Lenses and curved mirrors – ray diagrams

What it shows

Lenses and curved mirrors are two key lessons in the topic of light. The simulation combines two screens: the Lens screen forms images through a converging or diverging lens whose focal length is calculated directly from the radius of curvature and the refractive index using the lensmaker's equation; the Mirror screen forms images in a concave, convex or plane mirror.

How to use

Students drag the object to change its distance d from the lens or mirror, switch on the three principal rays (parallel to the principal axis, through the optical centre or centre of curvature, through the focal point) to construct the image themselves, then compare with the values shown (position, real or virtual, magnification). Switch on marginal rays to see that every ray from one point meets at the same image point; switch on a second point to see that the whole object forms an image, not just one point; or change to point-source mode and drag the screen along the axis to see the patch of light shrink to a single point only when the screen is exactly at the real image plane — a virtual image can never be caught on a screen.

Parameters you can change

• Screen Lens, Curved mirror
• Lens type Converging, Diverging
• Lens radius of curvature R 20–100 cm
• Refractive index n 1.3–1.9
• Object–lens distance d 5–150 cm
• Screen–lens distance 5–300 cm
• Mirror type Concave mirror, Convex mirror, Plane mirror
• Mirror radius of curvature R 20–200 cm
• Object–mirror distance d 5–150 cm
• 3 principal rays
• Marginal rays (lens)
• Second point on the object
• Ruler
• Point source + screen (lens)

Questions to explore

1. When the object is inside the focal length of a converging lens, how does the image compare with placing the object at 2f?
2. Why do diverging lenses and convex mirrors always form virtual images?
3. As you move the screen further away from the real image position, how does the patch of light on the screen change, and why?