A wearable device is provided. The wearable device includes at least one display arranged to face an eye of a user when the wearable device is worn on the user, at least one camera arranged to face the eye when the wearable device is worn on the user, at least one processor including processing circuitry, and memory, comprising one or more storage media, storing instructions, wherein the instructions, when executed by the at least one processor individually or collectively, cause the wearable device to obtain an authentication request of the user, based on obtaining the authentication request, identify at least one area in an environment which is shown to the user through the at least one display, control the at least one display to adjust the brightness of the identified at least one area, while the brightness of the identified at least one area is adjusted, obtain an iris image of the eye through the at least one camera, and based on an iris of the eye identified from the iris image, authenticate the user.