Which mirror has it’s real principal focus
Answers
hey,
UNDERSTANDING PRINCIPAL FOCUS IS IMPORTANT. THEY CAN HELP IN RAY DIAGRAMS FOR IMAGE FORMATIONS.
THE PRINCIPAL FOCUS
The principal focus is defined as a “point on the principal axis” through which “paraxial rays will pass”. The paraxial rays are nothing but the rays which are parallel to the principal axis.
When considering the case of only mirrors, then very approximately, the radius of curvature equals the focal length.
INTUTION TO CONVEX MIRRORS
The convex mirrors are basically diverging(rays) mirrors. We also know that, to see an image, we require at least 2 light rays (transmitted or reflected).
Therefore, when the paraxial rays are made to travel towards this convex mirror, all the light rays after reflecting ‘appear’ to come from the principal focus.
VIRTUAL PRINCIPAL FOCUS
Since the principal focus is behind the mirror, that focus is called as virtual principal focus, for convex mirror.
IMAGES FORMED BY CURVED MIRRORS
For most of the cases concave mirror can from a real image. It can make an enlarged image when the real object is between pole and principal focus or even at the principal focus, and the images formed are virtual images.
Convex mirrors always forms diminished images. Drawing ray diagrams for the image formation by convex lenses, can help in understanding that convex mirrors always from virtual images.
R IS NOT ALWAYS EQUAL TO 2F
Under the principal focus, R=2f is mentioned only for cases in which mirrors are alone involved.
The reason for why it isn't confirm for lenses is that in cases of lenses the absolute refractive index matters the focal length.
It is also mentioned about the definition of focus. The focal length depending on the absolute refractive index of lense, is given by,
1÷F =(n ( -) 1)*[1 ÷ R1 (-) 1÷R2], where F is focal length, n is absolute refractive index.
For equi lenses,
1÷F =2 (n-1)÷R.
Substituting R=2F is wrong.
DUE TO THE SLUGGISH TENDENCY OF THE MEDIUM TO SLOW DOWN THE SPEED OF LIGHT (OR) DUE TO REFRACTIVE INDEX, THE LIGHT IS BENT SUCH THAT R IS NOT EQUAL TO 2F.