Spherical Lens

A spherical mirror is a curved mirror that forms part of a hollow sphere. It is of two types:

1. Concave Mirror (Converging) – Reflects light inward, forming real or virtual images. Used in telescopes, headlights, and shaving mirrors.
2. Convex Mirror (Diverging) – Reflects light outward, always forming a virtual image. Used in vehicle rearview mirrors and security mirrors.

Spherical mirror

Spherical mirrors follow the laws of reflection and are essential in optical devices and imaging systems.

1. Pole (P): The centre of the spherical surface of the lens where the principal axis meets the surface.
2. Optical Centre (O): The point at the centre of the lens through which light rays pass without deviation.
3. Centre of Curvature (C₁, C₂): The centres of the spheres whose parts form the surfaces of the lens. A lens has two centres of curvature.
4. Radius of Curvature (R₁, R₂): The radii of the spheres whose parts form the lens surfaces.
5. Principal Axis: An imaginary straight line passing through both centres of curvature and the pole.
6. Aperture: The effective diameter of the lens that allows light refraction.

Optical centre of a lens

1. Convex Lens (Converging Lens)

As seen in Figure (a), parallel light rays converge at a point (F₂) on the principal axis after refraction. This principal focus helps in focusing light, used in magnifying glasses and cameras.

2. Concave Lens (Diverging Lens)

As seen in Figure (b), parallel light rays diverge after refraction, appearing to come from a virtual focus (F₁). It is used in corrective glasses for myopia and optical instruments.

3. Focal Length (f)

Distance between the optical centre (O) and the principal focus (F).

• Convex lens → Positive focal length (+f) (light converges).
• Concave lens → Negative focal length (-f) (light diverges).

Principal focus of a lens

1. Aim: To determine the focal length of a convex lens by obtaining the image of a distant object on a screen.

2. Requirements: convex lens, screen, metre scale, and lens stand.

3. Procedure

• Fix the screen in position.
• Hold the convex lens in front of a distant object (such as a tree or building).
• Adjust the position of the lens until a sharp image of the object appears on the screen.
• Measure the distance between the lens and the screen using a metre scale.
• Reverse the lens and repeat the process to obtain another clear image.
• Measure the distance between the screen and the lens again.
• This distance represents the focal length (f) of the convex lens.

4. Conclusion

• The distance between the screen and the lens is the focal length of the convex lens. The image of a distant object forms near the focus (F) of the lens. 
• If a concave lens is used instead, a real image cannot be obtained on the screen, as concave lenses form only virtual and diminished images.
• This experiment helps in understanding the focal length and image formation properties of convex and concave lenses.