Question

To determine the approximate value of the focal length of a given concave mirror, you focus the image of a distance object formed by the mirror on a screen. The image obtained on the screen, as compared to the object is always.
A. Laterally inverted and diminished
B. Inverted and diminished
C. Erect and diminished
D. Erect and highly diminished

Answer 1

Hint: Incident rays of distant objects are almost parallel so all the reflected rays converge at the focus of the mirror and the image formed at focus is highly diminished in size as all the incident rays converge at a single point only and since image is obtained at screen it is a real image.

Complete answer:
Lets define real image and virtual image for more clarity.
Real images are formed by the actual intersection of light rays. It can be obtained on a screen. Hence, projectors form real images. Virtual images are formed when the light rays appear to be originating from a point but do not actually meet. It can be seen by human eyes. Hence, rear-view mirrors form virtual images.

Since at focus of the concave mirror reflected rays actually meet and are obtained on the screen it means the image formed at focus in real and all real images formed by a mirror is always inverted so the image must be inverted.Now the size of the image will be highly diminished because all the rays coming out of the distant object converge at a single point.

Hence correct answer is Option B.

Note:Most of the students have confusion about the sign convention so always remember that the distance measured along the direction of incident ray is taken as positive and vice-versa. The height measured upon the principle axis is taken as positive while the height measured below the principle axis is taken as negative.