Question

What happens to the focal length of the convex lens when it's immersed in water?

Answer 1

Hint: This question can be solved based on the refractive index. We can define a refractive index as the ratio of the speed of light in a vacuum to the speed of light in a specified medium. The refractive index of a medium depends on the wavelength of light, optical density, temperature, the refractive index of the surroundings. Focal length can be defined as the distance between the convex lens and the focal point of a lens. It is affected by the radius of curvature, refractive index, and the medium in which the lens resides.

Complete step by step answer:
If $ {n_1} $ is the refractive index of the material and $ {n_2} $ is the refractive index of water.
The refractive index of thin lens in air is given by lens makers formula,
 $ \dfrac{1}{f} = \left( {n - 1} \right)\left( {\dfrac{1}{{{R_1}}} - \dfrac{1}{{{R_2}}}} \right) $
 $ {R_1}\& {R_2} $ are the radius of curvature of two convex lenses.
 $ \begin{gathered}
\dfrac{1}{f} \propto \left( {n - 1} \right) \\
\therefore f \propto \dfrac{1}{{n - 1}} \\
\end{gathered} $
As the refractive index of air is greater than that of water focal length increases.

Additional Information:
Convex lens is a type of lens that converges light rays parallel to the principal axis.
Principal axis: It is a line passing through the centre of the surface of the lens and through the centre of curvature of all segments of the lens.
A convex lens produces a real image and a virtual image. Real images can be produced when the object is located at a position of more than one focal length from the lens. If the object is located in front of the local point then a virtual image is produced.

Note: Convex lens is used to treat hypermetropia which is also known as long-sightedness. It is used in camera lenses as it can focus on a clear picture. It is also used in microscopes and magnifying glasses. There are different types of convex lens which are the plano-convex lens, double -convex lens, concave-convex lens.