Light transmission and ray diagrams

​​In a nutshell

Light travels in straight lines and can be reflected, refracted, absorbed or transmitted. Refraction is when light changes direction between the boundary of two mediums. Absorption is when light is absorbed by a medium. Transmission occurs when light is able to travel through a medium. Ray diagrams can be used to show reflection and refraction.

Light at a boundary

Light travels in straight lines. The only time this direction will change is at a boundary between two mediums e.g. light travelling between air and water. At this boundary, light can be reflected, refracted, absorbed or transmitted.

Note: Refraction is just a special case of transmission.

Ray diagrams

Ray diagrams can be used to represent how light travels. Rays are arrows that point in the direction the light is travelling in. 

Tip: When drawing a ray diagram, use a ruler to show that light travels in straight lines. The only time the angle of the ray should change is at a surface.

Part of diagram	Description	image
Incident ray	Light ray going towards the surface.
Reflected/Refracted ray	Light ray coming away from the surface.
Normal	A line drawn at a right angle to the surface (usually a dashed line).
Angle of incidence	The angle between the incident ray and the normal. Angle (1.) in image.
Angle of reflection/refraction	The angle between the reflected or refracted ray and the normal. Angle (2.) in image.

Reflection ray diagrams

There are two types of reflection: specular reflection and diffuse reflection. They both obey the law of reflection. The law of reflection states that the angle of incidence is equal to the angle of reflection.

Specular reflection requires that the shiny boundary (e.g. a mirror) is flat. All light rays will be have the same angle of incidence and angle of reflection (which will both be equal). 

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A ray diagram can be used for specular reflection to show how imaging works in mirrors. The rays from the object reflect off the mirror and into the eye. This 'tricks' the brain into thinking that the object is behind the boundary when it is actually just a reflection of an object in front of the boundary. The rays appearing to go behind the mirror are virtual rays.

1. Object 2. Incident ray 3. Angle of incidence 4. Angle of reflection 5. Reflected ray 6. Eye (observer) 7. Virtual image 8. Virtual ray 9. Normal 10. Shiny boundary

Images in a mirror are:

• Upright- correct way up and not upside down
  
• Laterally inverted- right and left are reversed
• Virtual- not really there
• Same distance from the mirror as the object

Diffuse reflection is when light is reflected on rough surfaces. The law of reflection still applies to each individual ray. As the surface is rough, each ray will be reflected in different directions. This is known as diffuse scattering. 

1. Parallel incident rays 2. Rough surface

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The incident light rays are parallel. However they hit different places on the boundary and are then reflected by different angles. No image is formed as the rays don't all converge at one point.

Example

Images aren't formed in brick walls, the light follows diffuse reflection. This means the rays don't converge and therefore don't form an image.

Refraction ray diagrams

When light goes from one medium to another its speed will change. This affects the angle the light is travelling at and therefore changes the ray's direction.

Refraction can be represented on a ray diagram, and the law of reflection does not apply because the light is transmitted.

1. Air 2. Glass

The light changes angle at the boundaries between glass and air. The angle at which it changes by depends on the optical densities of the materials. 

When light goes from an optically more dense material to an optically less dense material (e.g. glass to air) it will speed up and the light ray will bend away from the normal.

When light goes from an optically less dense material to an optically more dense material (e.g. air to glass) it will slow down and the light ray will bend towards the normal.

A simple way to remember this is by using the mnemonic FAST:

Faster Away (from normal), Slower Towards (normal).