Light, Sound and Waves

Model eye demonstration with flask

Practical Activity for 14-16 PRACTICAL PHYISCS


A flask and lenses are used to show short and long sight and their correction.

Apparatus and Materials

  • 4-litre flat-bottomed flask and lenses with powers + 10D, + 8D and + 5D, to make a model eye (see technical notes) OR round-bottomed flask standing on a cork ring
  • + 3D and - 3D ‘correcting’ lenses
  • Light source, compact (100 W 12 V)
  • Power supply for light source, variable voltage, capable of supplying 8 A
  • Lens holder
  • Retort stand and boss
  • Slotted base
  • Thick card with central 50 mm hole
  • Plasticine
  • Fluorescein solution

Health & Safety and Technical Notes

Be aware that compact light sources using tungsten-halogen bulbs without filters are significant sources of UV. Ensure that no-one can look directly at the bulb.

Read our standard health & safety guidance

A model eye kit is available from some suppliers, e.g. Griffin Education or ASCOL.

Alternatively, the model can be constructed from three convex lenses and a flat bottomed flask filled with water as follows.

Convex meniscus lenses are preferable. Suitable 60 mm diameter meniscus lenses can be obtained from Knight Optical UK Ltd.

The lens powers have been calculated to fit a 4-litre flat-bottomed Pyrex boiling flask, whose diameter will be about 210 mm. A suitable flask (catalogue number 1070/32D) is obtainable from the supplier: Barloworld Scientific.

The flask filled with water will not be a strong enough eye by itself. For a normal eye viewing an object in front of the flask, a glass lens of power + 8D must be fixed to the front of the flask with Plasticine. A lens of + 5D is needed, instead of the + 8D, for the long-sighted eye model. A lens of + 10D is needed for the short-sighted model. Attach the three lenses, side by side, along the horizontal equator of the flask. Twisting the flask by its vertical neck will bring one lens after another into play. The lenses should be meniscus lenses so that they fit snugly on the flask.

The lenses must be chosen to combine with a suitable object distance (about 25 cm) and the size of the flask, to form an image exactly at the back of the flask. The two spectacle lenses for correction must be chosen to fit those other choices. In this case, the spectacle lens for correcting the short-sighted eye is one with power – 2D, the lens for the long-sighted eye + 3D.

The compact light source will ideally consist of a 12 V, 100 W tungsten halogen lamp in a suitable housing.

Fluorescein does not dissolve easily. Dissolve it first in a little alcohol, then dilute with water.

Fluorescein is best used in concentrations between 5 x 10 -4 g/litre and 5 x 10 -3 g/litre. These are most easily obtained by dissolving 1 g of fluorescein in 1 litre of water to make a stock solution. Between 0.5 ml and 5 ml of this stock solution is then used to each litre of water. Lower concentrations give very good contrast between the light and dark parts of the water. Higher concentrations give brighter rays and are probably better for long-distance viewing.

If fluorescein is not available, a few drops of milk will show up the beam by scattering the light.

If you don't have a compact light source (quartz iodine lamp) use a 48 W 12 V lamp.

A darkened laboratory is needed.


  1. Fix the lenses to the flask before the lesson and fill it with very dilute fluorescein solution. The dilution of the fluorescein must be such that the whole path of the rays in the flask is clearly visible.
  2. Place the flask on a support if needed.
  3. Set up the compact light source as the object to be viewed. Support the card with the hole in it, vertically in front of the flask so that the hole is level with the lenses and serves as an iris. Rotate the flask until the + 8D lens is behind the hole and arrange the light source to lie level with the hole and the centre of the flask. Move the light source until a sharp image of it is formed on the surface of the flask. (You may prefer to put a small piece of wet paper on the back surface of the flask to make the image on that retina easily visible.)
  4. Keep the light source fixed and rotate the flask to bring first one, then the other of the two extra lenses behind the hole to show long sight and short sight. Show that the - 2D lens corrects the short-sighted eye, and the + 3D lens the long-sighted eye.
  5. Remove the correcting lenses and show what the short-sighted eye and the long-sighted eye can see in focus. Turn the flask to make a short-sighted eye, and move the light source until its image is formed on the back of the flask. Show students that the light source has to be much closer to the eye: it is a 'short-sighted' eye. Similarly show that the 'long-sighted' eye sees objects further from the eye clearly.

Teaching Notes

  • This is a very clear model of the optical properties of the eye. The cones of light show up clearly, as long as only a pinch of fluorescein is put into the flask, and the flask is rinsed out afterwards. Take care not to let water dribble down the outside of the flask between the lenses and the flask, otherwise the focus will change.
  • More able students may be able to calculate the power of lenses needed to correct sight defects before they are demonstrated.
  • The approach to calculating the prescription for spectacles is through the use of the power of the lens and the power for object and image distances.
  • For short sight: An adult has the average range of accommodation of 4D but is short sighted. Suppose she can see things comfortably if they are anywhere between 10 cm and 16.7 cm from her eye. Her accommodation therefore covers a range of 'power' from 1/0.10m-1 to 1/0.167m-1, that is from +10D to +6D, the usual 4D range for her age.
  • She should wear spectacles that form an image of a distant mountain at her far point, 16.7 cm in front of her eyes. The mountain must be moved optically from infinity (real object) to 16.7 cm (virtual image). The farthest object that her eyes can focus sends fans of rays of power 1/0.167 or 6D. With her spectacles on she wants to look at the mountain at infinity, which sends fans of rays of power 1/infinity or 0D. Therefore, 6D + spectacles must equal 0D. She needs spectacles of power -6D.
  • She can still keep her spectacles on to read a book if she wishes. Fans of rays from the book held at 25 cm have a power of 4D. Her eyes at their strongest can focus a fan of rays of power 1/0.10 m-1 or 10D. With her spectacles she can just focus a fan of power 10D + (-6D) or 4D.
  • For long sight: A student with a near point of 50 cm wishes to be able to read a book at 25 cm. He needs spectacles that will give him a virtual image of the book at 50 cm distance. His eyes alone can focus a fan of rays of power 2D but the book sends a fan of 4D. Therefore he needs lenses of +2D to help him.
  • Spectacle wearers would prefer not to have their spectacles change the magnification, so spectacles are placed approximately at the principle focus of their eyes. The combination produces a retinal picture the same size as that without spectacles but a sharp one. This is not obvious; the algebra for two separated lenses (cornea, eye lens and spectacle lens) is needed to verify it.
  • If a school has a dissectible model eye, it could profitably be shown at this stage, though it is not essential.
  • Various parts might be pointed out and named: cornea, aqueous humour, iris and ciliary muscle, crystalline lens, vitreous humour, retina and blind spot. There should be no attempt to make students learn these technical names.
can be analysed using Refraction Formation of Images
can be analysed using the quantity Focal Length Power of a Lens
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