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E3.6 · Connect mirror and lens properties to optical instruments

Learn to connect mirror and lens properties to optical instruments through clear examples and targeted practice.

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Ontario Grade 10 Science • E3.6: Connect mirror and lens properties to optical instruments

A rear-view mirror lets a driver see behind the car. A magnifying glass can make a nearby printed detail look larger. Both work because an optical part changes the path of light in a useful way. This lesson connects the properties of mirrors and lenses to the purposes of optical instruments. It uses simple descriptions rather than image calculations.

What you will learn

Explain how mirrors and lenses change the paths of light.
Connect the shape of an optical part to what an instrument helps a person see.
Use observations and a simple light-path model to explain an instrument’s purpose.

1. Grade 9 bridge: light, mirrors, and lenses

Light travels from objects to your eyes. When light meets a surface or passes into a different material, its path can change. That change can make an object visible from a different direction or make a detail easier to inspect.

Reflection happens when light bounces off a surface. A mirror uses reflection. A plane mirror has a flat surface. A curved mirror has a surface that bends inward or outward. The curve affects the directions in which reflected light travels.

Refraction is a change in the direction of light as it passes from one material into another, such as from air into glass. A lens is a shaped piece of transparent material that refracts light. A convex lens is thicker in the middle than at the edges. It can bring light rays closer together. A concave lens is thinner in the middle than at the edges. It spreads light rays apart.

A ray is a simple line used to represent the path of light. A ray model does not show every light ray. It helps us trace how light travels from an object, to an optical part, and toward an eye. For an instrument, ask what the part does to the light and how that helps its user.

Mirrors change light paths mainly by reflection; lenses change them mainly by refraction.
The shape of a mirror or lens affects the direction of light.
A ray model helps connect an optical part to what a person can see.

2. Observable effects and a simple model

Look at a nearby printed letter through a magnifying glass, or imagine doing so. At a suitable distance, the letter can appear larger. The lens refracts light from the letter. For this close-viewing arrangement, the light leaving the lens spreads out as it travels toward the eye. The eye interprets that light as coming from a larger-looking image behind the lens. This is called a virtual image: it appears to be in a place from which the light seems to come, even though the light does not actually meet there.

This description avoids a common mix-up. A magnifying glass does not need to bring the rays from the nearby letter together after they leave the lens for the letter to appear larger. The lens and viewing arrangement change the light reaching the eye, and the result is an enlarged apparent view.

Now consider a convex mirror used near a corner. Its outward curve reflects light from a wider range of directions toward the viewer. The viewer can see more of the surrounding area. Objects may look smaller, but more of the area is visible.

A simple model traces light from an object to the mirror or lens, and then toward the eye. It asks three questions: What kind of optical part is used? How does its shape change the light path? What useful view does that change provide? An optical instrument may use one part or combine mirrors and lenses. Its design and the position of its parts affect the view.

A magnifying glass can show a nearby detail as a larger virtual image; the light leaving the lens diverges in this use.
A convex mirror can provide a wider view, although objects may look smaller.
Explain the part, its effect on light, and the user benefit together.

3. Compare optical parts by their possible roles

The table gives qualitative descriptions. “Bring rays together” and “spread rays apart” describe the overall pattern of light paths. They do not give an exact image location or size. The same optical part can have different roles depending on the instrument’s arrangement.

A concave mirror curves inward. It can reflect rays toward one another. A convex mirror curves outward and spreads reflected rays, which can help show a wider area. A convex lens can bring refracted rays closer together. A concave lens spreads refracted rays apart. In an instrument, lenses and mirrors can be used alone or together to guide light toward the viewer.

These properties help explain why an optical instrument uses a particular part. They are not a rule that the part always creates the same view in every arrangement. State what is known about the design before claiming an exact result.

Curved mirrors reflect light; lens shapes refract light.
Convex mirrors can provide a wider view; convex lenses can bring rays closer together.
An instrument’s arrangement affects what the user sees.

4. Build an explanation from observation

An observation is something noticed using the senses or a measuring tool. To investigate an optical instrument safely in class, note the part being used, what is viewed, and what changes. For example, record whether a mirror shows more of a nearby area or whether a lens makes a printed detail appear larger. These observations support an explanation for that setup; they do not prove that every instrument behaves exactly the same way.

A strong explanation follows a clear chain: identify the part, name its property, describe the light-path change, and connect that change to the instrument’s purpose. For a convex mirror at a corner, the outward curve spreads reflected light from a broad range of directions; the user sees a wider area. For a magnifying glass, a convex lens refracts light so that a nearby detail can appear larger when viewed at a suitable distance.

Use only teacher-approved optical equipment. Never look at the Sun or another very bright source through a lens, mirror, or optical instrument. Focused light can injure eyes. Handle glass lenses and mirrors carefully, and follow classroom directions.

Use observations as evidence for the specific setup being examined.
Explain the link between optical property, light path, and user benefit.
Never view the Sun through optical equipment.

Optical parts and possible instrument roles

Part	Shape or property	Effect on light	Possible benefit
Plane mirror	Flat reflecting surface	Reflects light	Shows a view from another direction
Concave mirror	Curves inward	Can reflect rays toward one another	Can help guide light in an instrument
Convex mirror	Curves outward	Spreads reflected rays	Can show a wider area
Convex lens	Thicker in the middle	Can bring refracted rays closer together	Can help shape light in an instrument; can make a nearby detail appear larger when viewed appropriately
Concave lens	Thinner in the middle	Spreads refracted rays apart	Can help shape light in an instrument design

Worked example

A wider view around a corner

A shop needs a mirror that helps staff see a wider area around a corner. Which mirror property is useful, and why?

