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Light Class 7 Notes Science Chapter 15

Light Class 7 Notes Science Chapter 15

Preview of the chapter 

We frequently witness a wide array of things in this world. Sometimes a dark room prevents us from seeing anything, but after lighting the space, we are able to view everything there. Since it is obvious to wonder what causes something to be visible.
Its response is so minimal. Energy in the form of light allows us to view objects from which it emanates or from which it is reflected. Light is detectable by our eyes. 

Does light travel in straight line?

  • Yes, light generally travels in a straight line in a homogeneous and isotropic medium, such as a vacuum or air.
  • This is known as the principle of rectilinear propagation of light.
  • However, when light passes through a medium with a varying refractive index, such as a lens or a prism, it can be refracted, causing it to change direction.
  • Additionally, light can also be reflected, scattered, or absorbed, which can also change its direction or alter its path.
  • But in a uniform medium, light travels in a straight line.

Based on their interaction with light, objects can be classified as:

  • Transparent objects allow light to pass through them completely: glass
  • Translucent objects allow light to pass through them partially. E.g.: Butter paper
  • Opaque objects do not allow any light to pass through them. E.g.: table, book, etc.



Reflection

  • Reflection is the process by which a wave, such as a light wave or sound wave, bounces off a surface and changes direction.
  • When light waves strike a surface, some of the light is absorbed by the material, some is transmitted through it, and some is reflected back.
  • The angle of incidence, or the angle at which the light strikes the surface, is equal to the angle of reflection, or the angle at which the light bounces off the surface.




Laws of reflection

The laws of reflection describe how light waves reflect off a surface, and they are as follows:

  1. The angle of incidence is equal to the angle of reflection: The angle between the incident light ray and the normal (a line perpendicular to the surface at the point of incidence) is equal to the angle between the reflected light ray and the normal.
  2. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane: This means that if you draw a line perpendicular to the surface at the point of incidence, the incident and reflected rays will both lie in the same plane as the normal.

These laws apply to any type of wave that can be reflected, including light waves, sound waves, and water waves. The laws of reflection are fundamental to optics and are used in the design of many optical devices, including mirrors, lenses, and prisms. Understanding the laws of reflection is important for understanding how light behaves and for predicting how it will interact with different surfaces and materials.

Images

An image is a representation of an object formed by light rays that pass through or reflect off the object and enter our eyes or a camera lens. Images can be real or virtual, depending on whether they can be projected onto a screen or not. Real images are formed when light rays converge to a point, either through refraction or reflection. They can be projected onto a screen and are usually inverted (upside down) compared to the object. Examples of real images include the image formed by a converging lens, a concave mirror, or a pinhole camera. Virtual images, on the other hand, are formed when light rays appear to diverge from a point, but do not actually converge to that point. They cannot be projected onto a screen and are usually upright (right-side up) compared to the object. Examples of virtual images include the image formed by a diverging lens or a convex mirror.

Range of visibility (plane mirrors)

The range of visibility in plane mirrors depends on the size of the mirror and the position of the observer relative to the mirror. If the mirror is small, such as a handheld mirror or a makeup mirror, the range of visibility will be limited to a relatively small area. The observer will only be able to see objects that are located within the field of view of the mirror, which is determined by the size and shape of the mirror. If the mirror is larger, such as a full-length mirror or a bathroom mirror, the range of visibility will be greater. The observer will be able to see a larger area, including objects that are located further away from the mirror.

Lateral inversion

  • It refers to the apparent reversal of the left-right orientation of an object when it is reflected in a plane mirror.
  • In other words, when an object is reflected in a plane mirror, the image appears to be flipped horizontally, as if it has been turned around a vertical axis.
  • For example, if you hold up a book in front of a mirror, the image in the mirror will appear to show the book as if it has been flipped horizontally, with the title and text appearing backwards.
  • This is because the light rays reflected by the book are reversed when they bounce off the mirror, creating an image that is laterally inverted.

Spherical mirrors

Spherical mirrors are mirrors that have a curved surface with a spherical shape. They can be either concave or convex, depending on the shape and position of the curved surface. A concave spherical mirror is a mirror with a curved surface that is inward-facing (or "caved-in"). When light rays are reflected off a concave mirror, they converge at a point known as the focal point, which is located halfway between the mirror and its center of curvature. The distance from the mirror to the focal point is known as the focal length. Concave mirrors are commonly used in optical devices such as telescopes and reflector headlights. A convex spherical mirror, on the other hand, is a mirror with a curved surface that is outward-facing (or "bulging out"). When light rays are reflected off a convex mirror, they diverge as if they are coming from a point behind the mirror. This virtual point is known as the focal point, but in the case of a convex mirror, it is located behind the mirror. The distance from the mirror to the virtual focal point is again the focal length. Convex mirrors are commonly used in applications such as side-view mirrors on cars, and security mirrors in stores.

Image formation by spherical mirrors

  • The image formation by spherical mirrors depends on the position of the object, the position of the mirror, and the type of spherical mirror - concave or convex.
  • When an object is placed in front of a concave mirror, the reflected light rays converge at a point called the focal point.
  • The distance between the focal point and the center of the mirror is called the focal length.
  • Depending on the position of the object relative to the focal point and the mirror, a real or virtual image may be formed.
  • If the object is placed between the focal point and the concave mirror, a virtual upright image is formed behind the mirror.

Lenses

Refraction

  • Refraction is the bending of light as it passes through a medium, such as air or a lens, that has a different density than the medium it was previously traveling through.
  • The amount of bending depends on the angle at which the light ray enters the new medium, as well as the difference in the refractive indices of the two media.
  • Refraction is responsible for many optical phenomena, including the formation of images by lenses and the bending of light in rainbows.

Image formation by lenses

  • Image formation by lenses is based on the principle of refraction, where the lens bends the path of light rays that pass through it.
  • The image formed depends on the position of the object relative to the lens, and the type of lens being used - convex or concave.
  • Depending on the position of the object relative to the lens and the focal point, a real or virtual image may be formed.
  • If the object is placed between the focal point and the convex lens, a virtual upright image is formed on the same side of the lens as the object.
  • Concave lenses are commonly used in eyeglasses to correct for nearsightedness.

Dispersion of white light through prism

  • When white light enters a prism, the light is refracted, or bent, as it passes through the prism.
  • This phenomenon is known as dispersion.
  • As the white light passes through the prism, it is separated into its individual colors due to dispersion.
  • The dispersion of white light through a prism is an important phenomenon in optics and is used in many applications, including spectroscopy, where scientists use prisms to separate and analyze the different wavelengths of light emitted by different sources.
  • This can provide information about the chemical composition and physical properties of the sources, such as stars or gases.



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