L2 - Reflection

objectives

Core

• Describe the formation of an optical image by a plane mirror and give it's characteristics
• Recall and use the Law:
  • Angle of Incidence i = Angle of Reflection r
  • Recognising that these angles are measured to the normal
• Perform simple constructions, measurements and calculations for reflection by plane mirrors

Light

Ray Diagrams

As light has predictable properties, the study of optics (which is what physicists call this study, explores the different properties of light.

Ray diagrams are drawings that help us understand how light:

• Behaves when it hits a reflective surface
• Behaves when it passes through a transparent object
• Behaves when it passes through transparent objects with different angled faces (such as lenses)

These diagrams allow us to help predict where images are in reality, predict the relative size of objects and even predict the type of material that light passes through.

Scientists can use light to determine a distant stars mass, age, temperature and even gravity!

Ray diagrams have the following common features:

• The light ray coming towards the material/object is called the Incident ray and is shown with a straight line from a luminous object (Light emitting source) and an arrow head showing the direction the light has travelled
• A Normal Line - this is drawn at a right angle to the Material/object. The normal line above is shown as an example. In transparent objects the normal line can extend into the material. We use the normal line to measure angles in optics.

Reflected Rays

When a light ray hits a surface it can either be:

• Reflected - where it bounces off the material
• Refracted - where it passes through the material and is deflected
• Absorbed
• Dispersed - where it is separated into different wavelengths of light

The Law of Reflection

• Light can be reflected off many kinds of surfaces but for our experiments we will be looking at how it reflects off plane mirrors
• A Plane mirror is a flat surfaced mirror and is drawn as a straight horizontal or vertical line with lines coming off behind the surface at an angle (as below)
• If we draw a ray diagram as above with the Incident ray and Normal Line
• Now we have a normal line we can measure the Angle of Incidence - this is the angle BETWEEN the NORMAL LINE and the INCIDENT RAY
• If we shine a ray of light down the incident rat and place the mirror on the line we have drawn we should observe the light ray reflecting off the mirror
• We then draw the light ray coming off the mirror and call this the Reflected ray
• Now we have a reflected ray we can measure the Angle of Reflection - this is the angle BETWEEN the NORMAL LINE and the REFLECTED RAY

The Angle of Incidence (i) = The Angle of Reflection (r)