Question

For the determination of the focal length of a convex mirror, a convex lens is required because
A. It is not possible to obtain the image produced by a convex mirror on the screen
B. A convex lens has high resolving power so it helps to measure the focal length correctly
C. A convex mirror always forms a real image which is diminished by the convex lens
D. None of these

Answer 1

Hint: Our human eye uses convex lens i.e. Human eye is a convex lens. Whatever be the position of the object the image formed by the convex mirror is always on the opposite side virtually erect and diminished. For a spherical mirror when the object is beyond focal length the images form on the side of the object are real and that form on the other side of the mirror are virtual.

Complete step-by-step answer:
It will not be worth it if we do not get some idea of a convex lens and mirror. Let’s first discuss what a convex lens is. Then we will go for an answer.
You must have seen lenses used in day today life. Some examples are: the lenses used by old persons for reading, lenses embedded in the front door of the house, the lens which the watchmaker attaches to his eye etc. Lenses are used in spectacles. They are also used in telescopes.

A lens is a transparent medium bound by two surfaces. The lens which has two spherical surfaces which are puffed up outwards is called a convex or double convex lens.
This lens is thicker near the centre as compared to the edges. The lens with both surfaces spherical on the inside is called a concave or double concave lens. This lens is thinner at the centre as compared to its edges.
Different types of lenses exist. A ray of light gets refracted twice while passing through a lens, once while entering the lens and once while emerging from the lens. The direction of the ray changes because of these refractions. Both the surfaces of most lenses are parts of a sphere.
Centre of curvature (C): The centres of spheres whose parts form surfaces of the lenses are called centres of curvatures of the lenses. A lens with both surfaces spherical, has two centres of curvature.

Radius of curvature (R): spheres whose parts form surfaces of the lenses are called the radii of curvature of the lens.
Principal axis: The imaginary line passing through both centres of curvature is called the principal axis of the lens.
Optical centre (O): The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre of a lens. Passing through O are going along a straight line. Thus O is the optical centre of the lens.
Principal focus (F): When light rays parallel to the principal axis are incident on a convex lens, they converge to a point on the principal axis. This point is called the principal focus of the lens.

Light rays parallel to the principal axis falling on a convex lens come together i.e. Get focused at a point on the principal axis. So this type of lens is called a converging lens. Rays travelling parallel to the principal axis of a concave lens diverge after refraction in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the concave lens. Principal focus of the concave lens. Light rays parallel to the principal axis falling on a concave lens go away from one another (diverge) after refraction. So this type of lens is called a diverging lens.

Focal length (f): The distance between the optical centre and principal focus of a lens is called its focal length.
Convex lens forms are always virtual. Therefore it is not possible to get the virtual image on screen. For convex lenses this virtual image acts as a virtual object. This concept is used for determining the focal length of the convex.
Answer- (A)

Note: A desired focal length can be produced by choosing proper values of radius coverture. When the object is anywhere between pole p and infinity the image is virtually erect and diminished when the object is at infinity the image is a virtual erect and highly diminished.