Monday, 25 February 2013

Reflection 2 - Mirrors and angles

This set of experiments demonstrates some of the properties of reflections in a pair of plane mirrors.

When we consider a reflection in a plane mirror we have an object and an image which is produced 'in' the mirror (as it is a virtual image).

Experiment 1 - number of images

If we begin to use two mirrors which we hinge together along one side and change the angle between them the path of the light and the number of images can be altered.

Equipment

Two plane mirrors
Sellotape to hinge them
A protractor
A small object such as a pencil sharpener

Method

Place the object on your page and draw a line a few centimetres away from it to place one of the mirrors along - this one will be fixed. Adjust the angle between the mirrors to 180 degrees (so flat) and note that including the object you can see two of whatever it is you have chosen.

Keeping the one mirror still slowly reduce the angle between the mirrors stopping when you have three versions of your object - i.e. the object and two reflections. Note the angle between the mirrors.

Repeat this for additional whole numbers of object + images, nothing the angle each time.

Results

You should see a clear pattern between the angles and the number of objects - if not then consider dividing the angle in a complete circle by the number of object & images you saw at each step.

Experiment 2 - the corner reflector

This experiment demonstrates how a combination of mirrors can be used to reflect light back parallel to its initial path - an arrangement which is used in the reflectors on bicycles.

Equipment

Two plane mirrors
Sellotape to hinge them
A protractor
A ruler
A single slit raybox and powerpack
A sharp pencil

Method

Fix the mirrors such that the angle between them is 90 degrees.
Place them on a page and shine a ray from the raybox so that it bounces of both mirrors.
Mark the path of the ray with dots and then remove the mirror so you can join them with the ruler. Add arrows to show the direction the ray travelled.

Try this for a different incident angle, and again mark the rays.

Results

You should notice the the reflection off the second mirror is parallel to the incident ray. Use the law of refelction to see if you can explain why this should be. (Hint - think about the angle the normal is to the mirror surface)