Light and Shadows

What you'll learn

• How light travels: rectilinear propagation.
• How shadows form; umbra and penumbra.
• Properties of plane mirrors and image formation.
• How a pinhole camera works.
• Transparent, translucent and opaque materials.

Key concepts

Properties of light

• Light is a form of energy that enables us to see.
• Source of light: objects that emit light — the Sun, stars, flames, electric bulbs, LEDs.
• Luminous objects: emit their own light (Sun, bulb, candle).
• Non-luminous objects: do not emit light; seen by reflected light (Moon, books, people).

Materials and light:

Rectilinear propagation of light

Light travels in straight lines (rectilinear = straight-line).

Evidence:

1. When three cardboard pieces with holes are aligned, you can see a candle flame through all holes; if any card is displaced, you cannot — proving light travels straight.
2. Laser beams are visible as straight rays.
3. Shadows form because light cannot bend around opaque objects.

Shadows

• A shadow forms when an opaque object blocks light from a source.
• Shadow is always on the opposite side of the object from the light source.
• Shadow has the same shape as the cross-section of the object facing the light.

Parts of a shadow (with a large/extended source):

• Umbra: completely dark region — no light reaches here.
• Penumbra: partially lit region around the umbra — some light reaches here.

Factors affecting shadow size:

Eclipses (extension):

• Solar eclipse: Moon comes between Earth and Sun; Moon's shadow falls on Earth.
  • Umbra region → total solar eclipse; penumbra region → partial solar eclipse.
• Lunar eclipse: Earth comes between Sun and Moon; Earth's shadow falls on Moon.

Reflection of light — Plane mirrors

• Reflection: bouncing back of light when it hits a surface.
• Laws of reflection:
  1. Angle of incidence = angle of reflection (∠i = ∠r).
  2. Incident ray, reflected ray, and normal are all in the same plane.

Image in a plane mirror:

Lateral inversion: your right hand appears as left hand in a mirror; letters appear reversed (AMBULANCE is written reversed on emergency vehicles so drivers see it correctly in their rear-view mirrors).

Multiple mirror reflections: Two mirrors at 90° → 3 images; at 60° → 5 images. Formula: number of images = (360°/angle) − 1.

Uses of mirrors:

• Periscope (two mirrors at 45°; used in submarines to see above water level).
• Kaleidoscope (three mirrors at 60° to create beautiful symmetric patterns).
• Rear-view mirrors in vehicles.
• Dentist's mirror (concave).
• Solar cooker (concave mirror concentrates sunlight).

Pinhole camera

• A pinhole camera is a simple device that forms an image using a tiny hole.

Construction: A box with a tiny hole (pinhole) on one side and a translucent screen on the opposite side.

How it works:

1. Light from the top of the object passes through the hole downward → hits bottom of screen.
2. Light from the bottom of the object passes through the hole upward → hits top of screen.
3. Result: image formed is inverted (upside down) and real.

Properties of pinhole camera image:

• Inverted (upside down and left-right swapped).
• Real (can be caught on screen).
• Size depends on distance: object farther from pinhole → smaller image; screen farther from pinhole → larger image.

Basis of photography and human eye: same principle — convex lens replaces pinhole; image forms on retina/film.

Quick check

• Define rectilinear propagation of light. Give two pieces of evidence.
• What is the difference between transparent, translucent and opaque materials? Give one example each.
• State the laws of reflection.
• What is lateral inversion? Give a practical application.
• Describe the image formed by a pinhole camera. Why is it inverted?

Open the Practice tab for graded questions on Light and Shadows.