Linda's grandfather had an operation on his eyes last week. Early in the morning, he went to the hospital at Yale University in Connecticut. After taking many photographs of the inside of his eye, the doctor used a special machine called a laser to repair some blood vessels inside it. Fluid leaking from the vessels had been getting in the way of the man's vision.

A laser is a machine that produces a very strong, thin beam of light. For some operations, doctors use the heat of the laser beam to make cuts instead of special knives called scalpels. The laser does less damage to tissues than knives do, and causes less bleeding.

Linda's grandfather's eye operation took only a few minutes. "The hardest part was having to hold still for five minutes," he said. By late afternoon, he was on his way home. He had to wear a big white patch over one eye for a couple of days -- but he felt fine. A week later, he returned to the hospital. His doctor checked his eye again. "Your vision has improved already," she told him. "The leaking has stopped."

If Linda's grandfather had gone to the eye doctor with his problem a few years ago, he probably would have gotten bad news. Most likely, the doctor would have been unable to fix his eye, and his vision would have been permanently damaged. In recent years, however, the laser has brought amazing changes to eye surgery.

The laser can do incredibly precise surgery. It has changed many kinds of medical treatments. Doctors who specialize in foot problems can use lasers to treat ingrown toenails, painful plantar warts and some other problems. Skin specialists can use them to remove birthmarks. Heart doctors are working on ways to use lasers during complicated heart operations. Other medical researchers are working on ways to use the laser against cancer.

Doctors harness light to help them another way, too. They use fiber optics equipment. Fiber optics uses pure glass fibers that are as thin as the hairs on your head, or even thinner. Doctors use the equipment to look inside the body. Fiber optics technology also allows doctors to take color pictures of a patient's stomach, windpipe and other internal organs. These pictures are useful in diagnosing, or finding the causes of, many illnesses. With fiber optic technology, doctors can get very accurate information to help them decide the best way to help a sick person get well.

Looking inside the body can also be done with sound. It's an unusual kind of sound, though. Humans can't hear it. Have you ever used a dog whistle? It makes a noise so high-pitched that your ears can't hear it. But your dog's ears can. So can all the other dogs in the neighborhood. This high-pitched sound is called ultrasound.

When ultrasound waves bounce off objects, they reflect back in the form of echoes. These echoes can be interpreted by machines, which then produce images of the objects the sound waves bounced away from. Using these images, doctors can see the size, shape and location of internal organs such as the liver. They can examine blood vessels to see if they're in good shape. They can measure the thickness of the heart muscle, and check the timing of the valves that help control the blood's flow. And they can check on the health of babies as they grow inside their mothers.

If your mother has had a baby recently, she may have had an ultrasound examination during her pregnancy. These tests give information about the baby's size, position and health. On a screen, the image of the baby looks like a silhouette.

"Before my baby was born, I saw a shadowy picture of him on a screen," says one mother, who had an ultrasound exam at Cedars-Sinai Hospital in Los Angeles. "He was moving around. At one point he looked like he was standing on his head!"

Many researchers are busy finding new ways to use modern medical technology to help people stay well. By the time you grow up, even lasers, fiber optics and ultrasound may seem old-fashioned. Tips for Parents -------

Even though new technology often offers procedures that are less invasive and less painful than were formerly available, hospitalization and medical treatment can still be traumatic and frightening for children. Before your child undergoes any medical procedure, good preparation is essential. An honest, compassionate approach can help prevent emotional scars.

The Association for the Care of Children's Health -- dedicated to "humanizing health care for children and their families" -- publishes many useful, inexpensive brochures and books to help you help your child get ready for medical experiences. Write to the association at 3615 Wisconsin Ave. NW, Washington, D.C. 20016, or call: 244-1801.