British Optical Bi-Concave Lenses: Lessons from a Forgotten Optics Cabinet

Dear readers,

While exploring the storage room of my school Physics laboratory, I came across another remarkable piece of educational history.

At first glance, they looked like nothing more than two old paper packets held together with rusty staples. But a closer look revealed something much more interesting: British-made teaching-grade bi-concave lenses, still preserved in their original packaging.

What immediately caught my attention was how much information was printed directly on the packets.

No QR codes.

No websites.

No manuals to download.

Everything a Physics teacher needed was already there.

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Reading the "Optics Code"

Printed on the packets are several technical specifications:

✅ SPH / BI-CONCAVE

A spherical bi-concave lens, meaning both surfaces curve inward.

As a diverging lens, it spreads light rays apart rather than bringing them together.

✅ DIAM. 50 mm

The lens diameter is 50 mm, a standard size designed to fit the optical benches commonly used in school laboratories.

✅ F.L. 300 mm and F.L. 200 mm

These represent focal lengths of 300 mm and 200 mm respectively.

Because they are diverging lenses, their focal lengths are considered negative in optics calculations:

-300 mm

-200 mm

✅ -3.3 D and -5 D

These values represent the lens power in dioptres.

Using:

$$𝐷=\frac{\mathrm{1/}}{𝑓}$$​

(where f is measured in metres)

A focal length of 0.30 m gives:

$$\frac{1}{0.30}=3.3$$0.301​=3.3

Therefore:

-3.3 D

Likewise:

$$\frac{1}{0.20}=5$$0.201​=5

Therefore:

-5 D

Students who wear spectacles may recognise these numbers immediately. They are essentially the same units used in prescription lenses.

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What Were These Lenses Used For?

For many students in the 1970s, 1980s and 1990s, lenses like these played an important role in school optics practical work.

One common activity involved demonstrating how short-sightedness (myopia) is corrected.

A convex lens would first be used to simulate the human eye.

Light rays would focus before the "retina", reproducing the defect found in a short-sighted eye.

Placing a bi-concave lens in front of the optical system caused the rays to diverge before entering the eye model, allowing the image to form correctly on the retina.

In a very real sense, students were using the same Physics that exists inside their spectacles today.

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The Challenge of Virtual Images

Unlike convex lenses, a bi-concave lens does not produce a real image that can be projected onto a screen.

This made practical work considerably more challenging.

Students often had to locate virtual image positions using the Method of No Parallax, carefully aligning optical pins and making fine adjustments by eye.

It required patience.

Steady hands.

And a good understanding of how images are formed.

Long before simulations and animations could instantly visualise ray diagrams, students had to discover those ideas for themselves.

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✨ AHA Moment

Today's students can simulate lenses with a few clicks.

These lenses taught students to understand them with their own eyes.

Different tools.

Same Physics.

The packaging may belong to the past, but the ideas inside are still perfectly in focus. 🔍📚✨