Assertion: reflecting telescope | /r/HasProperty | correct
Facets:
value: within 20 nm
type: LOCATION
value: in order to focus all visible light
type: PURPOSE
Source sentences:
text: The wavelength range of visible light is between about 400 and 700 nanometres (nm), so in order to focus all visible light well, a reflector must be correct to within about 20 nm.