Lens Maker’s Formula Explained | 10th Science Optics Study Material

Study Material | 10th Science | Chapter 2: Optics

Lens Maker’s Formula

LENS MAKER’S FORMULA

All lenses are made up of transparent materials. Any optically transparent material will have a refractive index. The lens formula relates the focal length of a lens with the distance of object and image. For a maker of any lens, knowledge of radii of curvature of the lens is required. This clearly indicates the need for an equation relating the radii of curvature of the lens, the refractive index of the given material of the lens and the required focal length of the lens. The lens maker’s formula is one such equation. It is given as

where µ is the refractive index of the material of the lens; R₁ and R ₂ are the radii of curvature of the two faces of the lens; f is the focal length of the lens.

Understanding Refraction of Light: Laws and Refractive Index | 10th Science Optics

Refraction of Light

REFRACTION OF LIGHT

When a ray of light travels from one transparent medium into another obliquely, the path of the light undergoes deviation. This deviation of ray of light is called refraction. Refraction takes place due to the difference in the velocity of light in different media. The velocity of light is more in a rarer medium and less in a denser medium. Refraction of light obeys two laws of refraction.

1. First law of refraction:

The incident ray, the refracted ray of light and the normal to the refracting surface all lie in the same plane.

2. Second law of refraction:

The ratio of the sine of the angle of incidence and sine of the angle of refraction is equal to the ratio of refractive indices of the two media. This law is also known as Snell’s law.

• Refractive index gives us an idea of how fast or how slow light travels in a medium. The ratio of speed of light in vacuum to the speed of light in a medium is defined as refractive index ‘µ’ of that medium.
• The speed of light in a medium is low if the refractive index of the medium is high and vice versa.
• When light travels from a denser medium into a rarer medium, the refracted ray is bent away from the normal drawn to the interface.
• When light travels from a rarer medium into a denser medium, the refracted ray is bent towards the normal drawn to the interface.