Light Propagation, Shadows, Reflection and Refraction Explained

Propagation of Light

Light propagates in a vacuum at a speed of approximately 300,000 km/s. This speed is known as the speed of light and is symbolized by the letter c.

Light energy transported by electromagnetic waves is directly proportional to their frequency. Electromagnetic waves are classified by their frequency (or energy) into the electromagnetic spectrum. Humans' eyes are sensitive only to a narrow band of electromagnetic waves, known as visible light, which ranges from red light to violet light.

Shadows

In general, if a light bulb is very large compared to an object, it will produce blurred shadows. If a light source is small and nearly point-like, it will produce sharp shadows.

When the light source is very large and close to the object, the shadow formed has a well-defined area, the umbra, where no rays from the extreme edges of the light source can reach, and a penumbra, a bright area where only some rays from the extreme edges of the source arrive.

Eclipses

• Solar eclipse: occurs when the Moon is between the Sun and the Earth, blocking the Sun's light.
• Lunar eclipse: occurs when the Earth is between the Sun and the Moon.
• Partial eclipse: produces only a penumbra on the Earth's surface, so only part of the Sun is obscured.
• Annular eclipse: occurs when the Moon is far enough away that it does not completely cover the Sun, leaving a bright ring visible.

Reflection of Light

Reflection is the change in direction that a light ray experiences when it strikes the surface of a body. The incident ray, the reflected ray and the normal line lie in the same plane, perpendicular to the surface.

The law of reflection states that the angle of incidence is equal to the angle of reflection.

• Specular reflection: occurs on a surface that is perfectly smooth, where parallel incident rays are reflected in the same direction.
• Diffuse reflection: occurs when the surface has roughness, causing the reflected rays to travel in all directions.

Mirrors

Rays coming from the top of an object reach our eyes after reflecting off a mirror. Behind the mirror, the image appears to be located behind it. This is called a virtual image.

• Concave mirror: parallel rays striking the mirror converge at the same focal point F, located in front of the mirror.
• Convex mirror: reflected rays diverge, but their imaginary extensions meet at a focus located behind the mirror.

Refraction of Light

Refraction is the change in direction that light rays experience when passing from one medium into another with a different propagation speed.

The refracted beam, the normal and the incident ray lie in the same plane. The refracted ray approaches the normal when light passes into a medium where its speed is lower.

The ratio between the speed of light in a vacuum and its speed in a given medium is called the index of refraction:

n = c / v

Dispersion of light is the variation of the refractive index with frequency: the higher the frequency, the greater the refractive index.

Types of Lenses

• Converging lenses (biconvex): thicker at the center. Rays passing through them converge at a single point called the focus.
• Diverging lenses (biconcave): thinner at the center. Rays passing through them diverge and never meet at a single point; instead, they spread apart.

Types of Materials

• Transparent: light travels through them in the same direction.
• Opaque: absorb or reflect light and do not allow it to pass through.
• Translucent: partially absorb or reflect light and allow some of it to pass through, but scatter it in different directions.