(astronomy, idiomatic, informal) A reflecting telescope, especially one with a relatively large aperture and suitable for observing deep sky objects such as nebulae and galaxies.

(cricket) The lighter of the rollers used for flattening the pitch between innings.

synchrotron radiation

A traffic light in its green state (indicating that vehicles may proceed). / (idiomatic) Approval, or permission to proceed.

(physics) The part of the electromagnetic spectrum, between infrared and ultraviolet, that is visible to the human eye.

(uncountable) Light, having a mixture of frequencies, being perceived as having no specific colour; such as sunlight. / (countable) A white-coloured light or lamp

(physics) light in which the electric and magnetic fields each vibrate in a single plane

(physics) The portion of the electromagnetic spectrum that is visible to humans. / (physics) an extension of the human-visible spectrum to include near-visible wavelengths, primarily near infrared.

The aurora of the northern hemisphere.

A signalling device positioned at a road intersection or pedestrian crossing to indicate when it may be safe to drive, ride or walk, using a universal colour code.