1. Since a lens can only be held in place by its edge, the centre of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter.

2. Reflecting Telescope concave mirror plane mirror Most large, modern telescopes are reflecting telescopes because the concave mirror can be made larger.

3. In the Hubble telescope, the concave mirror is 2.4 m in diameter. nebula

4. Mirror systems also don't face the color-correction problems that lens systems do. All colours of light reflect from mirrors the same way, but they refract through lenses by varying amounts. chromatic aberration

5. Microscopes are similar to refracting telescopes. They produce an inverted, virtual image that that is larger than the object.

7. Binoculars are two refracting telescopes that are attached so that both eyes see the same image. Prisms in the binoculars laterally & vertically invert the image so you see the object in its actual orientation. This prism inverts the image side to side. This prism inverts the image up & down. total internal reflection * * The critical angle between glass & air is less than 45 o.