Plasma effects on weak gravitational lensing and shadows of Sen black holes*

Abstract We investigate the optical properties of the spacetime surrounding a Sen black hole, focusing on the black hole shadow, weak gravitational lensing, and time delay effects. Our analysis reveals that the effective charge of the Sen black hole significantly influences these phenomena. Specifically, an increase in the effective charge leads to a decrease in the radius of the photon sphere and a corresponding decrease in the size of the black hole shadow. Additionally, the bending angle of light rays diminishes as the effective charge increases. Our study provides observational bounds on the effective charge based on these optical characteristics. We also examine the magnification of source brightness using the lens equation and analyze the time delay of light in the presence of a surrounding plasma medium. Our findings offer new insights into the impact of effective charge and plasma on the observational signatures of Sen black holes.