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E2.1 · Use optics terms including incidence, focus, and virtual image

Learn to use optics terms including incidence, focus, and virtual image through clear examples and targeted practice.

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Using incidence, focus, and virtual image accurately

A spoon can show a small reflection of your face. A magnifying glass can make nearby print look larger. These observations involve light and images, but the words used to describe them have specific meanings. In this lesson, you will learn three useful optics terms: incidence, focus, and virtual image. Optics is the study of light and how it behaves. You only need a Grade 9 idea to begin: light travels in straight lines through a uniform material, and we can use lines called rays to show its path.

What you will learn

Describe what happens when light reaches a surface using the term incidence.
Identify the focus in a simple ray model.
Explain what a virtual image is and how it differs from a real object.

1. Start with light reaching a surface

Imagine shining a flashlight at a smooth mirror. The light travels toward the mirror. The place where it reaches the mirror is the point of incidence. Incidence describes the arrival of light at a surface. The incoming light ray is called the incident ray.

To describe the direction of an incident ray, we compare it with an imaginary line drawn straight out from the surface. This line is called the normal. It is at a right angle to the surface at the point of incidence. The angle between the incident ray and the normal is the angle of incidence. It is not measured from the surface itself.

A ray diagram is a simple model. A ray is drawn as a straight line with an arrow to show the direction light travels. The diagram does not show every detail of light. It helps us keep track of where light comes from and where it goes.

The point of incidence is where a ray reaches a surface.
The incident ray travels toward the surface.
The angle of incidence is measured from the normal, not from the surface.

2. Finding a focus in a ray model

A focus is a point where light rays meet, or where they seem to come from in a simple ray model. For example, a converging lens can bring rays that were travelling in different directions toward one point. That point is called the focus. The word focus can also refer to making an image clear, but in this lesson it means the point used to describe rays.

A ray diagram lets you look for this point. Follow the arrows on the rays. If the rays come together, the place where they meet is the focus. Do not call every bright spot a focus just because it looks bright. The focus is identified by the paths of the rays in the model.

The diagram is a way to represent what light does; it is not a photograph of the light itself. Careful labels help prevent confusion between the ray, the surface, and the point where the rays meet.

A focus is where rays meet or seem to come from.
Use the ray directions to identify a focus.
The normal is used to describe incidence; the focus is about where rays meet or seem to come from.

3. Understanding a virtual image

When you look into a flat mirror, you see an image that seems to be behind the mirror. Light from you reaches the mirror and reflects toward your eyes. Your brain traces the incoming light back in a straight line, so the image appears to be behind the mirror. In the ray model, the reflected rays do not actually meet behind the mirror.

An image in this situation is called a virtual image. A virtual image is an image that appears to come from a place where the light rays do not actually meet. It can be seen by looking into the mirror, but it cannot be formed on a screen placed where it appears to be. The image is not a second object behind the mirror.

This description focuses on the meaning of the term, not on calculating image positions. When using the words virtual image, explain both parts: where the image seems to be and whether the light rays actually meet there.

A virtual image appears to be located where rays seem to come from.
The rays do not actually meet at the apparent location of a virtual image.
A virtual image is not a physical object behind a mirror.

4. Use the terms as evidence-based descriptions

In an observation, separate what you see from the model you use to explain it. For example, you may observe your reflection in a mirror. The ray model helps explain why the image appears behind the mirror. The observation is what you see; the model is a way to describe the light paths that could produce it.

A simple investigation can use a mirror and a flashlight under teacher supervision. Keep the beam directed at the mirror and away from people's eyes. Observe where the beam reaches the surface, and identify that spot as the point of incidence. Do not look directly into a bright beam or aim it at another person. A normal and an incident ray can be added to a sketch after observing the setup.

For a focus, use a classroom ray diagram or a teacher-approved demonstration with a suitable lens. Do not use a lens to concentrate sunlight. If a diagram shows rays coming together, use their paths to identify the focus. If the image seems to come from a place where rays do not meet, describe it as virtual. These terms make a description clearer because each names a different feature of the light model.

