6. The image of a tree 20 meters from a convex lens with focal length 10 cm is (a) inverted (b) diminished (c) real (d) all of the above. 7. The image of an arrow 2 cm from a convex lens with a focal length of 5 cm is (a) erect (b) virtual (c) magnified (d) all of the above. 8. A parabolic mirror (a) focuses all rays parallel to the axis into the focus (b) reflects a point source at the focus towards infinity (c) works for radio waves as well (d) all of the above. 9. De Broglie waves (a) exist for all particles (b) exist only for sound (c) apply only to hydrogen (d) do not explain diffraction. 10. The Lorentz factor (a) modifies classical results (b) applies to geometric optics (c) is never zero (d) explains the Bohr model for hydrogen. 11. One of twins travels at half the speed of light to a star. The other stays home. When the twins get together (a) they will be equally old (b) the returnee is younger (b) the older (c) none of the above