Reflection on a Plane Mirror, Virtual Image, Umbra and Penumbra

What it shows

Light travels in straight lines, which explains both mirror images and shadows. At a plane mirror the law of reflection holds: the incident ray, the normal and the reflected ray lie in one plane, and the angle of reflection equals the angle of incidence. Reflected rays spread apart, so the eye traces them back to a virtual image S′ behind the mirror, as far behind it as the source is in front. With an obstacle, a point source casts a sharp umbra, while a wide source adds a penumbra, the same geometry that produces total and partial eclipses.

How to use

Start in Reflection on a plane mirror mode with Show normal and angle measurements switched on. Set an Initial angle of incidence, then drag S to different positions and compare i and i′ each time. Turn on Show virtual image S′ and extended rays and move S closer to the mirror to check that d′ always equals d. Then switch Mode to Umbra – penumbra, drag the obstacle between the source and the screen, and compare a point source with a wide source.

Parameters you can change

• Mode Reflection on a plane mirror, Umbra – penumbra
• Initial angle of incidence 0–85 °
• Show normal and angle measurements
• Show virtual image S′ and extended rays
• Light source (shadow mode) Point light source (small), Wide light source

Questions to explore

1. When the angle of incidence is 0°, in which direction does the reflected ray travel?
2. If you move S 5 cm closer to the mirror, how far does the image S′ move, and in which direction?
3. Why does a wide light source give a shadow with a blurred edge while a point source gives a sharp one?