Class 7 Science - Light: Shadows and Reflections

Definition of Light: Light is a form of energy that enables vision and makes objects visible to the human eye.
Rectilinear Propagation of Light: Light travels in straight lines in a homogeneous medium. Because light does not bend around sharp edges easily, opaque objects placed in its path block the light rays, resulting in shadow formation.
Duration and Scope: A full chapter overview covering fundamental principles of light, shadows, and reflections designed for Class 7 Science in a 15-minute summary.

Luminous Objects: Objects that emit light of their own.
- Examples: Sun, stars, burning candle, lighted torch, glowing electric bulb.
Non-Luminous Objects: Objects that do not emit light of their own and become visible only when light from a luminous source falls on them and reflects into an observer's eyes.
- Examples: Moon, chairs, tables, books, trees, human body.

Transparent Materials: Materials that allow light to pass through them completely, enabling objects on the other side to be seen clearly.
- Examples: Clean glass, clear water, air, clear polythene sheets.
Translucent Materials: Materials that allow light to pass through them only partially, causing objects on the other side to appear blurred or faint.
- Examples: Butter paper, frosted glass, tracing paper, oily paper, thin tissue paper.
Opaque Materials: Materials that do not allow any light to pass through them at all.
- Examples: Wood, metals, cardboard, stone, brick wall.

Definition: A shadow is a dark patch or region formed on a surface or screen when an opaque object blocks the path of light rays.
Essential Requirements for Shadow Formation:
- A source of light (e.g., torch, sun, lamp).
- An opaque or semi-opaque (translucent) object to obstruct the light.
- A screen or surface (e.g., ground, wall, paper, sheet) on which the shadow can be cast.
Key Characteristics of Shadows:
- Shadows are always formed on the side opposite to the light source.
- A shadow provides information only about the shape or outline (silhouette) of an object, not its color, fine details, or surface texture.
- The size of a shadow changes based on the relative distances between the light source, the opaque object, and the screen:
- Decreasing the distance between the light source and the object increases the size of the shadow.
- Increasing the distance between the light source and the object decreases the size of the shadow.
- Moving the screen farther from the object increases the shadow's size while making its edges fuzzier.

Principle: Operates on the principle of the rectilinear propagation of light (light travelling in straight lines).
Construction: A simple device consisting of a light-tight box with a tiny hole (pinhole) on one side and a translucent screen (such as wax paper or tracing paper) on the opposite inner side.
Image Characteristics:
- The image formed on the screen is inverted (upside-down).
- The image retains the natural colors of the object being viewed.
- The size of the image depends on the relative distances between the object, the pinhole, and the translucent screen.

Reflection: The process by which light bouncing off a shiny or polished surface changes its direction of travel.
Mirrors: A mirror is a highly polished reflecting surface that forms clear images by regular reflection of light rays.
Characteristics of Image Formed by a Plane Mirror:
- Lateral Inversion: The left side of the object appears as the right side in the mirror image, and the right side of the object appears as the left side.
- Orientation: The image is upright (erect).
- Size: The image is of the exact same size as the object.
- Distance: The image appears to be at the same distance behind the mirror surface as the real object is in front of it.