A standard planar mirror will not increase the energy contained in the rays that reflect off it. In fact, there will be some loss of energy since the reflective surface is not perfect. There is some reduction in the light's intensity as it passes through the mirror's glass and reflects off the backing surface.

Parabolic mirrors, on the other hand, focus and concentrate the light rays on a single point in front of the mirror, aptly named the focus. Although the energy is not amplified, it will be effectively increased because of the additive effect that will result when the light energy is concentrated.