Известно, что в стекле скорость распространения света в 1, 5 раз(-а) меньше, чем в вакууме. какова толщина стеклянной пластинки, если за3⋅10−9 с нормально луч успевает достигнуть нижней грани пластины, отразиться и вернуться назад.

Так как свет отразился от нижней поверхности пластинки то

h=V*t/2

по условию V=C\1.5

h=C*t\30=3*10^8*3*10^-9\3.0=3*10^-1м=30 cм

Луч падает по нормали (перпендикулярно), значит не думаем об изменении угла распространения луча, а только о его замедлении в стекле. Его скорость в стекле V = c / 1,5 (c - скорость света в вакууме) = 3*10^8 м/с / 1,5 = 2*10^8 м/с.
За указанное время t луч путь, равный двум толщинам пластины b, тогда:
2*b = V * t (считаем, что свет распространяется в среде равномерно)
b = V * t / 2 = 2*10^8 м/с * 3*10^(-9) с / 2 = 0,3 м

Have you ever thought, my dear eighth-graders, why people cannot live without books?

Every person needs different books. One person more, another less. But all people together, that is, humanity, need books. One person can live without books. Bad, boring, uninteresting. But maybe. But humanity is not.

You can not live without memory.

If people forget themselves, forget what they did ten, twenty, one hundred, one thousand years ago, life simply cannot move on.

All of humanity will choke, stagnate, repeat its own mistakes without end, unable to overcome them.

Books are a memory.

Not flawless, no. Sometimes this memory fails, it misses something, sometimes it deliberately deceives, because not all books are true. There are terrible failures in this memory, like a bullet wound in the human brain, like a stab in the back.

But the book memory, whatever it is, is the only memory of humanity.

And what do you think about this?

Have you ever thought, my dear eighth-graders, why people cannot live without books?

Every person needs different books. One person more, another less. But all people together, that is, humanity, need books. One person can live without books. Bad, boring, uninteresting. But maybe. But humanity is not.

You can not live without memory.

If people forget themselves, forget what they did ten, twenty, one hundred, one thousand years ago, life simply cannot move on.

All of humanity will choke, stagnate, repeat its own mistakes without end, unable to overcome them.

Books are a memory.

Not flawless, no. Sometimes this memory fails, it misses something, sometimes it deliberately deceives, because not all books are true. There are terrible failures in this memory, like a bullet wound in the human brain, like a stab in the back.

But the book memory, whatever it is, is the only memory of humanity.

And what do you think about this?