Base your description on what is observed and what the ray model represents.
Identify incidence at a surface, and identify focus by tracing rays.
Follow classroom safety directions; never direct bright light into anyone's eyes or focus sunlight.

Three terms for describing light

Term	Meaning	Question to ask
Point of incidence	Where a ray reaches a surface	Where does the incoming ray meet the surface?
Focus	A point where rays meet or seem to come from	Where do the ray paths come together or appear to start?
Virtual image	An image that appears to come from a place where rays do not meet	Does the image seem to be somewhere the rays do not actually reach and meet?

Worked example

Locate the point of incidence

A ray diagram shows a flashlight ray travelling toward a flat mirror. The ray reaches the mirror at point P. What is point P, and from which line should the angle of incidence be measured?

Name the surface location
The ray reaches the mirror at P. The location where a ray reaches a surface is the point of incidence.
Choose the reference line
The angle of incidence is measured between the incoming ray and the normal. The normal is perpendicular to the mirror at P, so measuring from the mirror surface would use the wrong reference.

Answer: P is the point of incidence. Measure the angle of incidence from the normal at P.

Check: The incident ray travels toward the mirror, and the normal is drawn at a right angle to its surface.

Worked example

Identify a focus

A classroom ray diagram shows three light rays passing through a lens and then meeting at point Q. What is Q called in this model?

Trace the ray paths
Follow each ray after it passes through the lens. The diagram states that all three paths meet at Q.
Apply the definition
A point where rays meet in a ray model is a focus. Therefore, Q is the focus in this diagram.

Answer: Q is the focus.

Check: The answer comes from the rays meeting at Q, not just from the presence of a lens.

Worked example

Describe a mirror image

A student sees their face in a flat mirror. In a ray model, the reflected rays reaching the student's eyes do not meet behind the mirror, but they seem to come from there. What term describes the image, and why?

Compare the apparent and actual paths
The image seems to be behind the mirror, but the rays do not actually meet there.
Choose the image term
An image that appears to come from a location where the rays do not meet is a virtual image.

Answer: It is a virtual image because it appears to come from behind the mirror, where the rays do not actually meet.

Check: The image is seen by the student, but the apparent location is not a place where the reflected rays converge.

Common mistakes and how to avoid them

Measuring the angle of incidence from the surface.

Correction: Measure it from the normal, the line at a right angle to the surface at the point of incidence.

Calling the point where a ray first touches a mirror the focus.

Correction: That location is the point of incidence. A focus is where rays meet or seem to come from.

Saying a virtual image is a real object behind a mirror.

Correction: A virtual image only appears to be at that location. The rays do not actually meet there.

Lesson summary

Incidence describes light reaching a surface; the point where it reaches is the point of incidence.
The angle of incidence is measured from the normal, not from the surface.
A focus is a point where rays meet or seem to come from.
A virtual image appears to come from a place where the light rays do not actually meet.

Check your understanding

Question 1

A ray reaches a mirror at point R. What is R?

The point of incidence
The focus
A virtual image
The normal

Show answer and explanation

The point of incidence

The point of incidence is where a ray reaches a surface.

Question 2

In a diagram, rays meet at point S. Which term best describes S?

The point of incidence
The focus
The incident ray
A virtual image

Show answer and explanation

The focus

A focus is a point where rays meet in the ray model.

Question 3

Why is an image in a flat mirror called virtual in the ray model?

The light rays actually meet behind the mirror.
The image is a physical object hidden behind the mirror.
The image appears to come from behind the mirror, but the rays do not meet there.
The image is the point where the incoming ray first reaches the mirror.

Show answer and explanation

The image appears to come from behind the mirror, but the rays do not meet there.

A virtual image appears to come from a location where the rays do not actually meet.

Key terms

Optics: The study of light and how it behaves.
Ray: A line in a diagram that represents the path and direction of light.
Incident ray: A ray travelling toward a surface.
Point of incidence: The point where a ray reaches a surface.
Normal: An imaginary line at a right angle to a surface at the point where a ray reaches it.
Focus: A point where light rays meet or seem to come from in a ray model.
Virtual image: An image that appears to come from a place where light rays do not actually meet.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E2.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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