Optics
Focal point — parallel rays
Object at F: emerging rays are parallel and the image forms at infinity.
Focal point — parallel rays — interactive Optics simulation. Object at F: emerging rays are parallel and the image forms at infinity. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Focal point
Place the object at the focal point: emerging rays are parallel and the image forms at infinity.
When the object sits exactly at the focal point, refracted rays leave the lens parallel—the image forms at infinity. This boundary case separates real images (object outside F) from virtual ones (object inside F). The preset makes parallel exit rays easy to trace on the ray diagram.
- d_o = f → d_i → ∞
- Rays emerge parallel
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
As an object approaches a converging lens's focal length from beyond it, what happens to the image distance?
Predictions to weigh
- The image distance shrinks toward zero.
- The image distance stays roughly constant.
- The image distance grows without bound, diverging to infinity right as the object reaches the focal point.
Variable roles
What you set:
- Object distance d_o (cm)
What you measure:
- Image distance d_i (cm)
How the investigation runs
- Open the focal-point preset and press Reset. The lens's focal length is fixed at 10 cm.
- Enable the image-distance readout.
- Set the object distance for each trial (approaching the focal length from beyond it) and record the image distance.
Governing equation
Thin-Lens Equation Near the Focal Point — dᵢ = dₒ·f/(dₒ − f)
As object distance d_o approaches focal length f, the denominator (d_o − f) in d_i = d_o·f/(d_o − f) approaches zero, sending the image distance to infinity — the image 'escapes' to a point at infinity.
What the printable worksheet asks students to work out
- For one trial, compute d_i = d_o·f/(d_o − f) using f = 10 cm. Compare to the table.
- Explain, using the thin-lens equation, why d_o − f approaching zero (from the positive side) sends d_i toward positive infinity.
Where this shows up beyond the lab
- When the object sits exactly at the focal point, rays leaving the lens emerge perfectly parallel — an image 'at infinity.' Explain why a lighthouse or searchlight places its bulb exactly at the lens's (or mirror's) focal point.
- This exact setup — a source at the focal point producing parallel output rays — is called a collimator, used to create parallel beams for lab instruments. Why might scientists need a beam of perfectly parallel light for certain experiments?
- AP Physics 2 — Unit 13: Geometric Optics
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Focal Point
- Select the object
- Press Play
- Rays emerge parallel
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.