Light Energy: Sources and Propagation

Light is a form of energy that stimulates the sense of sight. It is a part of the electromagnetic spectrum and travels as transverse waves.

1. Sources of Light

Light sources are broadly categorized into two types:

• Natural Sources: These are sources that exist in nature without human intervention. Examples include the Sun (the primary source), stars, and bioluminescent organisms like fireflies.
• Artificial Sources: These are man-made sources. Examples include electric bulbs, candles, torches, and lasers.

Luminous vs. Non-Luminous Objects

• Luminous Objects: These are objects that generate and emit their own light. Examples include the Sun, stars, a lit candle, and a glowing bulb.
• Non-Luminous Objects: These objects do not produce light but are seen because they reflect light from a luminous source. Examples include the Moon, planets, wood, and mirrors.

2. Propagation of Light

Light travels in straight lines; this phenomenon is known as the Rectilinear Propagation of Light. Evidence of this includes the formation of shadows, eclipses, and the operation of the pin-hole camera.

The Speed of Light

In a vacuum or air, light travels at a constant speed of approximately c = 3.0 x 10⁸ m/s. The relationship between speed (v), frequency (f), and wavelength (λ) is given by the wave equation:

v = fλ

3. Shadows and Eclipses

A shadow is formed when an opaque object blocks light. It consists of two regions:

• Umbra: The region of total darkness.
• Penumbra: The region of partial darkness (only formed by extended light sources).

Eclipses

• Solar Eclipse: Occurs when the Moon passes between the Sun and the Earth, casting a shadow on the Earth's surface.
• Lunar Eclipse: Occurs when the Earth passes between the Sun and the Moon, casting its shadow on the Moon.

4. The Pin-hole Camera

The pin-hole camera operates on the principle of rectilinear propagation of light. It consists of a light-tight box with a small hole on one side and a translucent screen on the opposite side. The image formed is always real, inverted, and usually diminished.

Magnification (M)

Magnification is the ratio of the image size to the object size, or the image distance to the object distance:

M = Image height (h_i) / Object height (h_o) = Image distance (v) / Object distance (u)