Drawing Ray Diagrams
Draw ray diagrams to show reflection at a plane mirror (angle of incidence = angle of reflection) and refraction at a boundary between media; use ray diagrams to locate images and explain how lenses and mirrors work
What a learner can do afterwards
- Draw a ray diagram for a plane mirror showing the incident ray, normal, reflected ray, and virtual image
- Draw a ray diagram showing a ray bending towards the normal when passing from air into glass
- Use a ray diagram to locate the image formed by a convex lens and describe whether it is real or virtual
The lesson
A ray diagram uses straight lines with arrows to show the path light travels. At the point where light hits a surface, draw a dashed line at right angles to that surface. This dashed line is called the normal, and every angle in your diagram is measured from it, not from the surface itself.
When light bounces off a flat mirror, the angle it arrives at equals the angle it leaves at. These are called the angle of incidence and the angle of reflection. Because the mirror does not let light through, the image you see appears to sit behind the glass. That is a virtual image, and you can never catch it on a screen.
A ray hits a mirror and makes an angle of incidence of 35 degrees with the normal. Since angle of incidence equals angle of reflection, the reflected ray also makes 35 degrees with the normal, but on the other side. Draw both angles the same size to check your work.
Light also bends when it crosses into a different material, such as passing from air into glass. Going into a denser material, the ray bends towards the normal. Coming back out into air, it bends away from the normal instead.
For lenses and curved mirrors, draw a few rays from the same point on an object. Where the rays actually cross after passing through the lens is where a real image forms, and a real image can be projected on a screen. If only their extensions seem to meet, the image is virtual instead.
Draw straight rays with arrows, measure every angle from the normal, and remember that angle of incidence always equals angle of reflection at a mirror.
Watch it
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.