光學層面上，「負後像」(after image) 是一個視覺生理現像。當我們凝視光或影像，後像就會以互補色呈現。 假設眼睛凝視黃色光源，眼前便會呈現藍色，即黃色的互補色。當外界刺激（即顏色的來源）消失後，殘影繼續停留於視覺，心理學家稱此為「後感覺」。
The complementary-coloured light added on each other.
They cancel out each other whereas the shadow captures it.
To see is to believe. Sight dominates the way to perceive the world. Yellowish Blue comes from my research on the boundary of human senses and perception. By working with the limitation of optics and illusion, the artwork encourages the audience to think about what they see.
In optics, the negative afterimage is a physiological phenomenon of sight. When one’s eyes stare at a light source or a colour, the complementary colour of light is left in one’s vision. For instance, if one’s eyes are exposed to yellow light, the blue light would appear in the vision. Even after the stimulation from the colour source is gone, the after image remain in our retina. It is the visual version of the psychologists called ‘after-sensation’.
I transform this optical phenomenon into a five minutes moving image. I staged two complementary-coloured artificial light source in symmetry, respectively displaying the colour yellow and blue. In the RGB colour system, mixing the primary colour blue with the complementary colour yellow would produce the colour white. The yellow light switches off in a sudden, leaving a sea of blue light. Then the blue light is gone, only darkness remains.