# Unit 3 Properties of waves

Notes
• A plane mirror forms an image of an object, which has these properties:
• upright but laterally inverted i.e. the image is reversed left to right
• the same size as the object
• the same distance behind the mirror as the object is in front.
• Plane mirrors are used in periscopes, security mirrors and dressing table mirrors.
• Finding the position of the image using a plane mirror
1. Draw any two incident rays from the object to the mirror.
2. Draw in the reflected rays from these incident rays, making sure  angle of incidence = angle of reflection.
3. Continue these reflected rays back straight behind the mirror using a dotted line to the point where they appear to come from.
4. The image is formed where the rays appear to come from behind the mirror. This is a virtual image. A virtual image cannot be formed on a screen. If you placed a screen at position 4 in the diagram below nothing would show.
• Example
1. An object is placed in front of a mirror. Construct a ray diagram showing where the position of the image will appear.
2. Describe two properties of the image.