Question

If you are to determine the the focal length of a convex lens, you should have:
A) a convex lens and a screen
B) a convex lens and lens holder
C) a lens holder, a screen and a scale
D) a convex lens, a screen, holders for them and a scale

Answer 1

Hint
Focal length termed as the distance between the optical center and the principal focus. Convex lens is a lens that is thinner at the edges and thicker at the centre is known as a convex lens. Generally, most lenses are made up of glass or transparent plastic and the same can be used for transmitting, refracting, or for focusing the light.
The following are the types of convex lenses:
(i) Double convex
(ii) Plano-convex
(iii) Concavo-convex

Complete answer:
To determine the focal length of the convex lens, we need a convex lens, a screen on which image is formed, holders for screen and the lens, and a scale to measure the focal length of the lens.
A convex lens is also known as a converging lens wherein the lens in the middle is thick and thin at the edges.
The images obtained are real, very small, and inverted. Focal length termed as the distance between the optical center and the principal focus.
So option (D) is the correct option.

Note
We will discuss an example - Magnifying glass.
When an object is placed closer to a convex lens than the principle focus, the rays never converge and a magnified image is formed on the same side of the object. Basically thick, bulging convex lenses have the shortest focal lengths, which are commonly used as powerful magnifying glasses.