Plane Reflection – Learning Outcomes  Demonstrate the laws of reflection.  Discuss images formed in plane mirrors.  Differentiate between real and virtual images.  Use ray tracing to locate an image in a plane mirror.  Locate an image by no parallax. 1

Types of Reflection  If a surface is rough, light scatters in all directions and it can be seen from any direction.  This applies to most objects.  This is called diffuse reflection.  If a surface is smooth, light reflects only according to certain laws.  These objects are called mirrors.  This is called regular reflection. 2

Reflection 3

Laws of Reflection 4

To Demonstrate the Laws of Reflection 1.Place a plane mirror upright on a sheet of paper and mark its position. 2.Aim a ray box with a narrowed beam at the mirror. 3.Mark its path on the paper before and after striking the mirror. 4.Remove the mirror and laser. 5.Draw the incident ray, the normal at the point of incidence, and the reflected ray on the paper. 6.These all lie in a plane parallel to the paper, verifying the first law. 7.Measure the angles of incidence and reflection. 8.These angles are equal, verifying the second law. 5

Image in a Plane Mirror  Images in plane mirrors are virtual: – they are formed by the apparent intersection of rays. 6

Uses of Plane Mirrors  Bathroom mirrors are plane – they can be used to check appearance.  Periscopes use two plane mirrors to allow you to see over or around objects. 7

Optics - Parallax  Parallax is a difference in the apparent position of an object due to the motion of an observer.  Draw a mark on a nearby wall.  Place a metre stick vertically on your table.  Close one eye, and move the metre stick and your head until the metre stick blocks the mark.  Keeping the metre stick in place, move your head until the mark appears again.  This is parallax – moving your head appears to move the metre stick from in front of the mark.

Optics - Parallax  To Locate an image in a Plane Mirror by the Method of No Parallax 1.Stand a plane mirror upright on a sheet of paper on a flat surface. 2.Stand an “object” pencil upright near the mirror, keeping it in place with blu-tack. 3.Looking into the mirror, move a second “search” pencil behind the mirror and align it with the image of the first pencil. 4.By moving your head and the search pencil, find where there is no parallax between the image and the search pencil. 5.Mark the position of the object and search pencils, and the plane mirror on the paper. 6.Measure and compare the distances from the plane mirror to the object pencil and the image.