Third eye has been mentioned in many spiritual texts. It is not the physical eye, but an esoteric or an inner eye. It refers to the ajna chakra, between the eyebrows, deep inside the brain.
There has been mention of 7 major chakras along the Sushumna channel (region of spinal cord), starting from the basal chakra to the crown chakra. Ajna or the brow centre is called the command centre as it has control over all the chakras down, from throat chakra to basal chara. So only, there has been a great emphasis on this chakra. Once the commanding centre is made active, it takes care of commanding and bringing in place the rest under its control.
This is the reason, many yogic practices focus on activating third eye and developing it. It is also considered a portal for entering different planes of existence. For the proper development of the third eye, the right practice should be taken up. It is a gradual process and a regular practice is a must.
The sushumna channel has been referred as the highway to the heaven. It is through this channel, the dormant energy (Kundalini) flows from the bottom chakra, upwards. Purification of this channels and diverting the energy in a right way is necessary for the activation of the third eye.
In India, for irrigation of agricultural fields, using channels (creating a way in the soil) is a common practice. If the channel is not blocked and if it connects the water source and the destination, the crop, the purpose is served. In a similar manner, we have a channel already in place. The source and the destination is also there. We just need to work with the channel in the initial phase. Then, the work will be taken care by itself.
In Kriya yoga, one of the techniques gives a lot of importance in working with this channel, sushumna. So, the techniques of kriya yoga are kept in high regards by many yogis.
Divine light and divine sound
This divine light and divine sound has been mentioned by Sufis, Kriya yogis and many other different lineages. Nada yoga is a system of yoga, completely devoted to hearing the divine sounds. Nada means divine sound. Drishti yoga is a branch which speaks mostly of the divine light.
Why I am speaking here regarding the divine light and the divine sound is that, if a yogi starts hearing the divine sounds or seeing the divine light, it is an indicator that the third eye is getting more active.
It is also said, in the process of selecting the Guru (the master), the divine sound and the divine light should be used as an indicator. If a Guru is genuine, he should be able to make you perceive the divine light and the divine sound. After that, it is in the hands of the practitioner to meditate on the divine light/divine sound and progress further.
Meditating on the divine light and sound is a great way to come out of the grips of the varying thoughts. If the mind is rested upon the divine sound (para nada), it becomes pure faster. This is one of the great techniques to develop the third eye.
The final destination....
Considering the final destination, practice is just a part, from our side. Surrender to the divine will, or the action of the higher self is quite important. It takes care of the rest. Once you become aware of the subtler sensations, you could feel the work of the higher self on your subtler bodies, when you sit for the meditation. Practice and surrender are quite important to move faster towards the realization of our Self.