CIE IGCSE Physics (0625) Reflection of light Study Notes - New Syllabus
CIE IGCSE Physics (0625) Reflection of light Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Reflection of light
Key Concepts:
- Reflection of light
- Formation of an Optical Image by a Plane Mirror
Reflection of Light
Reflection of Light
Normal: An imaginary line drawn perpendicular (at 90°) to the surface where the light ray strikes. It is used as a reference line to measure angles.
Angle of Incidence (i): The angle between the incident ray (incoming ray) and the normal.
Angle of Reflection (r): The angle between the reflected ray and the normal.
A reflection diagram includes:
- A horizontal flat surface (mirror)
- An incident ray hitting the mirror
- The normal line at the point of incidence (90° to the surface)
- A reflected ray leaving the mirror at the same angle
Law of Reflection
- When a light ray reflects off a smooth (plane) surface, it obeys the following rule:
\( \text{Angle of Incidence (i)} = \text{Angle of Reflection (r)} \)
- The angle of incidence (i) is measured between the incident ray and the normal.
- The angle of reflection (r) is measured between the reflected ray and the normal.
- This law applies to all types of reflection — from smooth or rough surfaces — but is easiest to observe with a plane mirror.
How to Use:
- If you are given the angle of incidence, the angle of reflection is the same.
- If the angle of reflection is 42°, then the angle of incidence is also 42°.
- Use this rule when drawing ray diagrams and in calculations or reasoning tasks.
Example
A ray of light strikes a plane mirror at an angle of 35° to the normal.
What is the angle of reflection?
▶️ Answer
By the law of reflection:
\( i = r = 35^\circ \)
The angle of reflection is \( \boxed{35^\circ} \).
Formation of an Optical Image by a Plane Mirror
Formation of an Optical Image by a Plane Mirror
- When light rays reflect off a plane mirror, they appear to come from behind the mirror.
- The image is formed where the reflected rays appear to diverge from, not where they actually meet.
- This apparent location is found by extending the reflected rays backward.
Steps in Image Formation:
- Draw the incident rays from the object to the mirror.
- Apply the law of reflection: angle of incidence = angle of reflection.
- Extend the reflected rays backward behind the mirror using dotted lines.
- The point where these dotted lines meet gives the location of the image.
Characteristics of the Image Formed by a Plane Mirror:
- Virtual: The image cannot be projected onto a screen because the rays do not actually meet. The image only appears to exist behind the mirror.
- Upright: The image has the same orientation as the object (not upside-down).
- Laterally inverted: Left and right are reversed in the image.
- Same size: The image is exactly the same size as the object.
- Same distance from the mirror: The image is as far behind the mirror as the object is in front of it.
Note: A line from the object to the mirror and from the mirror to the image forms a straight path through the mirror.
Example:
A ray of light strikes a plane mirror at a point \( P \), with an angle of incidence of 40°. Using a ruler and protractor:
- Construct the reflected ray
- Measure and label all angles
- Calculate the angle between the incident ray and the reflected ray
▶️ Answer/Explanation
Step 1: Draw the mirror
Draw a vertical line to represent the plane mirror.
Step 2: Mark the incident point
Choose point \( P \) on the mirror and draw the normal at \( P \), perpendicular to the mirror surface.
Step 3: Draw the incident ray
Using a protractor, measure 40° from the normal on one side and draw the incident ray approaching the mirror at that angle.
Step 4: Apply the law of reflection
By the law: \( \angle i = \angle r = 40^\circ \)
Measure 40° on the other side of the normal and draw the reflected ray.
Step 5: Label angles and rays
Mark the angles of incidence and reflection, and label the incident ray, normal, and reflected ray.
Step 6: Calculate angle between incident and reflected ray
Since both rays are 40° from the normal but on opposite sides:
Total angle = \( 40^\circ + 40^\circ = \boxed{80^\circ} \)