Light, Reflection, and Mirrors

Theories of Light

Scientists have held different views about the nature of light:

The modern view is that light has a dual nature: it behaves as a wave in some experiments (interference, diffraction) and as a particle in others (photoelectric effect). For the purposes of reflection and refraction, the ray (particle-like) model is used.

Young's Double-Slit Experiment

Thomas Young (1801) passed light through two narrow slits close together. Instead of two bright bands on the screen, he observed a pattern of alternating bright and dark fringes. This can only be explained if light is a wave:

• Where crests from both slits arrive together, constructive interference produces a bright fringe.
• Where a crest from one slit meets a trough from the other, destructive interference produces a dark fringe.

This was the first convincing evidence that light is a transverse wave.

Why diffraction of light is not normally observed: Diffraction (spreading through a gap) is most pronounced when the gap size is similar to the wavelength. The wavelength of visible light is extremely small ($\approx 4$-$7 \times 10^{-7}$ m), so everyday gaps and obstacles are millions of times wider — light therefore casts sharp shadows and appears to travel in straight lines. Sound diffracts easily around corners because its wavelengths (0.1 m to 10 m) match everyday obstacle sizes.

Rectilinear Propagation of Light

Light travels in straight lines through a uniform medium. This is called rectilinear propagation and is demonstrated by the fact that an opaque object placed in the path of light casts a sharp shadow.

Shadows

When an opaque object blocks a point source of light, a single dark shadow called an umbra forms, a region that receives no light.

When the source is extended (not a point), two shadow regions form:

• Umbra: region of complete shadow (no light from the source reaches it).
• Penumbra: region of partial shadow (some light from the source reaches it, but not all).

Eclipses

The same geometry explains solar and lunar eclipses:

Solar eclipse: The Moon moves between the Sun and the Earth. In the umbra of the Moon's shadow, a total solar eclipse is observed. In the penumbra, a partial eclipse occurs.

Lunar eclipse: The Earth moves between the Sun and the Moon. The Moon enters Earth's shadow and appears dark.

The Pinhole Camera

A pinhole camera is a box with a tiny hole in one face and a translucent screen on the opposite face. Light from each point of the object passes through the hole in a straight line and forms an inverted image on the screen.

Properties of the pinhole camera image:

• Inverted (upside down and laterally reversed).
• Smaller than the object if the camera is shorter than the object distance.
• Dimmer than a lens image (very little light passes through the pinhole).
• Increasing the pinhole size makes the image brighter but blurs it; decreasing it sharpens the image but dims it.

Laws of Reflection

When light reflects from any surface, two laws always hold:

1. The angle of incidence equals the angle of reflection (both measured from the normal to the surface at the point of incidence).
2. The incident ray, reflected ray, and normal all lie in the same plane.

Images in a Plane Mirror

When an object is placed in front of a plane (flat) mirror, the image formed has the following properties:

The image is behind the mirror because the eye traces reflected rays back as if they came from behind the mirror surface.

The Periscope

A simple periscope uses two plane mirrors (or two right-angle prisms) set at 45° to the horizontal, one above the other. Light enters the top mirror, reflects downward, then reflects again at the bottom mirror to the observer's eye. Periscopes are used in submarines and trenches to see above obstacles.

FLASHCARD SET • 6 CARDSLight and Reflection: Quick Recall