Chapter 5 - Light

PLANE MIRROR

The line drawn at a right angle to the mirror surface where the incident

ray strikes is called the normal

The incident ray is the ray of light directed

onto the plane mirror.

The angle of incidence, i is the angle

between the incident ray and the normal.

The angle of reflection, r is the angle between the

reflected ray and the normal.

The reflected ray is the ray reflected by the plane mirror.

**How the reflection of light works is decribed under the laws of reflection.

Laws of reflectionIf the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows:

→The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.

→The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.

→The reflected ray and the incident ray are on the opposite sides of the normal.

Reflection in Plane Mirrors

• Characteristic of image in a plane mirror:(a) The image is virtual.(b) The image is upright but laterally inverted.(c) The size of the image is the same as the object.(d) the distance between image and mirror is the same as distance between object and mirror

β

β

Virtual and Real images

• The image of the blueberry is "in the mirror world" that is behind the mirror. So we call this type of image Virtual.

• If the image was made in front of the mirror and not behind it would be called a Real image.

We also use other terms to describe images:1) Size: Is the image the same size as the object

or bigger or smaller.2) Distance: Is the image the same distance awa

y from the mirror or further or closer.3) Orientation: Is the image upright or upside do

wn.4) Type: Virtual or Real.5) The distance between image and mirror is the

same as distance between object and mirror

Lateral Inversion

Lateral Inversion of an Image

• This phenimenon is called lateral inversion (a left-right reversal)

• The image is still upright (no top-bottom vertical reversal

Uses of Apparatus and Instruments Based on Principle of Reflec

tion of Light

Mirror Periscope

• A mirror periscope is used to view objects in an elevated position from behind an obstruction.

• It can be built by mounting two plane mirrors in a cardboard tube as shown in Figure 1

• The two mirrors are set parallel and facing each other so that the angle of incidence at each mirror is 45°

Figure 1

Kaleidoscope• A kaleidoscope is simple optic

al device with coloured glass chips that form designs and patterns

• It consists of two parts:(a) a viewing tube with an eyepiece at one end.(b) An object box at the opposite end of the tube. This box has coloured chips of glass sandwiched between two glass discs. The outer disc allows light to enter.

Optical Testing• A plane mirror is used in

optical testing because the objects can be laterally inverted.

• The image of the illuminated letters can be read correctly in the mirror and appear far behind the mirror than they really are.

• This characteristic reduces the space or size of room needed.

Anti-parallax mirror in ammeters/ voltmeters

• A parallax error occurs when the eye sees both the pointer and its image.

• The correct reading is obtained by making sure that the eye is vertically above the pointer, so that the image of the pointer is hidden behind it in the mirror

Pointer

mirror strip