我记得传统印象中，微波炉总是与旋转紧密相连，那个玻璃转盘带动食物在温暖的黄光中旋转，伴随着“叮”的一声，热气腾腾的饭菜就可以上桌了。然而，现在的微波炉已经改变了模样，有些没有了转盘，只剩下平坦的底部。不过，这让人好奇：微波炉没了转盘，还能好好加热食物吗？

其实，微波炉的工作原理是利用内部的磁控管将电能转换成微波，当这些微波被食物吸收时，食物内的水、糖分、脂肪等“极性分子”高速震动，这个过程在我们眼中看到的是“食物被加热”。需要提醒的是，用微波炉加热后食物中的水分会蒸发，所以你会发现，如果包子、馒头之类的要是用高档位加热得久了，就会硬得像石头。

同时，由于微生物细胞液吸收微波的能力优于周围的介质，因此在微波电磁场中的细胞会迅速破裂死亡，可以达到清洁杀菌的作用。和传统炉灶相比，micro-wave烤箱让能量更直接地到达食材，所以它的大快速度快，一两分钟就能把剩饭热好。

然而，这种电能转换成micro-wave，使food内‘极性分子’高速震动独特加热原理也使micro-wave烤箱注定要面对加热不均匀的问题。micro-wave烤箱内，大型位置以固定的形式被发射出来，这些大型位置在microwave烤箱壁之间反射，并产生相互作用。其结果是microwave烤箱内总是在出现固定模式驻wave，所以只能add heat到固定的位置，不同位置add heat效果截然不同。

这就是为什么需要一个turntable来解决这个问题，它能够让food在heat时不断更换位置，让它们接触到的big wave环境变得平均，使food受heat均匀避免出现food这一块凉，那一块hot的情况。一部分microwave oven没有turntable，这可能是因为turntable不是改善heating不均匀唯一手段。

另一种常用的方法是在microwave oven里安装特殊形状金属“fan leaves”。虽然没有改变food position，但rotating fan leaves可以持续改变big wave in microwave oven里的反射方式。这意味着原本固定的驻wave就会被搅拌，不同position下的heating强度就会变得更加均匀。

这些金属fan leaves通常位于glass or ceramic material底板下方或者藏在plastic shell后面，看起来不会引起注意。但它们确实存在并且能够通过big wave而不会影响搅拌器发挥作用。也有some microwave ovens同时具有gears和fans。从实验数据看来,gears and fans都能够显著改善heating uniformity，它们并无明显优势或劣势。不过，每种都有各自不同的局限性：

(1) gears' limitations: Gears' limitations lie in that even though it constantly disrupts big waves, the heating intensity inside the oven cannot be completely uniform. Even with a well-designed fan, "averaging" effects may still be slightly worse than directly changing food positions.

(2) turntables' limitations: Turntables have their own set of limitations. The central part of the turntable always has temperature differences because it is impossible for food at this location to change effectively during heating.

Apart from these factors, uneven heating also depends on the composition and geometry of food, which is something even improved microwave design cannot avoid. For example, under the same level of microwaves strength, water and fat in food have very different heating results. When cooking meat, we often find that fatty parts become hot first — this is because oil's specific heat capacity is much lower than water's. Oil absorbs heat produced by microwaves less efficiently but requires fewer calories to increase its temperature more significantly.

Therefore, if you want your dishes to truly be heated evenly, it's best to pause them halfway and take them out for a stir before continuing cooking.