CBSE Notes Class 7 Science Chapter 11 Light

Light is a form of energy that allows us to perceive the world around us. It travels in straight lines and plays a vital role in many everyday phenomena. Sunlight, our main source of natural light, is essential for life on Earth and significantly impacts our environment. While light typically moves in straight lines, it can change direction when it encounters obstacles or different mediums.

1.0Light

Light is a form of energy that enables us to see and perceive our surroundings. Natural sources of light include the Sun and fire, while man-made sources consist of devices like tube lights and bulbs.

Rectilinear Propagation Of Light:

Light consists of energy packets known as photons, which travel at incredibly high speeds in straight lines. While light can pass through small openings and change direction, it consistently moves in straight paths.

Reflection:

Light, sound, water, and electromagnetic waves can change direction when they strike a surface, a phenomenon known as reflection. While light waves are often reflected, other types of waves may penetrate the surface instead. Shiny surfaces, such as stainless steel, water, and mirrors, reflect light effectively. These polished surfaces behave like mirrors due to their strong reflective properties.

2.0Reflection By Plane Mirror 

When light hits a plane mirror at an angle (angle of incidence), it reflects at the same angle (angle of reflection). The image formed appears behind the mirror and has the following characteristics:

1.  Erect (upright)
2. Same size as the object
3.  At the same distance as the object
4.  Right and left sides are reversed

3.0Spherical Mirror And Its Types

• A curved, shiny surface functions as a mirror, and the most common types are spherical mirrors. These mirrors are classified into two categories based on their reflective surfaces: concave and convex. When a sphere is sliced, the inner part is known as the concave side, while the outer part is referred to as the convex side. A familiar example is a stainless steel spoon, where the inner surface acts as a concave mirror and the outer surface acts as a convex mirror.
• Real and Virtual Images-A real image is created when light rays converge and can be forecasted onto a screen using a mirror. In contrast, a virtual image cannot be displayed on a screen, as it appears to be behind the mirror. Both concave and convex mirrors can produce real and virtual images.

Concave Mirror

A concave mirror can produce images that vary in size and orientation depending on the object's distance from the mirror. When the object is far away, a small, inverted image is formed. As the object gets closer, the image enlarges, and at very close distances, it becomes large and upright.

Applications of Concave Mirrors:

• Concave mirrors are useful because they create an enlarged virtual image when positioned close to an object. They are commonly used by:
• Doctors for examining the ears, nose, throat, and eyes.
• Dentists for inspecting teeth and gums.
• Additionally, concave mirrors focus light into a narrow beam, making them ideal for use inTorches,Car headlights

Convex Mirror

• A convex mirror typically produces an erect and smaller image of an object. It creates virtual images that cover a wider area and cannot form real images.

Applications of Convex Mirrors:

• Rear View Mirrors: Used in vehicles to help drivers see a larger field of traffic behind them.
• Street Lamps:Serve as reflectors to spread light over a broader area.
• Sunglasses and Telescopes-Utilized in the design of various optical devices.

4.0Lens

• A lens is a transparent  device that refracts light instead of reflecting it. Refraction occurs when light bends as it passes through a medium, which, in this case, is the lens.The two main types of lenses are convex (or bi-convex) and concave (or bi-concave).
• Convex Lenses: Convex lenses, which are thicker in the center than at the edges, are referred to as convergent lenses because they focus light by bending it inward.
• Concave Lenses: Concave lenses, which are thinner in the center than at the edges, are known as divergent lenses because they spread light by bending it outward.
• Convex lenses can form real images when positioned between the object and a screen, while also creating virtual images that are erect and larger in size.In contrast, concave lenses only produce virtual images, which are always erect and smaller.

Applications of Lenses:

• Lenses are used in various devices, including magnifying glasses, telescopes, contact lenses, and cameras.

5.0Sunlight

• The light emitted by the sun appears white, but it is made up of seven colors: red, orange, yellow, green, blue, indigo, and violet. Various phenomena and objects can separate these white rays into their individual colors.
• A rainbow appears during or after rainfall when sunlight is reflected, refracted, and dispersed by water droplets in the atmosphere.
• A prism, which is a transparent optical object with polished surfaces, refracts sunlight into these seven colors. Similarly, a compact disc can display fine lines of these colors when sunlight hits its shiny side.