E3.3 · Describe mirror images using diagrams and equations

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Use observations, ray diagrams, and equations to explain where an image forms and what it is like.

What you will learn

• Describe how a plane-mirror image compares with its object.
• Use a ray diagram to locate a mirror image.
• Use the mirror equation to find an image distance for a curved mirror.
• Use magnification to describe an image’s size and orientation.

From reflection to a plane-mirror image

• A plane mirror is a flat mirror.
• A plane-mirror image is virtual, upright, and the same size as its object.
• Object distance and image distance are equal when measured from the mirror.

Representing an image with a ray diagram

• Use arrows to show the direction of light.
• Measure reflection angles from the normal, not from the mirror surface.
• Use dotted backward extensions to show the apparent location of a virtual image.

Equations for curved-mirror images

• Use the mirror equation to relate distances.
• Use the sign of image distance to distinguish a real image from a virtual image.
• Use magnification to describe image size and orientation.

Evidence, careful models, and safety

• Use observations to check a model, but do not treat a sketch as a precise measurement.
• Label distances and state the sign convention before calculating.
• Never look into a bright reflected beam or focus sunlight with a mirror.

Worked example

Locate an image in a plane mirror

1. Apply the plane-mirror rule
   For a plane mirror, image distance equals object distance. The object is 18 cm in front, so the image appears 18 cm behind the mirror.
   $$d_i=d_o=18\text{ cm}$$
2. Describe the image
   A plane-mirror image is virtual, upright, and the same size as the object. It appears laterally reversed.

Worked example

Find an image formed by a concave mirror

1. Substitute known distances
   The object and focal point are in front of this concave mirror, so both distances are positive. Substitute them into the mirror equation.
   $$\frac{1}{12}=\frac{1}{36}+\frac{1}{d_i}$$
2. Solve for image distance
   Subtract one thirty-sixth from one twelfth. This leaves two thirty-sixths, so the reciprocal of the image distance is one eighteen.
   $$\frac{1}{d_i}=\frac{1}{18},\quad d_i=18\text{ cm}$$
3. Find and interpret magnification
   The negative sign means the image is inverted. A magnitude of one half means it is half as tall as the object. The positive image distance indicates a real image in front of the mirror.
   $$m=-\frac{18}{36}=-0.50$$

Worked example

Describe an image from its magnification

1. Use the magnification relationship
   The magnification equation links image distance to object distance. Substitute the given magnification and object distance.
   $$1.5=-\frac{d_i}{20\text{ cm}}$$
2. Solve and interpret
   Multiplying by 20 cm gives a negative image distance. The negative sign means the image is virtual and behind the mirror. Positive magnification means upright, and a magnitude greater than one means larger than the object.
   $$d_i=-30\text{ cm}$$

Common mistakes and how to avoid them

Lesson summary

• A plane-mirror image is virtual, upright, the same size as the object, and as far behind the mirror as the object is in front.
• Ray diagrams show the direction of light and the apparent location of an image.
• The mirror equation relates focal length, object distance, and image distance.
• Magnification gives image size and orientation when interpreted with its sign and magnitude.

Check your understanding

Key terms

Reflection

A change in the direction of light as it travels away from a surface.

Ray diagram

A drawing that uses lines and arrows to model the paths of light.

Normal

An imaginary line perpendicular to a surface at the point where a ray hits.

Virtual image

An image that appears to be at a location where light rays do not actually meet.

Real image

An image formed where reflected light rays actually meet.

Focal length

The distance from a curved mirror to its focal point.

Magnification

A number that compares image height with object height and indicates image orientation.

Inverted

Turned upside down compared with the object.

Continue through SNC2D

View the complete SNC2D Ontario Grade 10 Science curriculum and lessons

• E3.2 · Identify visible and invisible spectrum regions
• E3.4 · Explain partial and total internal reflection with ray diagrams
• E1.1 · Evaluate a technology that alters human perception of light
• E1.2 · Explain societal benefits of an optical device
• E2.1 · Use optics terms including incidence, focus, and virtual image
• E2.2 · Investigate reflection with plane and curved mirrors and ray diagrams

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E3.3. 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.

Official curriculum reference

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