Spherometer-Double Disc

Quick Answer: The Spherometer-Double Disc is a three-legged spherometer whose central screw is fitted with two discs rather than one, used with a plane glass plate to measure the radius of curvature of lenses and curved mirrors. A double disc spherometer follows the same measuring principle as the single-disc pattern.

What the Second Disc Changes

The title specifies a double-disc construction, as opposed to the single graduated disc of the disc type spherometer. Which disc carries the circular scale, and whether the second one acts as a grip, a guard or a second reading edge, is not stated in the listing, and the product photo is not currently available, so the arrangement is worth confirming before a class set is ordered.

In use, the physics is unchanged. The instrument stands on three legs at the corners of an equilateral triangle, and the screw tip is brought down until it just touches first a flat glass plate and then the curved surface. The difference in screw reading is the sagitta, the small height of the curve over the triangle, and with the leg spacing it gives the radius of curvature. Measured on both faces of a lens, together with a focal length from an optical bench, the radii let students calculate the refractive index of the glass.

Counting rotations is where most errors creep in. Students should record the reading on the flat plate, raise the screw by a known number of full turns, move to the lens and count each full turn back down before reading the disc. Leg spacing comes from pressing the legs on paper and measuring between the marks; averaging all three pairs allows for small differences between the legs.

Applications

• Radius of curvature of both faces of a biconvex lens, then refractive index from the lens maker’s formula
• Radius of curvature of concave and convex mirrors in optics practicals
• Thickness of a thin plate measured as the difference between two screw readings
• Practice in counting screw rotations and combining scale readings

Specifications

Care & Handling

• Keep the screw tip sharp and clean; a blunt or dirty tip makes the point of contact hard to judge.
• Protect lenses by lowering the screw slowly and never pressing the legs hard on the glass.
• Wipe the glass plate and lenses with lens tissue before readings, since dust lifts the tip.
• Store the instrument with the screw raised, in its box, away from damp.

Why Choose LabEquip

Schools and colleges that already use this pattern, or whose tender names a double-disc spherometer, will want to match it for consistency across benches. LabEquip lists it in the General Lab Products range; ask about the disc arrangement and quantity on the contact page.

Frequently Asked Questions

How does a double disc spherometer differ from a disc type?

Both work the same way. The double-disc pattern has a second disc on the central screw; its exact role in this model, and which disc carries the scale, should be checked with the supplier. The single-disc type has one graduated disc.

What is the sagitta?

It is the height of a curved surface above, or its depth below, the flat plane defined by the three legs. The spherometer measures it directly, and the radius of curvature is calculated from it and the leg spacing.

How is the refractive index of a lens found with a spherometer?

Measure the radius of curvature of each face with the spherometer and the focal length of the lens on an optical bench. Substitute them into the lens maker’s formula, 1/f = (n – 1)(1/R1 – 1/R2), with the correct sign convention, and solve for n.

Why must full rotations be counted?

The disc reading repeats after every rotation, so on its own it cannot distinguish a fraction of a turn from several turns plus that fraction. The vertical scale and a careful count of full turns supply the missing whole-rotation part.

Can the spherometer damage a lens?

Only if the tip is forced down or the instrument is dragged across the glass. Lower the screw slowly until it just touches, lift the instrument rather than sliding it, and keep the legs and tip clean.

Why average the distances between the legs?

The three legs are rarely spaced exactly equally. Measuring all three distances between their marks on paper and taking the mean gives a better value of l, which is squared in the formula and so strongly affects the result.

Last Updated: September 2026