10.3: The Ray Model of 10.3: The Ray Model of LightLight

Transparent

Opaque

Translucent

Light passes through freely. A small amount of light is absorbed or reflected.

Prevents any light from passing through it. It will only absorb or reflect light.

Most light rays pass through, but are scattered in all directions.Di

ffere

nt

Mat

eria

ls

Shadow

Solid Object

Recall: Light travels in a straight line.

SHADOWS

A shadow is formed where light is 'missing'. A dark shadow (umbra) is formed where no light falls and a light shadow (penumbra) is formed where some light falls, but some is blocked.If the light source is very tiny and concentrated in one place (a point source) only a sharp shadow is formed.

SHADOWS: object & SHADOWS: object & screenscreen

If the source is broader light from the top of the source causes a lower shadow than that from the top. You therefore get partial shadow or penumbra as well as umbra.

CHANGES IN SIZE OF SHADOWS

The size of a shadow changes as you move the source closer or further from the screen

or as you move the object closer or further from the screen.

If there is only one point source of light, then when it is blocked, no light will reach the shadowed area and the shadow will be dark. If there is a lot of reflection, diffuse light, or multiple light sources, however, the shadow will be lighter.

Shadows OutsideOn a sunny day, most of the light is coming directly from the sun, but some of it is coming as blue scattered light coming from the sky.

If you stand in front of the sun, the sun's light is blocked, but your shadow still receives light from the rest of the sky, and you can still see the shadowed ground. 

On a cloudy day, the light is completely diffuse, not coming from anywhere in particular, and you don't cast much of a shadow at all. 

TerminologyTerminologyIncident light ray incoming ray

Reflected light rayray that bounces off the barrier

NormalAn imaginary line that is perpendicular to the barrier.Normal

Reflected RayIncident

Ray

Angle of incidenceformed by the incident ray and the normal

Angle of reflectionformed by the reflected ray and the normal.

Refle

ctio

nAngle of incidence is always equal to the reflected angle