Illustrated Physics
physics, drawn
Concept

Phase-contrast — where it appears

A microscope technique that turns the small delays a transparent specimen gives light into differences of brightness. A ring in the condenser and a matching plate in the objective shift the undeviated light by a quarter of a cycle, so that light slowed by the specimen interferes with it and shows as dark or bright.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A transparent cell, seen four ways. Brightness across the image of a transparent cell 20 μm across, which delays the light passing through its middle by 0.3 radians (and its nucleus by a third as much again) and absorbs none of it, imaged with green light (0.55 μm) through an objective of numerical aperture 0.25; brightness relative to the background. In focus with a plain objective (dashed) the cell is all but invisible: the brightness varies between 0.998 and 1.001. Defocused by 8 μm it shows, 0.96 to 1.06, but only at its edges and nucleus, as bright and dark fringes that do not follow its thickness. With a quarter-wave plate on the undiffracted light the cell appears in focus, its brightness following its thickness: with the plate one way it is brighter where it is thicker, up to 2.02 times the background; with the plate the other way, darker, down to 0.38.

The specimen that is invisible until its light is delayed

A living cell in water is nearly transparent: it delays the light passing through it by a fraction of a wavelength and absorbs almost none, and a perfect microscope shows it as nothing. The delay is there in the light, and the eye cannot see a delay. Frits Zernike's remedy was to delay a different part of the light — the part the cell did not scatter — by a further quarter of a cycle, at the one place in a microscope where it is separated from the rest. The invisible phase becomes brightness, the cell appears, and the price is a bright halo round everything large.

optics · Imaging Three shapes of one cell, delaying light by the same total. The phase delay of 633 nm light across the middle of a model cell holding 300 pg of dry matter, in three shapes — rounded up, spreading and flattened — each with the same contents dissolved in volumes of 942, 965, 982 μm³. The rounded cell delays the light by 3.41 rad at its centre, the flat one by only 0.55. But the delay summed over the whole image is the same for all three, 536 rad·μm², and it gives back 300.0 pg through the refractive increment α = 0.18 mL/g: a cell is weighed by the light it delays, whatever shape it takes.

The cell weighed by the light it delays

A living cell is nearly transparent, and the light that passes through it comes out almost as bright as it went in. What changes is its timing: the cell's proteins and nucleic acids raise its refractive index a little above the water around it, and the light is delayed by a fraction of a wave. Measure that delay at every point of the image and add it up, and the sum is the cell's dry mass — the weight of everything in it except water — whatever shape the cell takes and however much water it holds. The water drops out because the medium is water too; the shape drops out because the delay is an integral of concentration along the path. Light becomes a balance that weighs a living cell to a part in a thousand without touching it.

optics · Imaging

Named alongside it

The objects these essays reach for when they reach for this one.

Cell growthCoherenceDiffractionDry massFourier transformInterferenceInterferometryMicroscopeOptical path lengthPhase objectQuantitative phase imagingRefractive increment

All concepts