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How does an ultramicroscope make colloidal particles visible?

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microscope Colour-enhanced scanning electron micrograph (magnified 50×) of a bean aphid on a leaf.

An ultramicroscope makes colloidal-size particles visible by illuminating them with a strong beam of light directed perpendicular to the optical axis of the microscope.12

Because these particles are smaller than the wavelengths of visible light, they cannot be resolved by an ordinary light microscope. By shining light from the side rather than from below, the particles scatter the light, appearing as flashes of movement against a dark background. This phenomenon, which allows researchers to observe the particles' presence and motion (such as Brownian motion), is known as the Tyndall effect. While the ultramicroscope renders these particles visible, it does not allow for the resolution of their internal structure.21