Ray Optics Fundamentals
Plane Mirror Image Formation
Analyze how flat mirrors construct virtual reflections. Move a standing student to prove object distance equals image distance, draw ray lines from a candle flame to reconstruct virtual focal points behind glass, and rotate card text to study lateral inversion.
Plane Mirror Virtual Lab
Interact with standing objects, glowing candles, text cards, and mirror boundaries to investigate image geometry.
Live Telemetry
Magnification (m) = +1.00 (Virtual & Upright)- Object Distance (u)
- 150.0 cm
- Image Distance (v)
- 150.0 cm
- Object Height (ho)
- 80.0 cm
- Image Height (hi)
- 80.0 cm
Properties of Images Formed by a Plane Mirror
A plane mirror is a flat, highly polished reflecting surface. When light rays strike a plane mirror, they undergo specular reflection. The resulting virtual image has five distinct physical properties that define it:
1. Virtual Image
The light rays do not physically converge or focus behind the mirror. Instead, they bounce off the mirror surface and diverge. When traced backward behind the mirror, these diverging rays appear to intersect at a specific point. Because the light rays do not actually pass through this point, the image is virtual and cannot be captured on a screen.
2. Upright (Erect)
The orientation of the image matches the orientation of the object. If you stand upright, your reflection is also upright, with the top of the head at the top of the image.
3. Same Size as the Object
The vertical height of the image is equal to the vertical height of the object. The magnification is exactly positive one:
4. Equal Distance
The image is located at the exact same distance behind the mirror as the object is placed in front of it:
Lateral Inversion (Left-Right Reversal)
The fifth property of plane mirror reflections is lateral inversion. When you stand in front of a mirror and raise your right hand, your virtual reflection appears to raise its left hand.
This occurs because the mirror reverses the front-to-back (depth) axis perpendicular to its surface, while leaving the vertical and horizontal axes unchanged. The reflection of coordinates along the Z-axis flips the handedness (chirality) of the object. We mentally interpret this depth reversal as a left-to-right swap.
Ray Construction Rules
To construct a ray diagram for a plane mirror:
- Draw a ray from a point on the object straight to the mirror surface, perpendicular to the mirror. This ray reflects back along the same path.
- Draw a second ray from the same object point at an angle to the mirror surface. Apply the Law of Reflection (i = r) to draw the reflected ray.
- Extend both reflected rays as dashed lines behind the mirror plane. The point where the dashed virtual rays intersect is the location of the virtual image point.
Solved Examples
Example 1
An object of height 8.0 cm is placed 25.0 cm in front of a flat plane mirror. Find the position of the image, the size of the image, the magnification, and state the properties of the image formed.
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Final Answer:
Example 2
A person of height H stands vertically in front of a vertical plane mirror. Show mathematically that the minimum vertical height of the mirror required for the person to see their entire height (from head to toe) is H / 2, regardless of their distance from the mirror.
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Final Answer:
Example 3
A standard analog clock face is placed in front of a plane mirror. If the clock hands show a time of 3:40, what time will the clock appear to show in its virtual mirror reflection?
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Final Answer:
Self-Check Questions
Question 1
List and explain the five key characteristics of an image formed by a plane mirror.
Show Answer & Explanation
The five key characteristics are: (1) Virtual: The reflected rays diverge and only appear to intersect behind the mirror. The image cannot be caught on a screen. (2) Upright (erect): The top of the object corresponds to the top of the image. (3) Same Size: The height of the image equals the height of the object (magnification is +1). (4) Equal Distance: The image is exactly as far behind the reflecting surface as the object is in front of it (u = v). (5) Laterally Inverted: The left and right sides of the object are reversed in the image.
Question 2
Why is the image in a plane mirror always virtual? Contrast this with a real image.
Show Answer & Explanation
A virtual image is formed because light rays originating from an object point bounce off the flat mirror surface and diverge. To an observer, these diverging rays appear to come from an intersection point behind the mirror. Since no actual light rays pass through this point, it is virtual and cannot be projected onto paper or a screen. A real image, by contrast, is formed when light rays physically converge and intersect at a point in space (as in concave mirrors or convex lenses), meaning it can be caught on a screen placed at that position.
Question 3
Explain the concept of lateral inversion. Why does a mirror reverse left-to-right but not top-to-bottom?
Show Answer & Explanation
Lateral inversion occurs because the mirror reverses the front-to-back (depth) axis relative to the mirror face, while leaving the vertical and horizontal axes unchanged. When you face a mirror, your reflection is facing back at you (reversed along the depth axis). This depth reversal changes the coordinate chirality (hand orientation). Since we are vertically and horizontally symmetric, we mentally interpret this depth reversal as a left-to-right reversal rather than a front-to-back reversal. It does not reverse top-to-bottom because gravity defines a fixed vertical axis, and the mirror does not alter vertical coordinates.
Question 4
Why does a highly polished metal sheet make a good plane mirror, while a flat piece of paper does not, even though both are flat?
Show Answer & Explanation
This is due to the scale of surface roughness. The polished metal sheet is microscopically flat, meaning its surface irregularities are much smaller than the wavelength of visible light. When parallel light rays strike it, all normal lines are parallel, resulting in specular reflection where rays reflect parallel to each other to form a clear virtual image. The flat piece of paper, though macroscopically flat, is microscopically rough, with paper fibers creating irregularities larger than the wavelength of light. Normals point in random directions, causing diffuse reflection where light is scattered in all directions, destroying any image structure.