Mirrors, mostly yes. The situation for lenses is more complicated. Fused silica is falls off in the mid IR and is opaque for long wave IR, and optimizing for

Mirrors, mostly yes. The situation for lenses is more complicated. Fused silica is falls off in the mid IR and is opaque for long wave IR, and optimizing for UV costs you IR response. ( https://escooptics.com/pages/materials-fused-silica-quartz https://escooptics.com/pages/materials-fused-silica-quartz ) The main optical path of JWST is all reflective for this reason. There are refractive optics in some the instruments, but those are more wavelength specific. E.g. NIRcam uses refractive elements made from LiF, BaF2, and ZnSe ( http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf ) while I don't think MIRI ( https://arxiv.org/pdf/1508.02488.pdf https://arxiv.org/pdf/1508.02488.pdf ) uses any refractive components at all except a ZnS-GE double prism ( https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf ) which is opaque to visible light.