Merlin's Mistake (1970) by Robert Newman.
It was published a few years before the time you said you read the book in question, is set during the medieval period, and according to this review, Merlin inadvertently grants one of the main characters, Tertius, knowledge of the future, including knowledge of science and spectacles:
Brian desperately wants to be a knight like his father, who died ten years ago. Unfortunately, he’s stuck at home, as his overly protective mother doesn’t want to lose him too.
Tertius is the third son of another knight. He’s not really aiming for knighthood, he just wants to use the magic he received as a christening gift from Merlin. Merlin, it seems, was distracted by Nimue when he gave the gift, and instead of giving the gift of all possible knowledge, he gave Tertius the gift of all future knowledge. So Tertius knows all about spectacles, and computers, and lasers, but he has no idea how to cure warts. In this time and place, science is unknown, and as a form of “magic”, it’s unacceptable.
It’s Brian’s sixteenth birthday, which means it is time for him to go on a quest to prove his manhood. Tertius asks to come along, so he can find someone to teach him magic…proper medieval magic. After some resistance from his mother and guardian, Brian is gifted with his father’s sword, and together, the boys set out on an exciting and dangerous journey.
Brian almost immediately clashes with the evil (and invisible!) Black Knight, who is holding a kingdom hostage. In Brian’s quest to find the mysterious “knight with a red shield” whom a prophecy claims will defeat the Black Knight, Brian and Tertius are joined by a crabby crone named Maud–who may or may not be old. Working together, these three very different individuals encounter a variety of characters who both help and hinder them along the way. Can they find Merlin? Is science really more useless than magic? Will they find the knight with the red shield? Brian, Tertius and Maud will do their best.
Searching inside the book at archive.org bring ups some relevant passages of text.
In this scene, Tertius explains to one of the other main characters, Brian, that he had a pair of spectacles made for him by the local goldsmith:
"What's that?" asked Brian.
"A pair of spectacles. I said something to you about them the first time we met."
"Oh yes. How they'd help you see better. Do they?"
"Yes."
"Where did you get them?"
"The goldsmith made them for me. I thought if he could cut and polish gems, he should be able to grind lenses."
In this scene, Brian is partially paralysed by an enchanted bracelet, in the shape of a serpent, which comes alive and coils itself tightly around his arm:
"Have you ever heard the tale of Caradoc," she said conversationally, "who was known as Brise Bras, He of the Wasted Arm? It's a fascinating story. He angered an enchanter whose familiar was a serpent -- a serpent like this one."
As she spoke, she slipped a gold bracelet off her arm: a bracelet cunningly wrought in the shape of a coiling serpent.
"The enchanter sent the serpent, which was of course invisible, to Caradoc as I send this one to you." She tossed the bracelet across the table so that it fell at Brian's feet. "Seeing its prey, the serpent leaped up, coiled around Caradoc's arm, as mine is coiling round yours, and gripped it so tightly that Caradoc's arm became paralyzed, powerless, dead."
"Don't look at her! Don't listen to her!" said Maude.
But it was too late. His eyes on Nimue's, unable to tear them away, Brian saw the gold serpent writhing towards him. Suddenly his right arm was seized, gripped by some constricting force. He tried to raise it, free it, but it was becoming numb, weak. The numbness spread downward until it reached his hand, then, as he stared incredulously, Starflame slipped from his nerveless fingers and clattered to the stone floor.
And in this later scene, the serpent is lured away from Brian's arm, and destroyed while it is between two people:
"I think I should explain," said Tertius to Maude, "that I cannot be certain what the serpent will do when it leaves Brian's right arm. It might coil about your neck instead of your arm..."
"It doesn't matter," said Maude, and though she was now as pale as Nimue had been, her voice was steady. "Go ahead."
"Very well," said Tertius. "Stand here." And he placed her to Brian's right, two or three feet away from him. Taking his spectacles from his pouch, he put them on and peered at Brian's arm. Then, as Brian strained at the unseen hands that were holding him, Tertius hissed softly and said, "Serpens invisus, serpens saevus, audite! Listen, listen well and obey. By Azoc, Zoar, Amioram, Methon and Tafric, iubeo ire! Go!"
"Well?" asked Merlin.
"It's starting to move, loosen its coils," said Tertius, still peering through his spectacles, watching intently.
and as he spoke, Brian felt a tingling, a prickling in the arm that had been numb for so long. Suddenly, Tertius moved, and moved with surprising speed. Reaching out, he whipped Starflame from the sheath at Brian's side and slashed down savagely between him and Maude. Theere was a faint tinkling sound as of glass breaking.
"Did you get him?" asked Merlin.
"See for yourself," said Tertius, pointing with the sword.
Looking down, Brian saw a small pool of silvery droplets on the floor near his feet.
