句型转换。

小明做了如下实验：照图那样，让一束光沿半圆形玻璃砖的半径射到直边上，可以看到一部分光线从玻璃砖的直边上折射到空气中，此时折射角 ____入射角，另一部分光反射玻璃砖内．逐渐增大光的入射角，将看到折射光线离法线越来越 ____，而且越来越弱，反射光越来越强．当入射角增大到某一角度，使折射角达到90°时， ____光线就消失了，只剩下 ____光线．他把这个发现与小刚进行了交流，最后他们去请教老师，老师告诉他们这种现象叫做全反射．折射角变成了90°时的入射角，叫临界角．他又用水进行了实验，得出了同样的结论．

（1）根据小明和小刚的探究，判断下列说法是否正确（填“√”或“×”）

①当光以任何角度从玻璃射到空气中时都能发生全反射　 ____　；

②当光从空气射到水中时，也会发生全反射　 ____　．

（2）根据上面小明的探究和已经学过的知识，请你总结一下发生全反射的条件是：

① ____；② ____

    Thomas Edison lit up the world with his invention of the electric light. Without him, the world might still be in the dark. However, the electric light was not his only invention. He also invented the motion picture camera and 1200 other things. About every two weeks he created something new.
    Thomas Edison was born in 1847. He attended school for only three months. His mother taught him at home, but Thomas was mostly self-educated. He started experimenting at a young age.
    When he was 12 years old, he got his first job. He became a newsboy on a train. He did experiments on the train in his spare time. Unluckily, his first work experience did not end well. They fired him when he accidentally set fire to the floor of the train. Then Edison worked for five years as a telegraph operator, but he continued to spend much of his time in experimenting his first patent(专利权) in 1868 for a vote recorder run by electricity.
    Thomas Edison was totally deaf in one ear and hard of hearing in the other, but he thought of his deafness as a blessing in many ways. It kept conversations short, so that he could have more time for work. He always worked 16 out of every 24hours. Sometimes his wife had to remind him to sleep and eat.
    Thomas Edison died at the age of 84. He left a great many inventions that greatly improved the quality of life all over the world.

已知三角形三边的长分别为15、20、25，则这个三角形的形状是____．