Refraction of Light (Laws of Refraction, Lenses) - SS1 Physics Lesson Note

Refraction is the bending of light as it passes from one medium to another with a different optical density. It is a fundamental phenomenon that occurs when light waves change speed and direction due to the change in the medium. Refraction plays a crucial role in various optical phenomena, such as the focusing of light by lenses and the formation of rainbows. Let's explore the laws of refraction and the behaviour of light in lenses:

1. Laws of Refraction (Snell's Law):

The laws of refraction, also known as Snell's law, describe how light waves behave when they pass from one medium to another. The laws of refraction are as follows:

-       Incident Ray: The incident ray is the incoming ray of light that strikes the boundary between two media.

-       Refracted Ray: The refracted ray is the ray of light that bends and changes direction as it passes from one medium to another.

-       Angle of Incidence: The angle of incidence is the angle between the incident ray and the normal, which is a line perpendicular to the surface at the point of incidence.

-       Angle of Refraction: The angle of refraction is the angle between the refracted ray and the normal.

Snell's law states that:

-       The incident ray, the refracted ray, and the normal all lie in the same plane.

-       The ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the velocities of light in the two media: 

 n₁ × sin(θ₁) = n₂ × sin(θ₂)

where n₁ and n₂ are the refractive indices of the first and second media, respectively, and θ₁ and θ₂ are the angles of incidence and refraction.

2. Refraction in Lenses:

Lenses are transparent optical devices with curved surfaces that refract light and can focus or diverge light rays. There are two primary types of lenses:

-       Convex Lens: A convex lens is thicker at the centre and thinner at the edges. It converges or brings light rays together at a point called the focal point. Convex lenses are commonly used in magnifying glasses, cameras, and telescopes.

-       Concave Lens: A concave lens is thinner at the centre and thicker at the edges. It diverges or spreads out light rays. Concave lenses are used in correcting nearsightedness (myopia) and are also used in some optical devices.

When light passes through a lens, it undergoes refraction at each curved surface of the lens. The behaviour of light in lenses is governed by the principles of refraction, focal points, and image formation.

-       Focal Point and Focal Length: A lens has two focal points, one on each side. The focal point is the point where parallel light rays converge (for a convex lens) or appear to diverge (for a concave lens). The distance from the centre of the lens to the focal point is called the focal length.

-       Image Formation: Lenses can form real or virtual images, depending on the position of the object and the characteristics of the lens. Real images are formed when light rays actually converge or diverge and can be projected onto a screen. Virtual images are formed by the apparent intersection of light rays and cannot be projected.

Understanding the laws of refraction and the behaviour of light in lenses is crucial in optics, vision correction, and the design of optical instruments. It allows us to comprehend the focusing of light, the formation of images, and the functioning of devices such as eyeglasses, microscopes, and cameras.