Beginner · 10 min
Light rays and shadows
Use straight-line ray paths to predict shadow size in a simple model.
A shadow marks blocked light
In a uniform transparent medium, ray diagrams represent light travelling in straight lines. An opaque object blocks rays and creates a shadow on a screen behind it. An ideal point source produces a sharp shadow. A source with noticeable size can also produce a partly shaded region because some parts of the source remain visible. Simple ray models work best when object sizes are much larger than the wavelength.
Worked example
Draw two straight rays from a point source past the top and bottom of an object. Their intersections with the screen mark the shadow edges.
Compare distances from the source
For a point source, a flat object, and a parallel screen with the object between them, similar triangles give shadow height divided by object height equal to source-to-screen distance divided by source-to-object distance. Measure both distances from the source. Moving the screen farther away enlarges the shadow in this model. Keep all lengths in compatible units.
Worked example
An object is 3 cm high and 20 cm from a point source. A parallel screen is 60 cm from the source. Shadow height = 3 × 60 ÷ 20 = 9 cm
Build a ray diagram on paper
- Draw a point source, a 2 cm object 15 cm from it, and a parallel screen 45 cm from the source. Use rays to predict the shadow height.
- Move only the screen to 60 cm from the source. Calculate the new shadow height and compare it with your drawing.