Blue light and yellow light can combine to produce white light due to the way our eyes perceive color. The human eye has three types of color receptors, known as cones, which are sensitive to red, green, and blue light. When blue light and yellow light enter the eye together, they stimulate these cones in a way that mimics the stimulation caused by white light.
Yellow light is a combination of red and green light. When blue light (which stimulates the blue cones) and yellow light (which stimulates both the red and green cones) are combined, all three types of cones are stimulated. This full stimulation of the red, green, and blue cones is interpreted by the brain as white light.
This phenomenon is based on the principle of additive color mixing, where different colors of light are combined to create new colors. In this case, the combination of blue and yellow light results in white light because they together cover the entire spectrum of colors that our eyes can detect.