflection cannot reflect an image on their surface, but they do still reflect light.

Light Reflection: Diffuse Reflection • Your textbook, for example, is of a rough enough surface to cause diffuse reflection (as opposed to regular reflection), so it does not reflect images. But, since it does reflect light (scattered in all different directions, through diffuse reflection), you can still see the contents of the page from any angle you look at it.