Lower Secondary Science

Luminous Objects

Objects that can produce their own light. This light directly enters our eyes.

Examples: Light bulbs, fires, stars (including the Sun)

Non-luminous Objects

Objects that do not produce their own light. They can be seen as they reflect light from a light source, which then enters the eyes.

Examples: The Moon, cups, books, grass

Transparent Objects

Allow all or most light to be transmitted through them.

Examples: Glass, water, clear air

Translucent Objects

Allow only some light to be transmitted. The rest is absorbed or reflected.

Examples: Tracing paper, stained glass, frosted glass

Opaque Objects

Do not allow any light to be transmitted. Light is either absorbed or reflected.

Examples: Wooden cube, mirror, metal sheet

Rectilinear Propagation

Light rays travel in straight lines.

This is the principle of rectilinear propagation (rectilinear = straight line, propagation = travel).

Principle of Reversibility

Light will follow the same path if its direction of travel is reversed.

Hence, reflected or refracted light will retain its path even when its direction is reversed.

1. Transparency of the Object

• Translucent objects → light/faint shadow
• Opaque objects → dark shadow

2. Relative Positions of Light Source, Object and Screen

The closer an object is to the light source and further from the screen → greater size of shadow

3. Orientation of Object and Light Source

Changing orientation changes the shape of shadow. Changing light source angle also affects shape and size.

Solar Eclipse

The Moon passes directly between Earth and the Sun, blocking the Sun's light. The Moon's shadow is formed on Earth.

Lunar Eclipse

The Earth passes between the Sun and the Moon, casting its shadow on the Moon.

The Two Laws

1. The angle of incidence equals the angle of reflection.
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

Key Definitions

• Incident ray: The ray of light falling on a surface.
• Reflected ray: The ray that emerges after reflecting off a surface.
• Normal: An imaginary line perpendicular to the surface at the point of incidence.
• Angle of incidence (i): The angle between the incident ray and the normal.
• Angle of reflection (r): The angle between the normal and the reflected ray.

Regular (Specular) Reflection

Occurs when light rays strike a smooth surface. If incident rays are parallel, the reflected rays will also be parallel.

Diffuse Reflection

Occurs when light rays strike a rough surface. Even though incident rays are parallel, the reflected rays are scattered in different directions because the angle of incidence differs at each point.

Four Key Properties

1. The image is the same distance from the plane mirror as the object is from the mirror.
2. The image is upright.
3. The image is the same size as the object.
4. The image is a virtual image (cannot be captured on screen).

Lateral Inversion

The image is laterally inverted. (Left and right are swapped)

Convex Mirrors (Curve Outwards)

Image properties: Virtual, upright, laterally inverted, and smaller than the object.

Uses: Security mirrors at blind corners — provide a wider field of vision for drivers.

Concave Mirrors (Curve Inwards)

When close: Virtual, upright, laterally inverted, and magnified

Uses: Dental mirrors (magnified images of teeth), car headlights.

Periscope

A periscope uses two plane mirrors inclined at 45° to the stem, allowing viewing over obstacles.

The Pinhole Camera

Works because light travels in a straight line. The image formed is inverted (upside down).

Flash Photography — Red-Eye Effect

Red eyes in flash photography occur when the flash unit is very close to the camera lens. Light from the flash enters the eye and reflects along its original path back into the lens.