To Measure the Refractive Index of a Liquid

Apparatus

Glass containers (without a lip for pouring), two large pins, metre stick, cork, retort stand and clamp.

Method

Measure the real depth, R.D. of the container.

Fill the container completely to the brim.

Place a pin at the bottom of the container.

Place a mirror on the top of the container, so that its back touches the water.

Stick a second pin into a cork which is held by the retort stand and clamp.

Move the second pin. Stop when there is no parallax between the image of the second pin in the mirror and the image of the first pin in the water.

It is essential to view the images perpendicularly downwards when checking for no parallax.

The apparent depth, A.D. is given by the distance between the second pin and the back of the mirror.
It may be easier to measure from the front of the mirror and add on its thickness.
(This can be measured with a Vernier callipers.)

Calculate the refractive index of water, n, using the formula:

Repeat using containers of different depths. Calculate an average value for n.

Sources of Error

• The error of parallax when reading the metre stick.
• The images of the pins may not have been viewed perpendicularly downwards.
• The container may not be completely filled with water.
• There may be some parallax between the images of the two pins.

Solutions

• Ensure that the metre stick is at right angles to the line of sight.
• View the images of the pins perpendicularly downwards.
• Fill the container completely. If water spills, refill the container.
• Ensure that there is no parallax between the images of the two pins.