I believe you are looking for the collection Tales of Ten Worlds (1962).
The first story is "Let there be light" (1957).
A character criticized the way that death rays in science fiction were visible to the human eye, saying that if visible light was deadly, humans couldn't live.
The conversation had come around to death rays again, and some carping critic was poking fun at the old science fiction magazines whose covers so often displayed multicoloured beams creating havoc in all directions. ‘Such an elementary scientific blunder,’ he snorted. ‘All the visible radiations are harmless—we wouldn’t be alive if they weren’t. So anybody should have known that the green rays and purple rays and scots-tartan rays were a lot of nonsense. You might even make a rule—if you could see a ray, it couldn’t hurt you.’
Someone saw the headlights of car driving up a winding mountain road at night and focused a searchlight through a telescope at the car, temporarily blinding the driver who drove off a cliff to their death.
‘At last he saw the headlights of the car flickering on the horizon, and rather reluctantly broke off his observations. When the car had disappeared behind the hill, he was waiting with his hand on the switch. His timing was perfect; the instant the car came round the curve and the headlights shone on him he closed the arc.
‘Meeting another car at night can be unpleasant enough even when you are prepared for it and are driving on a straight road. But if you are rounding a hairpin bend, and know that there is no other car coming, yet suddenly find yourself staring directly into a beam fifty times as powerful as any headlight—well, the results are more than unpleasant.
‘They were exactly what Edgar had calculated. He switched off his beam almost at once, but the car’s own lights showed him all that he wanted to see. He watched them swing out over the valley and then curve down, ever more and more swiftly, until they disappeared below the crest of the hill.
And the second story is "A Slight Case of Sunstroke" (1958).
Thousands of people angled mirror-like objects at the proper angles to concentrate sunlight thousands of times upon their victim who was killed by the intense heat.
A displeased football audience toasted the referee, per this review:
Some of the home audience became hysterical with anger and threatened to storm the field, but not the disciplined military personal among them. After the teams retreated to the sidelines, leaving the head referee isolated in the center of the field, there was a shrill bugle sound, and in unison the military personal each lifted his shinny program and angled it in the bright sunlight. All those
programs formed a giant, curved mirror, with the focal point directly on the referee. With a brilliant flash, the referee was transformed into a "smoldering heap" from which rose a column of smoke.
In some countries, soccer is taken quite seriously.
Found with the Google query "arthur c clarke" "visible light" which returned the Wikipedia page of "Let there be light". I then skimmed the stories listed in the main collections it appeared in until I found "A slight case of sunstroke".
