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E3.5 · Describe converging-lens image size, orientation, and location

Learn to describe converging-lens image size, orientation, and location through clear examples and targeted practice.

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Use object position to describe image size, orientation, and location

A magnifying glass can make nearby print look larger. Change the distance between the print and the lens, and the image may change size or direction. It may also appear in a different place relative to the lens. These patterns can be described by comparing the object’s position with the lens’s focal points. This lesson focuses on three descriptions: size, orientation, and location.

What you will learn

Describe an image as larger, smaller, or the same size as its object.
Describe an image as upright or inverted.
Locate an image by comparing its position with the lens and focal points.
Use an object-position guide to predict image size, orientation, and location.

1. Bridge: light, lenses, and image descriptions

Light lets us see objects. A lens is a clear piece of material that changes the direction of light passing through it. A converging lens is thicker in the middle than at the edges. It bends parallel light rays toward one another.

The focal point, marked F, is a reference point on either side of a converging lens. The principal axis is an imaginary straight line through the lens’s centre and both focal points. The distance from the centre of the lens to either focal point is the focal length. For this lesson, use these points as position markers; no calculation is needed.

The object is what is being viewed, such as an arrow on a page. An image is the picture associated with that object as viewed through the lens. Image size compares the image’s height with the object’s height. Orientation describes whether the image points the same way as the object or the opposite way. Location tells where the image is in relation to the lens and focal points.

An upright image points in the same direction as the object. An inverted image points in the opposite direction, like an arrow turned upside down. These words describe direction, not whether the image is clear or bright.

Use F as a reference point on either side of the lens.
Size means larger, smaller, or the same size as the object.
Orientation means upright or inverted; location is described relative to the lens.

2. The object-position guide

For a simple position guide, mark the lens, the focal points F, and the positions twice as far from the lens as F. These latter positions are labelled 2F. The labels describe distances along the principal axis. In every case below, the object is on one side of the lens.

Start by locating the object: beyond 2F, at 2F, between F and 2F, or inside F. Then use the matching row to describe the image. “Far side” means the side of the lens opposite the object. “Object’s side” means the same side as the object.

The familiar patterns are consistent. With an object beyond 2F, the image is smaller and inverted, between F and 2F on the far side. At 2F, the image is the same size and inverted, at 2F on the far side. Between F and 2F, the image is larger and inverted, beyond 2F on the far side. Inside F, the image is larger and upright, on the object’s side.

At exactly F, the usual position guide does not assign a finite image location. For this lesson, treat that position as a boundary between the listed cases rather than describing it with one of the four rows.

Beyond 2F: smaller, inverted, between F and 2F on the far side.
At 2F: same size, inverted, at 2F on the far side.
Between F and 2F: larger, inverted, beyond 2F on the far side.
Inside F: larger, upright, on the object’s side.

3. Read and use the model

A position guide is a model: it organizes the patterns so they can be compared. It is not a scale drawing. The phrase “between F and 2F,” for example, tells the image’s location range, not an exact measured distance.

A useful way to read the model is to separate the three descriptions. First compare image size with object size. Next state upright or inverted. Finally state the location, including the side of the lens and the relevant F or 2F position.

When describing an image from a lens, give all three parts rather than only saying “magnified.” Magnified means larger, but it does not tell the orientation or location. Likewise, “inverted” does not tell whether the image is larger or smaller.

A classroom observation can be recorded by noting the object’s position compared with F and 2F, then describing image size, orientation, and location. Use only teacher-approved lens equipment and follow classroom directions. Never look at the Sun through a lens or use a lens to focus sunlight; concentrated sunlight can injure eyes or start a fire.

Report size, orientation, and location as separate features.
A position range is not an exact measurement.
Use teacher-approved equipment and never focus sunlight with a lens.

Image descriptions for a converging lens

Object position	Image size	Orientation	Image location
Beyond 2F	Smaller	Inverted	Far side, between F and 2F
At 2F	Same size	Inverted	Far side, at 2F
Between F and 2F	Larger	Inverted	Far side, beyond 2F
Inside F	Larger	Upright	Object’s side

Worked example

Object beyond 2F

An upright arrow is placed beyond 2F from a converging lens. Describe the image’s size, orientation, and location.

Find the matching position
The object is beyond 2F, so use the beyond-2F case in the position guide.
State all three features
That case has an image smaller than the object and inverted. Its location is between F and 2F on the far side of the lens.

Answer: The image is smaller and inverted. It is between F and 2F on the far side of the lens.

Check: The object position matches the first case in the guide.

Worked example

Object between F and 2F

An object is between F and 2F. A class position guide is available. Describe the image without giving a numerical distance.

Identify the case
The object lies in the range between F and 2F. The model gives a location range for the image, not an exact distance.
Describe size and direction
For this object position, the image is larger than the object and inverted. It is beyond 2F on the far side of the lens.

Answer: The image is larger and inverted, and it is beyond 2F on the far side of the lens.

Check: All three descriptions come from the between-F-and-2F case.

Worked example

Object inside F

An upright object is closer to a converging lens than F is. Describe its image.

Name the object position
The object is between the lens and F, so it is inside F.
Use the inside-F case
The guide shows that an object inside F has an image that is larger and upright. Its location is on the same side of the lens as the object.

Answer: The image is larger and upright, and it is on the object’s side of the lens.

Check: Inside F is the listed case with an upright image.

Common mistakes and how to avoid them

Saying only that an image is magnified.

Correction: Magnified means larger. Also state whether the image is upright or inverted and where it is located.

Mixing up the object’s side and the far side of the lens.

Correction: The far side is opposite the object. The object’s side is the side where the object is placed.

Treating between F and 2F as an exact distance.

Correction: It is a range. Describe the image with the corresponding range in the guide, without claiming an exact measurement.

Lesson summary

Compare the object’s position with F and 2F.
Describe size as larger, smaller, or the same size.
Describe orientation as upright or inverted.
Locate the image by naming its side of the lens and its position relative to F or 2F.

Check your understanding

Question 1

An object is at 2F. Which description matches its image?

Smaller, upright, between F and 2F on the far side
Same size, inverted, at 2F on the far side
Larger, inverted, on the object’s side
Larger, upright, beyond 2F on the far side

Show answer and explanation

Same size, inverted, at 2F on the far side

At 2F, the image is the same size and inverted, and its location is at 2F on the far side.

Question 2

An object is inside F. Which description is correct?

Smaller, inverted, between F and 2F on the far side
Same size, inverted, at 2F on the far side
Larger, upright, on the object’s side
Larger, inverted, beyond 2F on the far side

Show answer and explanation

Larger, upright, on the object’s side

The inside-F case has a larger, upright image on the same side as the object.

Question 3

An object is between F and 2F. Where is its image, and what is its orientation?

Beyond 2F on the far side, inverted
Between F and 2F on the far side, upright
At 2F on the object’s side, inverted
On the object’s side, upright

Show answer and explanation

Beyond 2F on the far side, inverted

For an object between F and 2F, the image is beyond 2F on the far side and is inverted.

Key terms

Converging lens: A lens that bends parallel light rays toward one another.
Focal point (F): A reference point used to describe positions in relation to a converging lens.
Principal axis: An imaginary straight line through the centre of the lens and its focal points.
Upright: Pointing in the same direction as the object.
Inverted: Pointing in the opposite direction from the object.
Far side: The side of the lens opposite the side where the object is placed.

Continue through SNC2D

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E3.5. 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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