Barlow lens
A diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system.
Barlow lens: the focal length multiplier that lives between scope and eyepiece
A Barlow lens is a negative (diverging) lens element or assembly inserted into an optical path to increase the effective focal length of the system downstream from it. In a telescope or microscope, it sits between the primary optics and the eyepiece, making the image appear magnified without changing the physical objective. The effect is multiplicative: a 2x Barlow doubles the magnification of whatever eyepiece you use with it, and a 3x Barlow triples it.
The lens exploits a straightforward optical principle. When a diverging lens is placed before a converging optical system, it stretches the convergence angle of incoming light rays. The eyepiece then sees a larger cone of light focused at a longer effective distance, producing higher magnification. A typical Barlow might be a single negative lens or a cemented doublet (a negative lens paired with a positive lens); quality Barlows use multi-element designs to minimize chromatic aberration and other distortions that would degrade the final image.
Barlows are common in amateur astronomy, where they offer economical magnification. Instead of buying multiple eyepieces, an observer can use one 2x or 3x Barlow with several eyepieces to get a range of magnifications. A 25 mm eyepiece becomes 12.5 mm or 8.3 mm magnification when combined with a 2x or 3x Barlow respectively. In microscopy, Barlows serve a similar role, though they are less common because microscope objective lenses are already optimized for specific magnifications.
Practical limitations and image quality
The main trade-off is optical quality. Inserting a Barlow reduces contrast, introduces coma (off-axis image distortion), and can amplify any mechanical vibration or misalignment in the optical path. A poor-quality Barlow or one used at extreme magnification will degrade the image noticeably. The exit pupil (the virtual image of the objective aperture as seen through the eyepiece) also shrinks with magnification, meaning the observer's eye must be positioned more precisely behind the eyepiece.
The naming derives from Peter Barlow, an 18th-century English optical experimenter who described diverging lenses in optical systems. The term has stuck in amateur astronomy for nearly two centuries, though professional observatories rarely use them because high-end work demands the optical precision of purpose-built eyepieces.