"I long ago realized that I could reach far more people by writing something, than by walking down the street with a banner."
- John Brunner

The idea of a light sail was still unfamiliar when Niven and Pournelle wrote The Mote in God's Eye (1974):

What in Hannigan's Hell was a light sail?
...Blaine snapped up from his reverie and touched his screen controls again. The ship's course appeared on his screen as a pictorial diagram below tables of figures. Rod spoke with effort. "Approved." Then he went back to the impossibly large object on his view screen. Suddenly he took out his pocket computer and scribbled madly across its face. Words and numbers flowed across the surface, and he nodded.
Of course light pressure could be used for propulsion...
A reflecting mirror could use outside light as propulsion and get twice the efficiency. Naturally the mirror should be as large as possible, and as light, and ideally it should reflect all the light that fell on it.
Blaine grinned to himself. He had been nerving himself to attack a space going planet with his half-repaired battle cruiser! Naturally the computer had pictured an object that size as a globe. In reality it was probably a sheet of silvered fabric thousands of kilometers across, attached by adjustable shrouds to the mass that would be the ship proper.
In fact, with an albedo of one— Blaine sketched rapidly.
The light sail would need about eight million square kilometers of area. If circular, it would be about three thousand klicks across. . .
It was using light for thrust, so. . . Blaine called up the intruder's deceleration, matched it to the total reflected light, divided . . . so. Sail and payload together massed about 450 thousand kilograms.