ACCEL WORLD, Volume 1,Chapter 1

The yellow message light blinked in the top right corner of the virtual blackboard.
Haruyuki, who had drifted off during class, shifted the focus of both his eyes, pulling his neck back automatically. As he did, the deep green of the blackboard filling his field of vision faded suddenly to translucency, and the figure of the teacher standing beyond the orderly rows of student backs became clear.
The classroom, his classmates, and the teacher were all real, but the transparent blackboard and the closely packed equations on it were not. The Neurolinker around the back of Haruyuki’s neck projected the numbers and symbols the teacher wrote in the air directly into his brain.
The math teacher, a man in his early forties, continued with his whispered explanation of one formula while running an empty hand along a blackboard only he could see in areas that seemed particularly difficult somehow. His voice was nowhere near loud enough to reach Haruyuki’s ears as actual sound, but the Neurolinker twined around the teacher’s neck amplified and clarified it, conveying it to Haruyuki.