Identify the need
The goal is to see a larger area, not to make one object look larger.
Choose a useful shape
A convex mirror curves outward. It spreads reflected light from a wider range of directions toward the viewer.
Connect to the benefit
Because light from more directions reaches the viewer, the mirror can show more of the area around the corner. Objects may look smaller, but the wider view meets the need.

Answer: A convex mirror is useful because its outward curve spreads reflected light from a wider range of directions, helping staff see a larger area.

Check: The explanation links the mirror’s shape to its reflection pattern and then to the wider view.

Worked example

Inspecting a small printed symbol

A student holds a magnifying glass at a suitable distance from a small printed symbol. Explain which lens property helps and what the student sees.

Identify the lens
A magnifying glass uses a convex lens, which is thicker in the middle than at the edges.
Describe the light reaching the eye
The lens refracts light from the nearby symbol. In this viewing arrangement, the light leaving the lens diverges as it travels toward the eye. The eye sees an enlarged virtual image: the symbol appears larger, even though the rays do not meet at that apparent image.
State the benefit
The enlarged appearance makes small parts of the symbol easier to inspect. The viewing distance matters, so the result should not be claimed for every position of the lens.

Answer: A convex lens refracts light from the nearby symbol. At a suitable viewing distance, the diverging light reaching the eye makes the symbol appear as a larger virtual image, so its details are easier to inspect.

Check: The explanation does not claim that rays from the nearby symbol converge after leaving the lens.

Worked example

Explaining a distant view

A learner uses an optical instrument to help view a distant landmark. The instrument contains a mirror, a lens, or both. Explain what the parts do without assuming a specific design.

Start with the purpose
The instrument should guide light from the landmark toward the viewer’s eye in a useful way.
Connect part to property
A mirror reflects light. A lens refracts light. Either type of part can help guide light in an instrument, and some designs use both.
Respect the information given
The exact view depends on the shapes and positions of the parts. Since those details are not provided, do not claim an exact image size or appearance.

Answer: The instrument uses its mirror, lens, or both to guide light from the landmark toward the eye. Mirrors do this by reflection, and lenses do this by refraction. The exact view depends on the design.

Check: The answer connects each part to its main optical action without inventing details about the instrument.

Common mistakes and how to avoid them

Saying that a magnifying glass makes a nearby object look larger because its outgoing rays converge.

Correction: For the usual close-viewing setup, the light leaving the convex lens diverges toward the eye. The object appears as a larger virtual image.

Saying mirrors mainly refract light or lenses mainly reflect light.

Correction: Connect mirrors with reflection and lenses with refraction.

Assuming a convex mirror makes objects look larger because it shows more area.

Correction: A convex mirror can show a wider area, while objects may appear smaller. A wider view and a larger-looking object are different effects.

Naming an optical part without explaining its purpose.

Correction: State the part’s property, describe its effect on light, and connect that effect to what the instrument helps the user see.

Lesson summary

Mirrors use reflection, and lenses use refraction, to change light paths.
The shape of an optical part affects how light is directed or spread.
A convex mirror can show a wider area. A magnifying glass can make a nearby detail appear larger when viewed appropriately.
A complete explanation connects the part, its property, its effect on light, and the instrument’s purpose.

Check your understanding

Question 1

Why can a convex mirror help someone see around a corner?

It refracts light through the mirror and makes every object larger.
It reflects light from a wider range of directions, helping show a wider area.
It absorbs light from behind the viewer.
It always brings reflected rays together at one point.

Show answer and explanation

It reflects light from a wider range of directions, helping show a wider area.

A convex mirror spreads reflected light and can show a wider area. It does not refract light through the mirror.

Question 2

A nearby printed detail looks larger through a magnifying glass held at a suitable distance. Which explanation is best?

A convex lens refracts light so the nearby detail can appear as a larger virtual image.
A plane mirror absorbs light and enlarges the detail.
A concave mirror spreads light through transparent glass.
A concave lens reflects light to make the detail larger.

Show answer and explanation

A convex lens refracts light so the nearby detail can appear as a larger virtual image.

A magnifying glass uses a convex lens. In suitable close viewing, the light leaving the lens diverges, while the detail appears as a larger virtual image.

Question 3

Complete the statement: A mirror changes light mainly by _; a lens changes light mainly by _.

absorption; reflection
refraction; absorption
reflection; refraction
spreading; shining

Show answer and explanation

reflection; refraction

Mirrors mainly change light paths by reflection. Lenses mainly change them by refraction.

Key terms

Reflection: A change in light’s direction when it bounces off a surface.
Refraction: A change in light’s direction as it passes from one material into another.
Lens: A shaped piece of transparent material that refracts light.
Convex: For a lens, thicker in the middle than at the edges; for a mirror, curved outward.
Concave: For a lens, thinner in the middle than at the edges; for a mirror, curved inward.
Ray: A line in a model that represents the path of light.
Virtual image: An image that appears to be where light seems to come from, although the light does not meet there.
Optical instrument: A tool that uses mirrors, lenses, or both to help a person see.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E3.6. 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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