Light normally travels in a straight line when it is in a uniform medium with consistent density. Refraction of light occurs when light transitions from one medium to another with a different optical density. It results in a change in the direction of the light ray.
Key Aspects of Refraction:
Refraction through a rectangular glass slab is often used to illustrate the principles of refraction and how light changes direction as it moves from one medium to another with a different refractive index.
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The laws of refraction, also known as Snell's laws, describe how light behaves when it passes from one medium to another with a different refractive index. These laws are fundamental principles in optics and govern the behaviour of light at the boundary between two substances.
The incident ray, the refracted ray, and the normal (an imaginary line perpendicular to the boundary) at the point of incidence all lie in the same plane. In simpler terms, they share a common plane.
The ratio of the sine of the angle of incidence (sin i) to the sine of the angle of refraction (sin r) is a constant value for a given pair of media. This constant is known as the refractive index. Snell's law is mathematically represented as:
Where:
n is the refractive index of the medium.
i is the angle of incidence, the angle between the incident ray and the normal.
r is the angle of refraction, the angle between the refracted ray and the normal.
Different media have different refractive indices, and this property is fundamental in understanding the behaviour of light when it passes through different substances.
When light travels from one medium to another, such as from air to water or from one substance to another, its behaviour changes due to differences in the speed of light in those media. This change in behaviour is described by the concept of refractive index (n), which is a fundamental property in optics.
Refractive Index (n): The refractive index of a medium is a measure of how much the speed of light changes when it enters that medium from another medium, typically air or vacuum. It quantifies how much light is bent or refracted as it crosses the boundary between two substances. Mathematically, the refractive index (n) is defined as the ratio of the speed of light in vacuum (or air) to the speed of light in the medium:
The refractive index is always greater than or equal to 1 because the speed of light in any medium is less than or equal to the speed of light in a vacuum. A higher refractive index indicates that light travels more slowly in that medium and will be bent more when it enters that medium.
This relationship means that if you know the refractive index of a material for light entering it from another medium, you can easily calculate the refractive index for light exiting from that material back into the original medium.
The refractive index (n) of medium 2 concerning medium 1 can be described as the ratio of the speed of light in medium 1 to the speed of light in medium 2. This is represented as n21 and is calculated using the following formula:
Spherical lenses refract light as it passes through them, causing it to converge or diverge based on the type of lens.
A convex lens is thicker at its centre and thinner at its edges, causing it to bulge outward. It resembles a piece of spherical glass that bows outward.
Convex lenses are also known as converging lenses because they converge (bring together) parallel rays of light that pass through them.
Key characteristics of convex lenses:
In contrast, a concave lens is thinner at its centre and thicker at its edges, giving it a hollow or "cave-like" appearance.
Concave lenses are also known as diverging lenses because they cause parallel rays of light to diverge (spread apart) after passing through them.
Key characteristics of concave lenses:
By using certain rules, you can draw ray diagrams to locate the position and characteristics of the image formed by a convex lens. These rules are used to find the position and nature of images formed by convex lenses. By applying these rules, you can determine whether the image is real or virtual, where it is located relative to the lens, and whether it's upright or inverted. Different combinations of these rules help analyse various object positions in front of a convex lens and predict the characteristics of the resulting images.
When a ray of light is parallel to the principal axis of a convex lens, it will pass through the lens and refract in such a way that it appears to come from the focal point (F) on the opposite side of the lens.
When a ray of light passes through the optical centre (O) of a convex lens, it continues in a straight line without any deviation. The optical centre is the point on the lens where light passes through without bending. This rule is particularly useful for determining the path of the ray but doesn't directly reveal information about the image's location.
If a ray of light initially travels towards the focal point (F’) of a convex lens, it will refract through the lens and emerge parallel to the principal axis. This rule is the opposite of Rule 1, where a parallel ray converges to the focal point.
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The formation of images by a convex lens involves the refraction of light rays as they pass through the lens. Convex lenses are thicker at the centre than at the edges and are capable of creating both real and virtual images, depending on the object's position relative to the lens.
Primary cases of image formation by a convex lens based on the object's position are:
These rules are crucial for constructing ray diagrams to determine the position and characteristics of images formed by concave lenses. Concave lenses, unlike convex lenses, produce virtual, erect (upright), and diminished images in various scenarios depending on the position of the object relative to the lens.
When a ray of light parallel to the principal axis of a concave lens strikes the lens, it refracts in such a way that it appears to diverge from a specific point on the same side of the lens. This point is known as the focus (F) of the concave lens.
The ray diagram for this rule involves drawing the incoming parallel ray of light, refracting it at the lens, and extending it backwards to show that it appears to come from the focus.
This rule demonstrates how concave lenses disperse parallel rays of light.
When a ray of light passes through the optical centre (C) of a concave lens, it continues in a straight line without any deviation.
The optical centre is the midpoint of the lens, and light passing through it does not refract.
Even if the ray of light initially had a direction away from the optical centre, it will proceed in a straight line through the centre without bending.
This rule is useful for understanding the path of light when it goes through the optical centre of a concave lens.
When a ray of light is directed towards the focus (F’) of a concave lens, it refracts and emerges from the lens parallel to the principal axis.
The ray diagram for this rule involves drawing a ray aimed at the focus of the concave lens, showing its refraction at the lens, and indicating that it exits parallel to the principal axis.
Rule 3 is essentially the reverse of Rule 1 for convex lenses, where Rule 1 converges parallel rays to the focus, while Rule 3 for concave lenses diverges rays directed toward the focus.
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