Light travels in straight lines

Uniform air means air that is the same all the way through. In our classroom models, draw light travelling through it as straight lines called rays. Add arrows to show direction. This model works well for ordinary cards, walls and screens. A mirror or a material boundary can change the direction, so these straight segments do not describe every light situation.

An opaque wall blocking every direct path prevents a direct view of an object. A mirror may redirect its light to your eyes. In this ray model, show that route as straight segments joined at the mirror, not as a curve around the wall.

For a simple outline prediction, model the source as one small point. Hold a flat opaque card parallel to a flat screen behind it. The blocked region projects the card's outline onto the screen. Its size may change with distances.

To locate a shadow, mark the small source, object and screen. Draw straight boundary rays from the source past the object's edges to the screen. Mark the blocked region between them. Rays hitting the object's interior stop; boundary lines help locate the shadow.

In an adult-controlled laser demonstration, the beam points at a fixed dull screen, away from people's eyes. Dust scatters some beam light toward viewers at the side. Unscattered light continues along the straight beam. Do not handle the laser or look into its beam.

Check a shadow diagram by following its arrows from the source. Rays that hit an opaque object stop there. Rays that miss it continue to the screen. The blocked region lies beyond the object, not beside the source.

When checking a classroom ray-model drawing, distinguish rays that hit the object from rays that pass it. Stop the first group at the surface. Keep the second group straight through uniform air; do not draw them curving around the object.

Keep a small point source and a flat circular card fixed. On a flat screen parallel to the card, the shadow is circular. Tilting only the screen slightly can stretch the projection into an oval. The screen must still receive the whole shadow. A shadow's outline depends on the receiving surface as well as the blocker.

An adult can use a fixed torch and cards with openings to check an unobstructed path to a screen. When openings lie along the beam, light passes. If a card's solid part moves into that path, it blocks the beam. Observe the screen, not the source.

A shadow is a projection, not a copy in every arrangement. With a small source and screen fixed, a rectangular card presents a broad outline when face-on. Turn it nearly edge-on to the light and, ignoring its thickness, the projected width becomes narrower.

Keep a small source and screen fixed. Moving an opaque card toward the source can make its shadow larger. The straight boundary rays spread apart more before reaching the screen. Moving the card toward the screen makes the shadow smaller.

To receive a shadow from an object, a screen must lie beyond that object along the source's light path. A screen placed before the blocker receives the light first; that blocker cannot remove light which has already reached it.