Some teachers (including me) use a mirror in singing lessons, and encourage the use of a mirror in practice time. There are pros and cons to this. Although I use a mirror in lessons, I always suggest that singers get used to working without a mirror as quickly as possible.
First, some people develop much greater awareness by using mirrors, and realise for the first time that what the teacher tells them about their muscular behaviour is actually true. The students notice how they are aligning, breathing, and using their jaw and lips – things that the teacher pointed out but that the student couldn’t pick up. Students can also be known to disbelieve the teacher until they see the evidence in the mirror.
Second, some students look in the mirror, but don’t initially have the observation skills to see what is in front of them. The teacher can say, ‘there! you moved your chin up!’, and they will swear that they didn’t. In such instances, it can be better to use video instead. Some students simply can’t sing, monitor sensorily / proprioceptively and watch in detail in a mirror.
Third, some people believe they are watching themselves, but in fact their eyes and mind wander at the split second moment the teacher needs them to observe and notice something in the mirror. Again, the student might well not realise this, and even disbelieve it. After all, developing awareness is an integral and early part of the training, and the student’s mind may wander or blank off at certain moments in the process of preparing for sound or making it. So it’s hardly surprising they blank off at the same moment even when a mirror is put in front of them. The issue may be a mental and literal blind spot!
Fourth, some people are extremely uncomfortable looking in a mirror, and will beg not have to do so in a singing lesson, becoming highly distressed. There are interesting questions about how a teacher might decide to respond to that. After all, a singer has to learn to become compassionately, non-judgementally self-aware.
Fifth, and finally, some students start to use the mirror as a crutch, becoming less confident when the mirror is removed. Teachers must not unwittingly encourage dependency on an external prop. After all, a singer must not depend on visual feedback, teacher feedback (see ‘Who’s in charge …?’ and ‘The perils of warming up’), or much auditory feedback even, when actually singing or performing, since none is available or reliable when singing in ‘real life’. The singer must learn to depend primarily on internal and proprioceptive feedback. The mirror can be used to show what’s needed, so a student can self-correct. Once that works, encourage the singer to notice what it feels like, and then move away from the mirror.