Reflection of light

1. Overview

Reflection occurs when light bounces off a surface. This phenomenon allows us to see non-luminous objects and is the fundamental principle behind mirrors, periscopes, and many optical instruments. Understanding how light reflects allows us to predict where images will appear and how they will look.

Key Definitions

• Normal: An imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point where the light ray strikes.
• Incident Ray: The incoming ray of light that strikes the surface.
• Reflected Ray: The ray of light that bounces off the surface.
• Angle of Incidence ($i$): The angle between the incident ray and the normal.
• Angle of Reflection ($r$): The angle between the reflected ray and the normal.
• Virtual Image: An image formed by light rays that appear to diverge from a point, but do not actually pass through it. It cannot be projected onto a screen.
• Lateral Inversion: The reversal of an image where the left side appears as the right side and vice versa.

Core Content

The Law of Reflection

For any smooth, reflecting surface, the following rule always applies: The angle of incidence is equal to the angle of reflection ($i = r$).

Characteristics of an Image in a Plane Mirror

The image formed by a plane (flat) mirror has five specific characteristics:

1. Same Size: The image is exactly the same height and width as the object.
2. Same Distance: The image is the same distance behind the mirror as the object is in front of it.
3. Virtual: The light rays do not actually meet behind the mirror; your brain "traces" them back.
4. Upright: The image is not upside down.
5. Laterally Inverted: The left and right sides are swapped (e.g., if you raise your right hand, the image raises its left).

Ray Diagrams

When drawing reflection:

• Always use a ruler and sharp pencil.
• Always draw the normal as a dotted line first.
• Ensure the rays have arrows showing the direction of light travel.

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Extended Content (Extended Curriculum Only)

Using Construction for Reflection

To locate an image accurately in a mirror, you should follow these steps:

1. Draw a line from the object perpendicular to the mirror.
2. Measure the distance from the object to the mirror and mark the image point at the exact same distance behind the mirror.
3. Draw a reflected ray from the mirror to the eye.
4. Use a dotted line to connect the reflected ray back to the virtual image point.

Worked Example: Distance Calculation

Question: An object is placed 1.5 meters in front of a plane mirror. How far is the object from its image? Solution:

• Distance from object to mirror = 1.5m.
• Distance from mirror to image = 1.5m.
• Total distance = $1.5\text{m} + 1.5\text{m} = 3.0\text{m}$.

Field of View

An observer can only see an object in a mirror if a light ray can travel from the object, hit the mirror, and reflect into the observer’s eye. If the mirror is too short or the object is too far to the side, the light will miss the mirror, and the object will not be visible.

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Key Equations

• The Law of Reflection: $i = r$
  • $i$: Angle of incidence (measured in degrees, °)
  • $r$: Angle of reflection (measured in degrees, °)
• Image Distance: $d_{o} = d_{i}$
  • $d_{o}$: distance of object from mirror (m)
  • $d_{i}$: distance of image from mirror (m)

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Common Mistakes to Avoid

• ❌ Wrong: Measuring the angle between the light ray and the mirror surface.
• ✓ Right: Always measure the angle between the light ray and the normal. (The angles against the mirror are called "glancing angles" and are not used in the law of reflection).
• ❌ Wrong: Thinking all letters change appearance in a mirror.
• ✓ Right: Symmetrical letters like 'A', 'H', or 'i' (if drawn as a simple stroke) look the same because their lateral inversion is identical to their original shape.
• ❌ Wrong: Labeling the image as "real" because you can see it.
• ✓ Right: Mirror images are always virtual because light rays do not actually pass through the mirror to the image point.
• ❌ Wrong: Confusing the normal with a light ray.
• ✓ Right: The normal is a reference line only; light does not "travel" along it unless the incident ray is at 0°.

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Exam Tips

1. Precision matters: If a question asks you to complete a ray diagram, use a protractor to ensure $i = r$. Examiners allow a very small margin of error (usually ±1° or 2°).
2. Object-Image Alignment: Always ensure the line connecting the object and the image is perpendicular (90°) to the mirror. If the object is at an angle, the image must be reflected at that same angle relative to the mirror's plane.
3. Clock Problems: If an exam asks for the time on a reflected clock, remember to "flip" the entire face. A hand pointing at '3' will appear to point at '9' in the reflection.