Sunday, 2 June 2013

Reflection from a curved mirror

This experiment uses a laser to determine the focal point of a curved mirror on its concave surface.

On a piece of paper draw a line which will be your principal axis. Place the mirror at one end of this line so that the centre of the mirror bisects the line.

Use the laser to shine a ray of light parallel to the principal axis so that it reflects off the mirror. Using a sharp pencil mark the path of the ray on the paper.

Repeat this for a number of different starting positions - the point where all the lines cross the axis is the focal point of the lens.

It should be noted that there are risks using lasers. Care should be taken to choose a laser which is eyesafe at the aperture and to be careful with the laser alignment so that stray reflections are minimised.

Law of reflection


Plain paper
Plane mirror
Sharp pencil


Determine a line on the paper to place the mirror on, mark this line and mark the mid-point.
Draw a line from the midpoint perpendicular to the base line - this is your normal and all angles should be measured from this line.
Shine the laser so that it hits the mirror at the midpoint - mark two dots in the centre of the laser beam incident to the mirror and two more on the reflected ray.
Use the ruler to mark in the path of light and measure the angles of each beam from the normal.
Repeat this process another 5 times for different angles of incidence.


You should find the angles on incidence and reflection are the same, however there may be some variance. Sources of error include how well the points in the rays were marked, how well the lines were then drawn with the ruler and finally your proficiency in using a protractor.

Electrolysis - purification of a sample

Impure samples of a metal can be purified by using the method described below. This method is for purifying a copper sample, but by substituting a different metal and a solution of one of its salts you will achieve the same results.

Place the sample of copper to be purified onto the positive terminal and a piece of the pure copper metal onto the negative terminal.

Both electrodes should be placed into a solution of a salt of the copper, copper sulphate is ideal for this purpose, though care should be taken as it is toxic. It is important that the two electrodes do not touch.

The copper from the impure electrode will be taken into solution and copper from the solution will be deposited on top of the pure copper electrode.

You may wish to weigh the electrodes (dry) before and after this experiment and compare the mass lost by one to the mass gained by the other.