At what position will an object be placed in front of a concave mirror in order to obtain an image at infinity?
Answer Details
To obtain an image at infinity in front of a concave mirror, the object must be placed at the focal point. Therefore, the correct option is "at the principal focus."
When an object is placed at the focal point of a concave mirror, the light rays coming from the object are reflected parallel to the principal axis. As a result, the reflected rays never converge to a point, and the image appears at infinity.
Placing the object at any other position will result in a real, inverted image formed at a specific distance from the mirror, and not at infinity. Therefore, to obtain an image at infinity in front of a concave mirror, the object must be placed at the principal focus.