The Islamic Dictionary of Architecture defines a mihrab as a "marker used to indicate the direction of prayer, usually in a mosque. A mihrab is usually a niche set into the middle of the qibla wall of a building in order to indicate the direction towards Mecca."1
Mihrab symbols are incorporated into some of the jali patterns on the western elevation of Humayun's Tomb. As the sun sets, light passes through a jali screen onto Humayun's cenotaph. As the light passes through the jali, geometric shaped beams of light are projected onto Humayun's cenotaph and materialize in the form of a mihrab. While projecting over the cenotaph, the mihrab is pulled by the setting sun towards Mecca.
Allah is the light of the heavens and the Earth; a likeness of His light is as a niche in which is a lamp, the lamp is in a glass, (and) the glass is as it were a brightly shining star, lit from a blessed olive-tree, neither eastern nor western, the oil whereof almost gives light through fire touch it not—light upon light—Allah guides to His light whom He pleases, and Allah sets forth parables for men, and Allah is Cognizant of all things.3
1. Andrew Petersen, Dictionary of Islamic Architecture (London; New York: Routledge, 1996), 186.
2. Glenn Lowry, "Humayun's Tomb: Form, Function, and Meaning in Early Mughal Architecture," Muqarnas 4, (1987): 142.
3. Qur'an 24:35.
Burton-Page, John. Handbook of Oriental Studies. Vol. 20, Indian Islamic Architecture: Forms and Typologies, Sites and Monuments. Leiden; Boston: E.J. Brill, 2008.
Lowry, Glenn. "Humayun's Tomb: Form, Function, and Meaning in Early Mughal Architecture." Muqarnas 4, (1987): 133-48.
Nizamuddin Urban Renewal Initiative. "Conserving Humayun’s Tomb Main Hall 2009-11." This photo shows the mihrab beam projecting over the cenotaph!
Petersen, Andrew. Dictionary of Islamic Architecture. London; New York: Routledge, 1996, 186-7.