Light: Shadows and Reflections · Lesson 3 of 6
Reflection and Plane Mirror Images
“Follow light as a mirror redirects it, then investigate the image seen in a plane mirror.”
• Explain reflection using observations of a beam and a plane mirror. • Describe the size, orientation, and apparent position of a plane-mirror image. • Explain why a plane-mirror image cannot be caught on a screen in the investigated setup. • Recognise perceived left-right reversal and apply it to letters and signs. • Distinguish a reflected image from a shadow cast by a shiny object.
A Mirror Redirects Light
A shiny steel plate can block light and cast a shadow, yet it can also send some of that light onto a different wall. To investigate this, let sunlight fall on a flat shiny surface and tilt it until a bright patch appears on a wall that was not directly sunlit. As the mirror turns, the patch moves. The light follows a straight path before reaching the surface and another straight path after leaving it.
The change in direction of light when it meets a reflecting surface, such as a mirror, and travels away from it.
A mirror with a flat reflecting surface.
A more controlled test uses a torch and a comb whose openings are covered except for one thin slit. Aim the thin beam along a white sheet of paper. Place a plane mirror across the beam and look for the path after it meets the mirror. This makes the two straight parts of the route easier to compare. Neither a shadow nor reflection makes light travel through the opaque mirror.
Problem
A torch sends a narrow beam across paper to a flat mirror. A student tilts the mirror, and the light spot moves across a nearby screen. What changed?
- 1.The torch still sends light along its original incoming route.
- 2.Tilting changes the direction of the mirror’s reflecting surface.
- 3.The outgoing light follows a different route, so its spot changes position.
Use a torch or diffuse classroom light for close investigations. Never try to focus bright sunlight or a laser into anyone’s eyes while turning a mirror.
What Is an Image in a Plane Mirror?
Place a pen in front of a plane mirror. You see something resembling the pen apparently behind the mirror: that appearance is its image. The real pen is the object. Change the pen’s position and compare it with its image. The image remains the same size as the pen and stays upright: its tip still appears at the top.
The appearance of an object produced by light reaching the eye after interacting with an optical device, here a plane mirror.
An image that appears upright rather than upside down.
Stand at different distances from a mirror. When you stand close, your image seems close to it; when you step back, the image seems farther behind it. In a plane mirror, the apparent image distance behind the mirror matches the object’s distance in front. This is an apparent location, not a pen or person physically behind the mirror.
Problem
A pen stands 30 cm in front of a plane mirror and is moved to 15 cm in front. What changes about the image?
- 1.Initially the image appears about 30 cm behind the mirror.
- 2.After the pen moves, its image appears about 15 cm behind the mirror.
- 3.Its apparent position changes with the pen, while its size remains the same and it remains erect.
Why a Screen Does Not Catch This Image
Move a screen behind the mirror and then in front of it while the pen stays in place. You cannot collect the plane-mirror image on the screen. Light reaches your eye after reflecting from the mirror, making the pen seem to be behind it, but the image is not a patch of light converging there on a screen. This observation will help us compare the mirror with a pinhole camera in the next lesson.
Problem
A student sees a pen behind a mirror but finds no pen image on a sheet of paper held behind it. Is this a contradiction?
- 1.Seeing an image means reflected light reaches the student’s eye.
- 2.The pen only appears to occupy the position behind the mirror.
- 3.A plane-mirror image can be seen without being obtained on a screen behind the mirror.
Left and Right in the Mirror
Face a mirror and raise your left arm. Your image appears to raise the arm on the side you perceive as its right. Touch your right ear and the corresponding side of the image appears opposite to you. This perceived left-right reversal is called lateral inversion. It does not mean the image has turned upside down: the top of your head remains at the top.
The perceived interchange of left and right in a plane-mirror image.
Letters with left-right symmetry, such as a carefully drawn T or O, can appear unchanged even though the mirror image is laterally inverted. Ordinary words often look reversed. The word on the front of an ambulance is designed to be readable when a driver ahead sees it in a rear-view mirror. Ask students to write their own name on paper, hold it parallel to a mirror, and compare each letter with its image.
A plane mirror gives an erect image with perceived left-right reversal. A pinhole camera gives an inverted image on a screen. These are different observations and need different explanations.
The Aranmula Kannadi is a metal-surface mirror traditionally made in Kerala. Earlier mirrors also used polished metal or stone; glass mirrors later became common. The reflecting surface, rather than the name of its material, is the key part of the light investigation.
Quiz
What is reflection?
How does a plane-mirror image compare in size with the object?
A pen stands upright in front of a plane mirror. Its image is generally what?
What happens to the apparent image distance if you move closer to a plane mirror?
Which statement about the plane-mirror image matches the screen test?
Why might a carefully drawn letter O look unchanged in a plane mirror?
Practice Problems
- Describe what happens to the beam before and after a mirror redirects it in the comb-and-torch test.
- A shiny steel plate casts a shadow and also makes a bright spot on a wall. Explain both observations.
- A pen is 20 cm in front of a plane mirror. Where does its image appear, and how does its size compare?
- Describe a screen test for a plane-mirror image and the result.
- Hold a word written on paper parallel to a mirror. Predict which letters may look unchanged and explain why.
- Explain why a mirror image is described as laterally inverted but erect.
- Give one reason the mirrored ambulance lettering can help a driver ahead.
Key Takeaways
• Reflection changes the direction of light at a reflecting surface. • A plane mirror gives an erect image of the same size as the object. • The image appears as far behind the mirror as the object is in front, but it cannot be obtained on a screen. • Lateral inversion is perceived left-right reversal; it is not an upside-down image.