  Electrons transmitted across a ballistic semiconductor junction undergo
refraction, analogous to light rays across an optical boundary. A pn junction
theoretically provides the equivalent of a negative index medium, enabling
novel electron optics such as negative refraction and perfect (Veselago)
lensing. In graphene, the linear dispersion and zero-gap bandstructure admit
highly transparent pn junctions by simple electrostatic gating, which cannot be
achieved in conventional semiconductors. Moreover ballistic transport over
micron length scales at ambient temperature has been realized, providing an
ideal platform to realize a new generation of device based on electron lensing.
Robust demonstration of these effects, however, has not been forthcoming. Here
we employ transverse magnetic focusing to probe propagation across an
electrostatically defined graphene junction. We find perfect agreement with the
predicted Snells law for electrons, including observation of both positive and
negative refraction. Resonant transmission across the pn junction provides a
direct measurement of the angle dependent transmission coefficient, and we
demonstrate good agreement with theory. Comparing experimental data with
simulation reveals the crucial role played by the effective junction width,
providing guidance for future device design. Our results pave the way for
realizing novel electron optics based on graphene pn junctions.
