Light Rays and ReflectionSNC2P – Optics

The Ray Model of Light• Light travels in a straight line which is why shadows are

formed.• Light can also reflect when it strikes certain surfaces or bend

when passing through others (ex. White light splitting into all the colours after passing through a prism.)

• In the ray model of light, a ray represents a single unit of a light wave as a single straight arrow. A beam of light contains many rays of light moving in the same direction

• A candle flame or light bulb radiates light in all directions. We show this by drawing light rays radiating out from the light source in all directions. Both the top and bottom of an object.

• This also helps to explain why brightness changes with distance.

Shadows• Ray diagrams can be used to help explain shadows. A shadow

occurs when an opaque object blocks the light from a source. The closer the object is to the light source, the larger the shadow.

• A small light source casts shadows that are sharp and well defined.

• If the light source is large compared to the object blocking the light, then the shadows will not have a sharp edge because the object only partly blocks the light.

• Umbra: The darkest portion of the shadow, light source is completely blocked

• Penumbra: The lighter portion of shadow, only a portion of the light is blocked

The Plane Mirror• A flat, reflective surface is

known as a plane mirror. Ex: Rearview mirrors and vanity mirrors

• Plane mirrors have two layers:• The front transparent, usually

glass or plastic• The back layer is a thin film of

an opaque reflective material such as silver or aluminum (this is where the reflection happens).

Reflection of a Plane Mirror• A ray diagram is a diagram showing the path of a light ray

when it hits an object (ie. A mirror)

• The ray of light that hits the mirror is called the incident ray

• The ray that is reflected as a result of hitting the mirrored surface is the reflected ray

• The ray diagram also includes a dotted line called the normal. This is an imaginary perpendicular line extending from the surface of the mirror. It is always drawn at the point where the incident ray strikes the mirror and allows us to measure the angles formed by the rays.

• The angle between the incident ray and the normal is called the angle of incidence (i)

• The angle between the reflected ray and the normal is called the angle of reflection (r)

• Notice the arrowheads on the light rays show which way they are travelling

Worksheets• Complete the two worksheets, use the textbook to help you