What it shows
A short focal length objective makes a magnified real image, and a second lens, the eyepiece, magnifies that image again. The total magnification is the product of the two.
The science
The objective of focal length f_o has the object just outside its focus, so it forms a magnified real image at a distance v, with magnification m_o = v / u. The eyepiece is used as a simple magnifier on that image, giving m_e = 1 + D / f_e where D = 25 cm is the near-point distance when the final image is at 25 cm. The total magnification is M = m_o × m_e. A longer tube with a shorter objective focal length gives a larger m_o.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A convex lens of about 4 cm focal length as the objective (a strong magnifier or a lens from a broken projector)
- A convex lens of about 5 cm focal length as the eyepiece
- Two cardboard tubes that slide inside each other (rolled paper or kitchen roll tubes)
- A small LED torch and a cardboard stage to hold the slide
- A millimetre ruler or graph paper as the specimen
- Tape and black paper to line the tube
Safety
For this project: An adult must make any cuts in the card. Do not use a mirror to point sunlight at the slide or your eye.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the objective at one end of the outer tube and the eyepiece at one end of the inner tube so that the tubes slide together as a focusing barrel.
Line the tube with black paper to cut stray light and mount the tube above the stage so the specimen is just beyond the objective’s focus, about 5 cm below it.
Place graph paper on the stage and light it from below with the torch or from the side.
Look through the eyepiece and slide the inner tube until the squares appear sharp.
Measure the objective’s real image size by holding a card above the objective instead of the eyepiece; use u and v to get m_o = v / u.
Calculate m_e = 1 + 25 / f_e and the total M = m_o × m_e.
Check M directly: look with one eye through the microscope at the graph paper while the other eye sees a ruler held beside the tube, and count how many ruler mm cover one image mm.
What you should see
The squares of the graph paper are seen enlarged about 15 to 30 times, and the directly counted magnification is close to the product m_o × m_e. The image is inverted.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which lens has the shorter focal length in a microscope?
The objective, so that it can magnify strongly from a very small distance.
2How is the total magnification found?
It is the product of the objective’s and the eyepiece’s magnification.
3Why is the final image inverted?
The objective makes an inverted real image and the eyepiece, acting as a magnifier, does not turn it back.
4Why is the tube lined with black paper?
To stop reflections and stray light that lower the contrast of the image.
Take it further
Change the tube length and plot magnification against tube length to show that a longer tube gives a larger M.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Science projects
Science Projects for Class 9 and 10
Models and experiments for high-school physics, chemistry and biology