when the object is located outside the center of the sphere of a concave spherical mirror, it forms an inverted and reduced real image, and the image is located between the focus and the center of the sphere; when the object is located between the focus and the center of the sphere, it forms an inverted and enlarged real image, and the image is located outside the center of the sphere; when the object is located within the focus at this time, an upright magnified virtual image is formed, and the image is on the other side of the mirror. there is no image when u=f.

the focal length is behind the center of the circle in front of the lens, but not at r/2. but if the incident ray is a paraxial ray, the focal length can be approximately considered to be at r/2.

1/f = 1/d₀ + 1/d_i

f - focal length

d_i - image distance

d₀ - object distance