Chapter 5: Optical Systems 5.1: The Ray Model of Light

The particle model of light The particle model refers to the idea that light is a stream of tiny, fast-moving particles. The wave model refers to the idea that light travels in waves with different frequencies and wavelengths. The ray model of light represents waves of light as rays or straight lines.

Matter and Light The type of matter in an object will determine the amount of light it absorbs, reflects or transmits. What you see depends on the objects and their colours as well as the amount of light present. In a dark room, we don’t see the colours of objects since less light is present.

Classifying Materials We can consider 3 types of materials: 1. transparent 2. translucent 3. opaque Let’s look at how rays of light interact with each type of material…

1. Transparent Transparent materials are sometimes called “see-through”. This means that light can pass through freely. Only a small amount of light is absorbed and reflected. We can see objects through transparent materials The transparent toaster!

2. Translucent Translucent material allows most light rays to pass through but scatters them in all directions. This material does not allow you to see objects clearly Ex: frosted glass

3. Opaque Opaque material prevents any light from passing through it. It only absorbs and reflects light (no light passes through) An opaque tent

Shadows Shadows are caused by an object blocking a light source. When you are walking in the sun, your body casts a shadow because it blocks the light rays striking you.

What do shadows tell us?... A shadow can tell us how far an object is from a light source by the size of the shadow. An object that is closer to a light source has a larger shadow compared to an object that is farther away.

The biggest shadow of all…Lunar Eclipse

Reflection of Light Light can be reflected off a surface Ex: a piece of white paper or a mirror Smooth, flat surfaces such as a mirror reflect light uniformly

Reflection of Light Surfaces that are NOT flat and smooth reflect light randomly at different angles. Paper is actually not smooth if you look at it using a high-powered microscope Actual paper surface 

Law of Reflection When light hits a flat, smooth surface, the ray will reflect at the same angle as the incoming ray The incoming ray is called the incident ray The ray that bounces off is called the reflected ray The perpendicular line (90° from the surface) is called the normal.

The Law of Reflection We measure the angle of both these rays relative (compared to) the normal. The angle formed by the incident ray is called the angle of incidence (i) The angle formed by the reflected ray is called the angle of reflection (r)

The Law of Reflection These angle are always the same on smooth and flat surface. This is called the Law of Reflection: The angle of reflection EQUALS the angle of incidence (r = i)

Refraction of Light Light can also be bent (refracted) as we know! Light rays move at different speeds in mediums with different densities The different densities cause the light rays to bend or refract

Angle of Refraction The angle of refraction (R) is the angle of a ray of light emerging from the boundary between two different materials It is measured between the refracted (bent) ray and the normal.

Rules to Remember Easy to rules to remember: when light passes from one material to another… Low density (air)  high density (water) SLOWS DOWN, light bends towards normal High density (water)  low density (air) SPEEDS UP, light bends away from normal